Zinc: a multipurpose trace element
Zinc (Zn) is one of the most important trace elements in the body and it is essential as a catalytic, structural and regulatory ion. It is involved in homeostasis, in immune responses, in oxidative stress, in apoptosis and in ageing. Zinc-binding proteins (metallothioneins, MTs), are protective in situations of stress and in situations of exposure to toxic metals, infections and low Zn nutrition. Metallothioneins play a key role in Zn-related cell homeostasis due to their high affinity for Zn, which is in turn relevant against oxidative stress and immune responses, including natural killer (NK) cell activity and ageing, since NK activity and Zn ion bioavailability decrease in ageing. Physiological supplementation of Zn in ageing and in age-related degenerative diseases corrects immune defects, reduces infection relapse and prevents ageing. Zinc is not stored in the body and excess intakes result in reduced absorption and increased excretion. Nevertheless, there are cases of acute and chronic Zn poisoning.
KeywordsZincMetallothioneinsZinc supplyAgeingAntioxidantsZinc toxicity
Zinc (Zn) is a trace element essential for cell proliferation and differentiation. It is a structural constituent of many enzymes and proteins, including metabolic enzymes, transcription factors, and cellular signalling proteins. There is increasing evidence for a direct signalling function of Zn at all levels of cellular signal transduction (Beyersmann 2002).
Zinc is an important element in preventing free radical formation, in protecting biological structures from damage and in correcting the immune functions. More specifically, Zn is an essential element for thymic functions by means of a Zn-dependent thymic hormone called thymulin required for T-cell maturation and differentiation (Goldstein 1984; Mocchegiani et al. 2000).
Zinc deficiency increases the levels of lipid peroxidation in mitochondrial and microsomal membranes and the osmotic fragility of erythrocyte membranes. Zinc deficiency also produces impaired haemostasis due to defective platelet aggregation, a decrease in T-cell number and the response of T-lymphocytes to phytomitogens (Keen and Gershwin 1990; Tapiero and Tew 2003). Zinc deficiency produces growth retardation, anorexia, delayed sexual maturation, iron-deficiency anemia, and alterations of taste (Barceloux 1999).
Zinc-binding proteins, such as metallothioneins (MTs), belong to the family of intra-cellular metal-binding proteins that are present in virtually all living organisms and they play a key role in the Zn effect upon the immune system. MTs are protective against stress and increase in ageing (Mocchegiani et al. 2001). While MTs do not appear to be essential for life, as evidenced by apparently normal reproductive capacity and long-term survival of mice lacking functional MT genes, there is evidence for a survival advantage of MT production in situations of stress, including exposure to oxyradicals and toxic metals, inflammation, infection and low Zn nutrition (Coyle et al. 2002).
In this paper, we review the Zn as a multipurpose trace element, the toxicity of Zn, its role as antioxidant, in apoptosis, in stress conditions, in immune responses and in ageing and age-related diseases. Also, in this paper the efficacy of supplementing Zn in ageing and in age-related degenerative diseases is described.
Biology of zinc
Zinc is a ubiquitous trace element. It is one of the most important trace elements in the body and it is indispensable to the growth and development of microorganisms, plants, and animals. It is found in all body tissues and fluids in relatively high concentrations, with 85% of the whole body zinc in muscle and bone, 11% in the skin and the liver and the remaining in all the other tissues. The average amount of Zn in the adult body is about 1.4–2.3 g Zn (Calesnick and Dinan 1988).
In multicellular organisms, all Zn is intracellular, 40% is located in the nucleus, 50% in the cytoplasm, organelles and specialized vesicles and the remainder in the cell membrane (Tapiero and Tew 2003). The potential importance of Zn to the gene can be appreciated from the fact that a great percentage of the Zn content of rat liver is found in the nucleus and a significant amount of Zn is incorporated into nuclei in vitro (Cousins 1998). Mechanistically, Zn is involved in the processes of genetic stability and gene expression in a variety of ways including the structure of chromatin, the replication of DNA and transcription of RNA through the activity of transcription factors and RNA and DNA polymerases, as well as playing a role in DNA repair and programmed cell death (Falchuk 1998).
Of the trace metals found in humans, only iron is more abundant. However, if hemoglobin-bound iron is not considered, then Zn becomes the most abundant transition metal (Vasak and Hasler 2000).
