Characterization of peroxidase enzyme and detoxification of phenols using peroxidase enzyme obtained from Zea mays L waste
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Phenol is one of the greatest menaces among the industrial pollutants. A treatment method which utilizes agricultural wastes in a simple manner has become the need of the hour. In this investigation, details on the extraction, optimization of parameters and study of detoxification potential of the peroxidase enzyme obtained from baby corn waste have been elaborated. The enzyme has been extracted from both corn silk and husk, and studies have been conducted on both the samples. Estimation of amount of protein and enzyme activity has shown promising results, and the conditions optimized are easily attainable at larger scales, making this work feasible for scale-up.
KeywordsDetoxification Phenol Zea Mays L waste Peroxidase enzyme
Management and processing of the waste from different industries are the major problem in the different parts of the world (Anupama et al. 2013). The harmful impact of molecules that are generated from various different process industries pollutes air and water by its dangerous and mutagenic properties (Busca et al. 2008).
Major organic pollutants include phenol and its derivatives like chlorophenol, cresols, orthophenols, nitro phenols, etc., produced by various industrial activities, such as coal mining, petroleum refining, resin, plastic and pharmaceutical production, wood preservation, metal coating and textile dyeing (Varsha et al. 2011). Production and degradation of pesticides also release huge quantities of phenols and its derivatives to the environment. Phenols are poisonous and harmful to organisms even at low concentrations, and as a result detoxification of phenol has wide prospect. In addition to toxic effects, phenolic compounds also generate an oxygen demand in different sources of waters and pass on taste and smell to water with tiny concentrations of their chlorinated compounds. Ground waters are infected by phenolics as a result of the constant release of these harmful compounds from petrochemical, coal conversion and phenol-generating industries. Owing to its harmful effects, the waste water containing phenolic compounds must be treated before discharging these compounds into the water streams (Roostaei and Tezel 2004).
Methods to remove phenols from industrial wastewater are classified as conventional and advanced methods. Conventional methods include steam distillation, liquid–liquid extraction, adsorption, solid-phase extraction, wet air oxidation, catalytic wet air oxidation and biodegradation for the removal of phenols. Highly developed technologies are also there for treatment of phenol such as electrochemical oxidation, photo-oxidation, ozonation, Fenton reaction, membrane processes and microbial and enzymatic treatment (Mohammadi et al. 2015). Enzymes have several favourable characteristics. They have either narrowed or far-reaching specificity. Due to its specificity, enzymes show their functionality even in mixed compounds. Enzymes extensively alter structural and toxicological properties of contaminants into mild inorganic harmless end products. Furthermore, enzymes may present balance over traditional technologies and also over microbial remediation. All these characteristics turn enzymes into eco-friendly catalysts, and enzymatic techniques are environmentally affable processes (Rao et al. 2010). Peroxidases are the enzymes that catalyse the variety of organic and inorganic compounds at the expense of peroxide, usually H2O2. Peroxidases tarnish phenolic compounds to simple radical species, which spontaneously produce insoluble oligo- or polymeric derivatives. These insoluble, polymerized products may be isolated using uncomplicated techniques like sedimentation and filtration techniques (Regalado et al. 2004). Maize silk is the name given to the long styles and stigmas on flower pistils. The stigmas are fine and soft, yellowish to green or purple threads of female flowers. Silk of some maize genotypes contains abundant phenolics, such as flavonoids, tannins and hydroxycinnamic acid esters. Besides their role in defence against pathogens and in host–plant resistance to insects, phenolics are implicated in free radical scavenging, inhibition of lipid peroxidation and defence against UV radiation. Of particular interest in maize are chlorogenic acid and some flavones with similar adjacent hydroxyl ring structure because they are implicated in corn earworm resistance (Elliger et al. 1980; Duffey and Stout 1996) and are considered as silk antibiotics. These adjacent hydroxyls undergo enzymatic oxidation resulting in the production of quinones, which condense with themselves or proteins to produce brown-coloured complexes. Two polyphenol oxidase enzymes, laccase (o- and p-diphenol oxidase, EC 18.104.22.168) and catechol oxidase (o-diphenol oxidase, EC 22.214.171.124), and POD (EC 126.96.36.199) oxidize a large number of aromatic structures at the expense of O2 or H2O2, respectively (Pütter and Becker 1983; Mayer 1987; Pourcel et al. 2007). Oxidized phenolics protect sensitive plant tissues from biotic and abiotic stress (Hurrell et al. 1982; Duffey and Stout 1996).
Enzymatic method is highly efficient and low cost compared to physical, chemical and microbial methods. Zea mays L (Baby corn) husk and silk are cheap, cost-effective alternative for phenol treatment in effluent water, especially in industries located close to agricultural lands, because the crude phenol-degrading enzyme (most likely, peroxidase) is present in the sample, can be isolated and extracted from both husk and silk and therefore has immense potential in the complete degradation of phenol.
Materials and methods
Zea mays L (baby corn) sample was obtained from the local market. Corn silk is a common name for the glossy, frail, weak fibres that develop as part of ears of corn (maize); the tuft or tassel of silky fibres protrudes from the tip of the ear of corn. The ear is enclosed in modified leaves called husks.
