The structure–activity relationship review of the main bioactive constituents of Morus genus plants

Morus genus plants are mainly distributed in the temperate to tropical areas over the world and include 17 species and two subspecies. Due to their excellent pharmacological activity, security in food additives and high value in the national economy, Morus genus plants have drawn more and more attention in recent years. In the light of the references published over the last few decades, flavonoids, benzofurans, stilbenes, and Diels–Alder adducts have been reported to be the main bioactive constituents of Morus genus plants. This review summarizes the compounds with excellent bioactivities isolated from Morus genus plants as well as their structure–activity relationships (SARs), which might be useful for the further research and development of Morus genus plants. The aromatic heterocycles with excellent bioactivities isolated from Morus genus plants as well as their structure–activity relationships (SARs) were summarized.


Introduction
Morus genus plants comprise flowering trees belonging to Moraceae family and are distributed in the temperate to tropical areas over the world. According to the webpage of https ://www.thepl antli st.org, the Morus genus comprises 17 species and two subspecies. In traditional Chinese medicine (TCM), the leaves, root, bark, stems, and fruits of M. alba are used for treatment of rheumatism, cough and inflammation, and the leaves and fruits of M. alba serve as foodstuff around the world. Recently, not only M. alba, but also other Morus genus plants' (such as M. alba var. tatarica, M. lhou, M. australis, M. yunnanensis, M. cathayana, and M. nigra) chemical components and their biological activity have been evaluated [1][2][3].
This paper intends to provide an review of biological chemical constituents present in Morus genus plants and summarize their structure-activity relationships (SARs) on α-glucosidase, lipase, tyrosinase, β-secretase, acetylcholinesterase and cytotoxicity, which may be benefit for nutritional supplements development and structure modification of lead compound from Morus genus plants.

Polyphenols from Morus genus plants
Morus genus plants have a diverse polyphenol profile that includes flavonoids, benzofurans, stilbenes, and Diels-Alder adducts with hydroxyl, methoxyl, glycosyl or prenyl substitution moieties as shown as Fig. 1.
For benzofurans from Morus genus plants, the major substitution type is hydroxyl or methoxyl at C-7, C-3′ or C-5′, while the prenyl group substitution often occurs at C-7, C-2′ and C-4′, and the cyclization is always linked between 4′-prenyl and 3′-OH.
Stilbenes, whose chemical structures are similar to those of benzofurans, are another kind of compounds in Morus genus plants. The substitution types and positions are similar to those of benzofurans. According to the different numbering rules, the oxygen groups at C-2, C-4, C-3′ and C-5′ and prenyl groups at C-2′ and C-4′ have the most common substitution patterns.
Diels-Alder adducts are another kind of polyphenols in Morus genus plants, most of them contain flavonoid groups, and the C-2 and C-3 of the flavonoid unit can be replaced by prenyls and their analogs.

SARs of bioactive compounds from Morus genus plants
As a widely used complementary and alternative medicine, Morus genus plants, especially mulberry leaves, were used as adjuvant for blood sugar and TG management, neuroprotection, as anti-tumor agent, immunity regulation, and so on. According to their clinic effects, bioactive compounds in Morus genus plants were screened and the SARs to key enzymes partly characterized.

SARs of α-glucosidase inhibition
Type 2 diabetes is a kind of metabolic disease affecting more and more people all over the world. It is characterized by high postprandial glucose level, high fasting glucose level, insulin resistance, and relative lack of insulin. As one of the effective treatments for high postprandial glucose level, α-glucosidase inhibitors were used to lower the digestion of carbohydrates and reduce the absorption of glucose from the intestine [12,14]. 1-Deoxynojirimycin and its analogs obtained from leaves of Morus genus plants have been considered to be a classical effective α-glucosidase inhibitor [7,8], and flavonoids, benzofurans and Diels-Alder adducts were also found to play significant roles.

SARs of lipase inhibition
Hyperlipemia is a high risk factor of obesity, heart disease, diabetes and retinal vascular disease with an abnormally high triglyceride (TG) level in the blood. Pancreatic lipase (PL) is an enzyme that catalyzes triglyceride to fatty acids and glycerol. Inhibition of PL can decrease TG hydrolysis and reduce free fatty acid absorption from intestine to blood and finally lower blood TG level [18,19]. Orlistat, a PL inhibitor, was used as an anti-obesity drug to inhibit dietary fat absorption and reduce cardiovascular risk factors.
Several  [21]. Through the observation of the structures of bioactive compounds, it was found that if 3′-OH in 2-arylbenzofurans were methylated, their PL inhibitory activity may be decreased (41 vs 42, 43 vs 44) (Fig. 3).