The minimum Zn requirements of humans compatible with satisfactory growth, health, and well-being vary with the type of diet consumed, climatic conditions and the existence of stress imposed by trauma, parasitic infestations, and infections. In general, the recommended daily dietary Zn requirement is estimated at 15 mg/day (Tapiero and Tew 2003) and the tolerable upper intake level of Zn recommended is 25 mg/day (SCF 2003).
Zinc is virtually nontoxic to living organisms. It is the only pre-, post-, and transitional element that is neither cytotoxic nor systematically toxic, nor is it carcinogenic, mutagenic, or teratogenic. Zinc is not stored in the body and excess intakes result in reduced absorption and increased excretion. Nevertheless, there are documented cases of acute and chronic Zn poisoning (SCF 2003).
Acute toxicity is infrequent in humans. Brown et al. (1964), described several cases of food poisoning resulting from storage of food or drink in galvanized containers. Ingestion or administration of even very large amounts of Zn and all its compounds does not have adverse long-term consequences. In man adverse systemic reactions are only observed in very rare instances i.e. either on inhalation of Zn oxide fumes (metal fume fever) or accidental ingestion of large amounts of the metal (Vallee 1995; Bertholf 1988; Vallee and Falchuck 1993).
Chronic and subchronic toxicity
A minimum risk concentration (MRL) of 0.3 mg Zn/kg/day was established for intermediate oral exposure based on the development of reduced haemoglobin and serum ferritin concentrations. These reductions, including reduced erythrocyte superoxide dismutase (SOD) activity, developed in women were given daily supplements of 50 mg Zn as zinc gluconate for 10 weeks (Yadrick et al. 1989; Barceloux 1999). Prolonged intakes of Zn supplements ranging from 50 mg/day up to 300 mg/day have been associated with leucopaenia, neutropaenia, sideroblastic anaemia, decreased concentrations of plasma copper and decreased activity of the copper containing enzymes, SOD and ceruloplasmin, altered lipoprotein metabolism and impaired immune function (Sandstead 1995). Many of these biochemical and physiological changes are similar to those observed during copper deficiency.
Zinc plays three major biological roles in the organism: as catalyst, and as structural and regulatory ion (Mocchegiani et al. 2000).
Zinc is required for the biological function of more than 300 enzymes. In particular, Zn is essential and directly involved in catalysis and co-catalysis by the enzymes, which control many cell processes including DNA synthesis, normal growth, brain development, behavioural response, reproduction, fetal development, membrane stability, bone formation and wound healing (Barceloux 1999; Mocchegiani et al. 2000). Catalytic Zn sites generally comprise three coordinating protein ligands plus one water molecule: histidine nitrogen and/or glutamate as aspartate oxygen and cysteine sulphur atoms are the usual donors with a frequency of His > Glu > Asp > Cys for all active sites containing a single, catalytic Zn atom (Vallee and Auld 1990). Cocatalytic Zn sites are characteristic of multi-Zn enzymes containing two or more Zn and/or Mg atoms adjacent to one another. These contain Asp or Glu bridges between at least two of the three metal atoms, the most characteristic feature of this group, resulting in a multi-metal site in which the metal atoms are very close to one another (Vallee 1995).
Zinc, due to its physico-chemical properties, plays structural and functional roles in several proteins involved in DNA replication and reverse transcription and it is critical for the function of a number of metalloproteins (Mocchegiani et al. 2000; Tapiero and Tew 2003). The structures of nineteen mono-Zn enzymes have been determined by X-ray crystallography and/or NMR and now serve as standards of reference for the Zn ligands of their respective protein families. Within any given enzyme family the metal binding site is characteristic (Vallee 1995). Zinc ions are hydrophilic and do not cross cell membranes by passive diffusion. In general, transport has been described having both saturable and non-saturable components, depending on the Zn concentrations present. Zinc ions exist in the expression of genetic information, in storage, synthesis and action of peptide hormones and structure maintenance of chromatin and biomembranes (Tapiero and Tew 2003).
The biological essentiality of Zn implies the existence of homeostatic mechanisms that regulate its absorption, distribution, cellular uptake and excretion (Vallee and Falchuk 1993). Zinc regulates both enzymatic activity and the stability of the proteins as an activator or as an inhibitor ion (Mocchegiani et al. 2000). To regulate the availability of Zn dynamically, eukaryotes have first compartmentalized Zn and at the same time they have the metallothionein/thionein pair, which controls the pico- to nanomolar concentrations of metabolically active cellular Zn (Maret 2003). Zinc has also been found to modulate cellular signal transduction processes and even to function as a modulator of synaptic neurotransmission in the case of the Zn-containing neurons in the forebrain (Beyersmann 2002).