The AR-grade chemicals used in this work and supplier details are as follows: ammonium sulphate (Qualigens), calcium chloride (Merck), guaiacol (Srichem), hydrogen peroxide, catechol, phenol (Nice Chemicals), anhydrous sodium carbonate (Rankem), sodium bicarbonate, 4-aminoantipyrine, boric acid, potassium ferricyanide, potassium dihydrogen phosphate (HiMedia Laboratories), di-potassium hydrogen phosphate (S D Fine-Chem Ltd), BSA (Merck), sodium hydroxide (Merck), sodium bicarbonate, copper sulphate, sodium potassium tartrate (Merck) and Folin reagent (Merck).
Extraction of crude enzyme
100 g of silk from baby corn was carefully washed with distilled water, drained and ground into coarse paste. It was then filtered through dirt-free muslin cloth, and the filtrate was collected. A specified amount of filtrate was measured, and ammonium sulphate was added to precipitate the proteins (crude enzyme) from the filtrate. The solubility of globular proteins increases upon the addition of salt (ammonium sulphate), an effect termed salting-in. At higher salt concentrations, protein solubility usually decreases, leading to precipitation. This effect is termed salting-out. The filtrate was subjected to stirring for about 2.5 h followed by cold (4 °C) centrifugation for 15 min at 8000 RPM. The pellets were collected, and the supernatant liquid was reused for the same procedure at different salt concentrations. Five cuts (0–20%, 20–40%, 40–60%, 60–80% and 80–100%) of precipitations were carried out, and the resulting solids (proteins) were stored separately. The same procedure was repeated for the husk sample too.
Proteins can be separated from salt by dialysis through a partially permeable membrane. Molecules having size significantly higher than the pore diameter are retained inside the dialysis bag, whereas smaller molecules and ions navigate the pores of such a membrane and emerge in the dialysate outside the bag. The crude enzyme solution or the pellets that were extracted were filled in dialysis bags made up of cellulose esters (3 kDa) subjected to magnetic stirring for 48 h for the removal of salt that was added to precipitate the proteins. The partially purified enzymatic solutions were then obtained.
Quantification of protein
“Lowry’s test (1951) was used for estimation of amount of protein in the solutions”. The Lowry’s test has been one of the most commonly used methods for determining protein concentration. Folin reagent together with a copper sulphate solution is mixed with a protein solution, and it results a blue purple product, which can be quantified by UV spectrophotometer at 640 nm.
Estimation of enzyme activity
Peroxidase activity in the extracts was measured as described by Loukili et al. (1999). Enzyme activity was tested using Guaiacol as substrate where the effect of peroxidase enzyme on guaiacol in the presence of hydrogen peroxide was tested. UV spectrophotometric method of analysis was used at a wavelength of 470 nm to quantify the activity on the basis of the production of coloured complex due to the action of the enzyme.
Optimization of physical parameters
Once the presence and activity of the peroxidase enzyme were determined, various parameters like volume of enzyme, concentration of the substrate, temperature and pH were varied and its effects were tested. The volume of enzyme solution was varied between 10 and 200 µl, concentration of the substrate between 20 and 200 millimoles, the temperature between − 20 and 40 °C and the pH between 3.5 and 9.
Both Michaelis–Menten and Lineweaver–Burk plots were plotted, and the enzyme kinetics was studied. The concentration of guaiacol, which is taken as a substrate, was varied, and the activity was tested. Substrate concentration was varied between 20 and 200 millimoles. The plot of activity versus substrate concentration gave the Michaelis–Menten data, and the double reciprocal plot of the same was used as the Lineweaver–Burk plot.
The partially purified enzyme was used to detoxify phenol in the presence of hydrogen peroxide. The study was conducted at different concentrations of hydrogen peroxide and for different time periods. Quantification of phenol at both the initial and final stages was carried out by the 4-AAP method, where a reaction mixture formed a coloured complex with the phenol in the solution. The amount of phenol was indirectly estimated by the amount of coloured complex produced. This was analysed by UV spectrophotometric analysis at 520 nm using Agilent Technologies (USA) Cary 60 UV–Visible spectrophotometer.
Results and discussion
Estimation of total protein in the enzyme extract
Optimization of cuts depending on enzyme activity
Optimization of temperature for the highest activity
Optimization of pH for the highest activity
Molecular weight determination
SDS-PAGE was carried out according to the method of Laemmli (1970) with 10% polyacrylamide gel in a vertical slab gel apparatus for crude enzyme fraction (50 µg) from baby corn husk and baby corn silk (50 µg). Fractions to be analysed by SDS-PAGE were mixed with equal volumes of 2× non-reducing sample buffer, and the mixture was heated for 1 min at 70 °C and cooled. The samples were loaded onto the wells of 4% stacking gel and were resolved in 10% polyacrylamide gel at 75 V using the regular SDS tank buffer. After reaching the maximum electrophoretic front, the power supply was stopped. The bands were visualized by staining with Coomassie Brilliant Blue R-250.
Thermodynamic concept of enzyme kinetics
Considering all the optimized conditions and the extent of detoxification, it can be concluded that the peroxidase enzyme obtained from Zea mays waste can be used as an effective alternative to other methods. The enzymes obtained from both silk and husk show similar activity. However, husk sample can be obtained in bulk. Both the samples can be used together as they are easy to procure and feasible to be used in an industrial scale.
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