SARs of tyrosinase inhibition
Tyrosinase is an oxidase distributed in fungi, plants and animals widely. Generally, tyrosinase is known to be involved in melanin synthesis that gives skin, hair, and eyes color. In tumor cells, tyrosinase expression level is significantly upregulated than in normal cells along with increase of tyrosinase activity. Tyrosinase inhibitors have attracted considerable attention in improving tumor immunity, such as imatinib, an antitumor drug against chronic myelogenous leukemia.
Morus genus plants-enriched polyphenol had been used as a kind of non-toxic natural tyrosinase inhibitor to whiten skin [22,23]. Early in 1998, Shin et al. [24] have studied the tyrosinase inhibitory activity of the hydroxystilbenes isolated from the twig of M. alba. The SARs summarized in the reference suggested that the existence of hydroxyl groups in oxyresveratrol and their positions might be important for its inhibitory activity, and the substitution of methyl groups would negatively influence the inhibitory effects. Moreover, Zheng et al. [25] have reported that the substituted position of prenyl/geranyl groups is the key moiety in the tyrosinase inhibitory activity of flavonoids from roots of Morus nigra, monoisoprenyl-substituted flavone compounds as well as 2-arylbenzofuran derivatives. Free hydroxyl group may be the active unit, especially for 4′-OH of flavanone, 2-or 4-OH of stilbenes and 4-OH resorcinol skeleton. Intact prenyl group might lead to higher tyrosinase inhibitory activity.
The comparison of the activities of kuwanon G (47) and moracenin D (48) with kuwanon L (49) from the roots of M. australis indicated that the prenyl groups may not be the key active units of a Diels-Alder adduct of chalcone with flavonoid, though the type of prenyl groups would influence the tyrosinase inhibitory activity. When the prenyl group was hydroxylated, a higher inhibition effect appeared (IC 50 values: > 200, 4.6 ± 0.1, and 58.8 ± 1.5 μM for 47, 48, and 49, respectively) (enzyme: from mushroom; substrate: l-tyrosine) [27] (Fig. 4).  Fig. 1) for SARs study of lipase inhibitory activity. Relative PL inhibition (%) was calculated as 100-(activity of sample with substrate-negative control of sample without substrate)/(activity of without sample and with substrate-negative control of without sample and substrate) × 100%

SARs of β-secretase and acetylcholinesterase inhibition
Alzheimer's disease (AD) is a neurodegenerative disease that is characterized by memory impairment and loss of recognition, with neuropathological changes such as cerebral β-amyloid angiopathy, neurofibrillary tangles, and glial responses [5,28].
β-Secretase (BACE) is an integral membrane aspartyl protease that initiates the production of amyloid protein and plays a crucial role in AD occurrence and development. BACE inhibitor can prevent the buildup of β-amyloid and show benefits to AD therapy.
Cho et al. [29] reported the inhibitory effects on human BACE-1 (substrate: oligopeptide) of eight compounds obtained from the stem bark of M. lhou, and the SARs suggested that the prenyl groups in flavones and the hydroxyl in ring B played an important role in the BACE-1 inhibitory activities.
A significant feature in the AD brain is the high level of acetylcholinesterase (AChE) associated with β-amyloid plaques. AChE, a key enzyme in cholinergic transmission, promotes the hydrolysis of acetylcholine. AChE inhibitor, such as donepezil, is primarily used to keep acetylcholine levels as high as possible despite cell damage and destruction, to treat memory and learning deficit symptoms.

Conclusions
Morus alba has a long history of medicinal and edible usage in China. The relatively mature studies on its phytochemistry and pharmacology and clinical trials have led to enormous economic value [3].
On the basis of reports and reviews published, the SARs of α-glucosidase, lipase, tyrosinase, β-secretase, and acetylcholinesterase, and cytotoxicity of compounds obtained from Morus genus plants have been summarized. Therefore, prenyl and hydroxyl substituted flavonoids compounds, benzofurans, stilbenes, and Diels-Alder adducts were found to possess significant bioactivities, which may provide some references for the seeking of effective substances with anti-diabetic, anti-obesity, neuroprotective and anti-cancer effects. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.