Zinc and MTs
MTs are low-molecular weight metal-binding proteins with 61 aminoacids; among them 20 are cysteines. MTs play pivotal roles in metal-related cell homeostasis because of their high affinity for metals, in particular zinc and copper (Bremner and Beattie 1990; Mocchegiani et al. 2000; Stefanidou and Maravelias 2005.)
The order of binding affinity of metals to MTs in vitro has been demonstrated to be Zn < Cd < Cu < Hg (Holt et al. 1980). Although the metals, Zn, Cu, Cd, Hg, Au and Bi all induce MT, Zn is the primary physiological inducer since the other metals, except Cu, can be regarded as environmental toxicants (Coyle et al. 2002). Moreover, MTs, play an important regulatory role in Zn uptake, distribution, storage, and release. MTs may also play a role in Zn absorption by competing with or supplying Zn to a variety of transporter proteins (Vasak and Hasler 2000).
It is well known that MTs bind Zn more tightly than most other Zn proteins and Zn bound to MTs represents 5–10% of the total Zn in human hepatocytes (Buhler and Kagi 1974; Maret 2000). However, it is not clear whether the binding of Zn to MT is only the result of MTs synthesis or the consequence of MTs function (Kondoh et al. 2003a). Cu- and ZnMT appear to be degraded differently, since the rate of degradation of MTs is determined by the identity of the metal atom bound to the protein, and half-lives of the metals. Thus, Zn is rapidly released from the protein and is therefore able to participate in cellular function and to induce further MTs synthesis. However, Zn clearly has protective effects independent of MTs. This is perhaps not surprisingly given that there are over 300 enzymes known to be Zn dependent, in addition to other actions, including membrane stabilization (Coyle et al. 2002).
MTs are not only metal-binding proteins, but also potent scavengers of heavy metals, including cadmium, mercury, zinc and copper by forming trimercaptide linkages and MTs induced by chemical stressors has been shown to detoxify harmful heavy metals and reactive oxygen species (ROS) produced by the stressors (Kondoh et al. 2003a). It has been shown that the ability of MTs to capture hydroxyl radicals, which are primarily responsible for the toxicity of ROS, is 300-times greater than that of glutathione, the most abundant antioxidant in the cytosol (Sato 1992). Thus, MTs protect biological structures and DNA from oxidative damage, by distributing Zn, since Zn undergoes rapid inter- and intra-cluster exchange, maintaining the MTs tissue concentrations (Cousins 1985; Davis et al. 1998; Kondoh et al. 2003b). Long-term deprivation of Zn renders an organism more susceptible to injury induced by oxidative stress (Tapiero and Tew 2003). There is considerable evidence that Zn metabolism is altered in Alzheimer’s-disease and other neurodegenerative diseases (Aschner 1996), since it has been found that MT-3 was deficient in extracts from Alzheimer’s-disease brains. In this situation, MTs may both protect neurons from oxidative stress as well as modulate neurotransmission (Coyle et al. 2002; Stefanidou and Maravelias 2005).
MTs are protective against stress (Kagi 1993) and bind Zn with higher binding affinity (kd) than thymulin (Mocchegiani et al. 1998). MTs increase in ageing and show preferential binding with Zn in the aged, rather than with copper in the young (Hamer 1986; Mocchegiani et al. 2000). High MTs levels obstruct antioxidative responses after exposure to H2O2 in the brain of trisomy 16 mouse (experimental model of Down’s syndrome) (Scortengagna et al. 1998). Oxidative damage and impaired immune responses in Down’s syndrome subjects (accelerated ageing) are restored by supplementing Zn (Chiricolo et al. 1993; Fabris et al. 1993). Therefore, the biological role of MTs is crucial in antioxidative and immune responses during ageing and age-related diseases (Mocchegiani et al. 2000).
Studies using transgenic mice suggested a role for MT-1 and MT-2 isoforms in Zn metabolism. Zn in response to various types of stress such as acute endotoxin inflammation and starvation has been confirmed using MT-null (MT—/—) mice (Philcox et al. 19952000). Seven atoms of Zn are known to be bound to 1 mol of MT (Kondoh et al. 2003a). These results have led to the proposal that MTs protect the cell from high Zn levels by the production of Zn7-MT and that, on the other hand, it offers a driving force for Zn-uptake by the transient production of apo-MT (Tapiero and Tew 2003).
