Characterization of tyrosinase- and polyphenol esterase-catalyzed end products using selected phenolic substrates

The oxidative end products that result from the biocatalysis of tyrosinase (PPO) and/or a polyphenol esterase (PPE) extract have been investigated simultaneously in model systems containing selected phenolic compounds as substrates. The spectrophotometric scanning of brown color, formed in the presence of both PPO and PPE, showed a decrease in the absorbance compared to that obtained with PPO only. Graphical analyses of the iterative spectra of oxidized phenolic end products by PPO confirmed the presence of, at least, three kinetically related absorbing species. HPLC analyses of the end products, obtained by the biocatalysis of PPE or PPO activity, indicated the presence of two main groups of compounds: colored ones of λ(max) at 294-324 nm and colorless products of λ(max) at 264-290 nm. PPE produced both compounds when selected substrates were used as substrates, whereas PPO produced only one type of oxidation product. However, when both enzymes were incubated together, the nature of the end products was similar to that obtained with PPE only

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Rhus vernificera laccase-catalyzed oxidation of phenolic compounds, i.e., (+)-catechin, (-)-epicatechin and catechol, was carried out in selected organic solvents to search for the favorable reaction medium. The investigation on reaction parameters showed that optimal laccase activity was obtained in hexane at 30 °C, pH 7.75 for the oxidation of (+)-catechin as well as for (-)-epicatechin, and in toluene at 35 °C, pH 7.25 for the oxidation of catechol. Ea and Q10 values of the biocatalysis in the reaction media of the larger log p solvents like isooctane and hexane were relatively higher than those in the reaction media of lower log p solvents like toluene and dichloromethane. Maximum laccase activity in the organic media was found with 6.5% of buffer as co-solvent. A wider range of 0-28 ?g protein/ml in hexane than that of 0-16.7 ?g protein/ml in aqueous medium was observed for the linear increasing conversion of (+)-catechin. The kinetic studies revealed that in the presence of isooctane, hexane, toluene and dichloromethane, the Km values were 0.77, 0.97, 0.53 and 2.9 mmol/L for the substrate of (+)-catechin; 0.43, 0.34, 0.14 and 3.4 mmol/L for (-)-epicatechin; 2.9, 1.8, 0.61 and 1.1 mmol/L for catechol, respectively, while the corresponding Vmax values were 2.1 10-2, 2.3 10-2, 0.65 10-2 and 0.71 10-2 A/g protein min); 1.8 10-2, 0.88 10-2, 0.19 10-2 and 1.0 10-2 A/g protein min); 0.48 10-2, 0.59 10-2, 0.67 10-2 and 0.54 10-2 A/g protein min), respectively. FT-IR indicated the formation of probable dimer from (+)-catechin in organic solvent. These results suggest that this laccase has higher catalytic oxidation capacity of phenolic compounds in suitable organic media and favorite oligomers could be obtained. " 2008 Elsevier B.V. All rights reserved.",,,,,,"10.1016/j.molcatb.2008.07.006",,,"http://hdl.handle.net/20.500.12104/42473","http://www.scopus.com/inward/record.url?eid=2-s2.0-68649123619&partnerID=40&md5=b555e402d4692d4f7bb3d3463d886698",,,,,,"01-abr",,"Journal of Molecular Catalysis B: Enzymatic",,"8