Glyoxylate aminotransferases are unique among aminotransferases (EC 2.6.1.) because they catalyze the conversion of glyoxylate to glycine and this reaction is often considered to be physiologically irreversible. Glutamate: glyoxylate aminotransferase (GGAT, EC 2.6.1.4.) and serine: glyoxylate aminotransferase (SGAT, EC 2.6.1.45.) are the most extensively studied plant glyoxylate aminotransferases. The most important metabolic function of these two enzymes is to catalyze the transamination of glyoxylate to glycine in peroxisomes during photorespiration of higher plants. In this review some other possible metabolic roles of GGAT and SGAT are also discussed. Purification and substrate specificity of GGAT, SGAT and other glyoxylate aminotransferases are considered. The physical and kinetic properties of these enzymes such as their molecular weight, subunit composition, pyridoxal phosphate requirement, effect of pH and cations on activity and their mechanism of action are reviewed. Effects of natural metabolites on GGAT and SGAT activity with special consideration of mechanism of the inhibition by glyoxylate in the presence of ammonium ion are discussed as well.
Activity of mushroom polyphenol oxidase (PPO) toward 6 substrates and inhibitory effect of cysteine, 2-mercaptoethanol, benzoic acid and sodium metabisulphite were determined. The o-diphenols which appeared to be the best substrates were: catechin, DOPA (L-3,4-dihydroxyphenylalanine) and chlorogenic acid. Affinity of PPO crude preparation substrates to enzyme, expressed as inverse value of Michaelis constant was lower then affinity of catechol. Inhibitory effect depended on specifity of inhibitors and their concentration. Electrophoretic patterns of PPO of mushrooms reveals slow and fast moving 4 isoforms when DOPA was used as a substrate, 2 bands for catechin and chlorogenic acid while only one band showed activity toward tyrosine and p-cresol.