A widely accepted but still disputed functional model for Zn7-MTs includes Zn donation to specific storage sites, or to proteins. In the past, a number of in vitro studies showed that Zn7-MTs and the apoprotein (thionein), through Zn transfer or removal, affect the activity of Zn-dependent proteins and can be involved in modulation of the DNA-binding ability of Zn-finger transcription factors (Vasak and Hasler 2000).
In inflammation, the liver is thought to be the main site of Zn regulation via the induction of ZnMT in the liver (Sobocinski et al. 1978; Cousins 1985; Bremner and Beattie 1990; Kondoh et al. 2003a). MTs in other tissues, including the small intestine (Coyle et al. 1999, 2000), and pancreas (De Lisle et al. 1996; Rofe et al. 1999) may restrict Zn loss from its major route of excretion, the gut (Coyle et al. 2002).
In addition to the above, MTs induction by dietary supplementation of Zn acetate (25–50 mg, three times per day) (Henderson et al. 1995) is a recommended therapy for long-term management of patients with Wilson’s disease, an inherited disorder of Cu accumulation and toxicity (Brewer et al. 1989; Brewer 2000). Zn induces intestinal MT, which sequesters Cu in the mucosal cell and prevents its transfer into the circulation. Intestinal cells turnover approximately every 6 days, thus removing the MT-bound Cu in the stool. Hepatic MT is also temporarily increased, presumably in the form of nontoxic CuMT. In the long term (>18 months of Zn treatment), hepatic Cu concentrations remain the same or lower than pretherapy levels, and there is normal liver function (Coyle et al. 2002).
Zinc as antioxidant
A free radical is any molecule that contains one or more unpaired electrons. Zinc does not hinder directly a free radical reaction, rather it exerts its effects in an indirect manner by stabilizing the cell membrane structure, contributing to the structure of the SOD and maintaining the metallothionein tissue concentrations (Tapiero and Tew 2003).
The role of Zn in protecting biological structures from damage by free radicals may be due to several factors: maintaining an adequate level of MTs (which are also free radical scavengers), as an essential component of SOD, as a protective agent for thiols and in preventing the interaction between chemical groups with iron to form free radicals (Cousins 1985; Davis et al. 1998; Tapiero and Tew 2003).
Zinc is recognized to be important for stabilizing DNA and it appears to reside longer in the nucleus than in any other cell compartment. Accordingly, it is possible that as intracellular levels of Zn rise, more iron will be displaced from the nucleoproteins and less OH-driven DNA damage will occur (Dreosti 1991). An interesting extension of this notion is the proposal that an underlying mechanism for the carcinogenicity of several heavy metals may be the displacement of Zn from Zn-finger transcription factors and the release of oxygen radicals on the DNA where they are bound (Sarkar 1995; Hartwig 1998; Dreosti 2001). Support for this view comes from the recent finding that displacement by iron of Zn from the two Zn-fingers in the estrogen receptor transcription factor will, in the presence of H2O2 and ascorbate generate highly reactive free radicals causing cleavage of DNA in the estrogen responsive element (Conte et al. 1996; Dreosti 2001).
Zinc deficiency, after prolonged reduction of intake or excessive uncompensated losses, has been described both in animals and humans. Long-term deprivation of Zn renders an organism more susceptible to injury induced by oxidative stress. More specifically, Zn deficiency increases the levels of lipid peroxidation in mitochondrial and microsomal membranes and the osmotic fragility of erythrocyte membranes, while the presence of Zn prevents lipid peroxidation. Thus, Zn may play a role in protecting the cell from oxidative stress (Vallee and Falchuk 1993; Tapiero and Tew 2003).
Zinc and apoptosis
Zinc deprivation by starvation or by Zn chelation leads cells to death and the mode of cell death in many cell types is apoptosis, whereas Zn may also become cytotoxic if its extracellular concentration exceeds the capacity of the Zn homeostatic system. Elevated extracellular Zn concentrations lead to the breakdown of the Zn transporting system of the plasma membrane. The resulting enhanced intracellular Zn concentration activates the apoptosis (Beyersmann 2002).
Apoptosis is a major mechanism of programmed cell death involved in several biological events during tissue development, remodelling or involution. It is a regulated biological mechanism required for the removal and deletion of superfluous, mutant or moderately damaged cells in response to toxic agents (Nath et al. 2000). Rather than the cellular ‘homicide’ that occurs in necrotic cell death, apoptosis is a pathway of cellular ‘suicide’. Apoptosis is morphologically distinct from cell death due to lysosomal breakdown and/or necrosis (Kumar et al. 2003). Apoptosis occurs in two phases: in the first, the biochemical signalling pathways commit a cell to apoptosis and in the second, the executional phase is characterized by morphological changes leading to cell death (Tapiero and Tew 2003).
The precise mechanisms underlying the triggering of apoptosis are not clear but damage to DNA and activation of the p53 gene appears to be an important component of the process as also is activation of certain members of the caspace family of proteases. Zinc is involved to some extent in both processes (Dreosti 2001). The specific DNA-binding domain of p53 has a complex tertiary structure that is stabilized by Zn (Verhaegh et al. 1998). Modulation of binding of p53 to DNA by physiological concentrations of Zn might represent a pathway that regulates p53 activity in vivo (Palececk et al. 1999).
Apoptosis is induced by several extracellular or intracellular stimuli with an important role for trace metals like Zn or calcium (Seve et al. 2002). The dysregulation of apoptosis is central to pathogenic mechanisms in many diseases such as neurodegenerative disorders, acquired immune deficiency syndrome, autoimmune disease and cancers (Thornberry and Lazebnik 1998; Tapiero and Tew 2003).
Increased apoptosis in vivo may occur as direct or indirect consequence of a decrease in intracellular Zn concentrations. Therefore, cellular Zn is described as an inhibitor of apoptosis, while its depletion induces death in many cell lines (Seve et al. 2002). This Zn depletion activates some apoptosis-specific proteases (caspases) that may activate the apoptotic endonucleases which produce the typical internucleosomal fragmentation of DNA (Beyersmann 2002). The caspases-3, −8 and −9 are responsible for the proteolysis of several target proteins like poly (ADP-ribose) polymerase or transcription factors. Caspase-6 is the most sensitive apoptosis-related molecular target of Zn. It cleaves and activates the proenzyme form of caspase-3 and is also responsible for the cleavage of lamins and therefore, is directly involved in nuclear membrane dissolution (Tapiero and Tew 2003). The balance between life and cell death is maintained by several Zn channels, controlling the intracellular Zn movements and the free amount of the metal (Seve et al. 2002).
Zinc and the immune system
Zinc is considered crucial for immune responses. It influences and interacts specifically with components of the immune system, a highly proliferative system (Wellinghausen and Rink 1998). Zinc is relevant for immunocompetence, because it bounds to enzymes, proteins and peptides with different binding affinity (Mocchegiani et al. 2000). Zinc is transported to cells bound to proteins, predominantly albumin, a2-macroglobulin and transferrine, but only free Zn ions seem to be biologically active (Vallee and Falchuk 1993). The function of a2-macroglobulin is regulated by Zn itself. Zinc alters the structure of a2-macroglobulin and enhances its interaction with cytokines and proteases, thus indirectly influencing immune function. Impairment of immune function has been attributed to Zn deficiency and may be the most common cause of secondary immunodeficiency states in humans (Tapiero and Tew 2003).
Decreased chemotaxis by neutrophils and monocytes and impairment of cell-mediated immune responses, including thymic endocrine activity, natural killer (NK) activity, and cytokine production are observed in Zn deprived diets (Wellinghausen et al. 1997). In addition, the liver extrathymic NK activity is also reduced in ageing because of the major quota of Zn ions bound with liver MTs in aged and immunosuppressive persons (Mocchegiani and Muzzioli 2000b; Mocchegiani et al. 2000). In particular, Zn is essential for thymic functions by means of a Zn-dependent thymic hormone called thymulin (ZnFTS) (Dardenne et al. 1982) required for T-cell maturation and differentiation (Goldstein 1984). Zinc is also important for liver extrathymic T-cell functions (Mocchegiani et al. 1997) prominent in ageing (Abo 1993) and, in turn, Zn, thymulin and peripheral immune responses show age-related declines (Mocchegiani and Fabris 1995; Mocchegiani et al. 1995a, 2000).
Zinc-binding proteins, such as MTs, may play key roles in the Zn effect upon the immune system. The correlation between plasma MTs and the immune system indicates that the extracellular Zn-MT stimulates lymphocyte proliferation. Mitogens and extracellular membrane-bound MTs act synergistically to induce T- and B-lymphocyte proliferation. If only MTs are present in the cell membrane, proliferation occurs only in B-lymphocytes and their differentiation is activated (Lynes et al. 1990; Borghesi et al. 1996). Nevertheless, Zn is the foremost biological cargo of MTs, that maintain Zn and act as a component of cellular defense against partially reduced oxygen (Thornberry and Lazebnik 1998; Telford and Fraker 1995). Since Zn ion bioavailability is essential for immune functions, attention is addressed to the role of MTs in immune responses, including NK cell activity, because of MTs increments in ageing. Moreover, low Zn ion bioavailability is a risk factor for infection relapses in the elderly, who display decreased immune functions (Mocchegiani and Muzzioli 2000a).
MTs play a pivotal role in Zn-related cell homeostasis because of their high affinity for this trace element (Kagi and Schaffer 1988), which is in turn relevant against oxidative stress (Evans and Halliwell 2001) and for the efficiency of entire immune system including NK cell activity (Mocchegiani et al. 1998). MTs sequester Zn ions during transient stress-like condition, as it may occur in young-adult age (Kelly et al. 1996), because residual Zn ions must be maintained due to a consistent loss of Zn by urine and faeces; (Mocchegiani et al. 1998). But, at the same time, MTs also dispense Zn to Zn-dependent enzymes and proteins (Kagi and Schaffer 1988).
Zinc deficiency produces impaired haemostasis due to defective platelet aggregation, a decrease in T-cell number and the response of T-lymphocytes to phytomitogens. In fact, Zn is the only naturally occurring lymphocytic mitogen (Keen and Gershwin 1990; Tapiero and Tew 2003). Deficiencies in Zn also accompany many diseases such as gastrointestinal disorders, renal disease, sickle cell anaemia, alcoholism, some cancer types, AIDS, burns and others (Keen and Gershwin 1990; Mocchegiani et al. 1995; Fraker et al. 2000). In ageing, loss of immunological responses may have various origins among them decline in neuroendocrine function (Fabris et al. 1997) and increase in apoptosis regulated by Zn (Sunderman 1995). Aside from the restoration/stimulation of immune surveillance, Zn was shown to correct the immune function depressed by tumour burden (Singh et al. 1992). Several studies have demonstrated that Zn inhibits phagocytosis, reducing the release of oxyradicals and production of superoxide and hydrogen peroxide. The increase in ROS is apparently due to a G-protein coupled system activated by Zn (Takeyama et al. 1995; Tapiero and Tew 2003).
Clinical observations have also shown that serum Zn concentrations were significantly lower in patients with oesophagitis than control subjects (Kadakia et al. 1992). In patients having oesophagitis and receiving H2-receptor antagonists, oesophageal tissue concentrations of Zn approached normal values. In the stomach, Zn deficiency increases gastric secretory volume, acid and pepsin and promotes or aggravates stress-induced gastric lesions and decrease in mast cell count (Cho et al. 1987). Thus, any state manifesting systemic Zn deficiency will promote or potentiate the development of gastric mucosal damage by increased mast cell histamine and acid-pepsin secretion (Tapiero and Tew 2003).
It has been shown that Zn supplementation in old mice is capable to restore immune efficiency with no interference on already high MTs levels (Mocchegiani et al. 2002). As such, more Zn ions bioavailability occurs with subsequent extension of the maximum life span due to significant reductions of deaths from infections (Mocchegiani et al. 1998). Thus, supplementing Zn might induce MTs to regain their role of protection and it may arrest tumour growth and lead cancer cells to cell death by means of p21 overexpression (Liang et al. 1999). This phenomenon has been recently suggested for increased a-2 macroglobulin concentrations in cervical carcinoma (Mocchegiani et al. 1999a).
Zinc and ageing
In ageing MTs preferentially bind Zn rather than copper and they are unable to release Zn. Indeed, during ageing the stress like-condition is persistent provoking a sequester of intracellular Zn with subsequent low Zn ion bioavailability for immune efficiency and for the activity of Zn-dependent enzymes and proteins (Mocchegiani et al. 2000, 2002).
Low Zn ion bioavailability and high MTs levels are present in ageing and stress (Mocchegiani et al. 1997, 2000), thus, Zn and MT homeostasis is crucial for NK activity of thymic (Muzzioli et al. 1992) or liver (Tsukahara et al. 1997) origin during ageing. Moreover, low Zn ion bioavailability and high MTs levels consist risk factors for infection relapses in the elderly, since the old organism becomes a ‘low responder’ to external harmful stimuli with the appearance of age-related degenerative diseases (cancer and infections) (Mocchegiani et al. 2000; Ebadi and Swanson 1988). Low Zn ion bioavailability and reduced T-cell sub-populations (CD4+) are the major risk factors for the appearance of infections in elderly and infection relapse in old infected patients (Mocchegiani et al. 1999b).
Since cognitive functions are impaired in hypothyroidism (Vara et al. 2002), which is a usual event in Down’s syndrome and elderly (Fabris et al. 1997), and Zn affects thyroid hormones receptors (Darling et al. 1998) and brain function (synaptic transmission) (Weiss et al. 2000), it is evident that a low Zn ion bioavailability may also trigger impaired cognitive functions, via altered thyroid hormones turnover. Such an assumption is also supported by high MTs and low Zn ion bioavailability (Mocchegiani et al. 2002a) coupled with impaired cognitive functions (Sawin et al. 1998) in experimental hypothyroidism (propylthiouracil treated mice).
Since Zn-bound MTs increase also in the brain from old mice (Mocchegiani et al. 2001) and from trisomy 16 mouse (experimental model of human Down’s syndrome) with no protective role by MTs against stress (Scortegagna et al. 1998), it is thus very suggestive to interpret Zn-bound MTs homeostasis as pivotal in general for ageing, including brain and cognitive functioning.
The supply of Zn is considered as necessary in ageing (Fortes et al. 1998; Lesourd 1997), although some authors report very limited beneficial effect of Zn in immune efficiency in elderly people (Bogden et al. 1990). In those cases where Zn is considered as necessary, its role is duplicate. First, it induces major Zn ion bioavailability by faster MT degradation; second, it avoids the continuous sequestration of intracellular Zn by MTs. Therefore, a plateau in MTs protein production is maintained with a consequent role of protection as Zn donors (Mocchegiani et al. 2002; Klaassen et al. 1994).
Nevertheless, the efficacy of Zn supply seems to be strictly related to the dose and length of the treatment. Long treatment or high doses of Zn may provoke a Zn accumulation with subsequent damage on immune efficiency (Mocchegiani et al. 2001), whereas, physiological dose of Zn (12 mg Zn++/day) (USDA 1976) for short period (1 month) restores immune efficiency in Down’s syndrome individuals (Fabris et al. 1993) and in HIV patients (Mocchegiani et al. 1995b), with reduction (50%) of infectious episodes in Down’s syndrome (Fabris et al. 1993) and no appearance of the first opportunistic infection in HIV subjects (Mocchegiani and Muzzioli 2000b).
From all these considerations, it is thus clear that a Zn supply may be useful to reduce infection relapse and to restore immune efficiency in elderly and, at the same time, in preventing age-related degenerative diseases (Mocchegiani et al. 2000). Such an assumption is supported because of no increments on already high MTs in old Zn treated mice (Mocchegiani et al. 2002). Moreover, MTs may be possible genetic markers of immunosenescence, since high MTs levels are present in ageing (Mocchegiani et al. 2000, 2001).
Zinc appears to be a multifunctional molecule compatible with satisfactory growth, health, and well-being. Zinc is involved in various immune responses, in oxidative stress, in apoptosis and in ageing. Zinc, via thymulin, is one of the mechanisms by which Zn may impact upon the immune responses.
The dysregulation of cellular Zn during stress, ageing and other diseases may affect cell survival. Nevertheless, the role of MT in maintaining Zn and the divalent metal concentrations is obviously critical to cell survival. Zinc bound MTs homeostasis is pivotal for health longevity with a potential role of biological and genetic marker of immunosenescence and in general of ageing, since high MTs levels are present in ageing.
An overall estimation of all experimental and clinical observations on the biological role of Zn seems to lead us to the conclusion that Zn supply may be useful in reducing infection relapse and in restoring immune efficiency in ageing and in preventing age-related degenerative diseases. Nevertheless, there are documented cases of acute and chronic Zn poisoning.