Biocatalytic characteristics, product formation and putative pathway of P-coumaric acid oxidation by a crude peroxidase from onion

Onion peroxidase (POD) has been known to act on some o-diphenol acids, such as caffeic and hydrocaffeic acid, but data on monophenols are rather scarce. This study is aimed to investigate the use of a crude peroxidase preparation from onion solid wastes for oxidising p-coumaric acid, a widespread phenolic acid, various derivatives of which may occur in foods. The highest enzyme activity was observed at a pH value of 5, but considerable activity was also retained for pH up to 6. Favourable temperatures for increased activity varied between 30 and 50 degrees C, 40 degrees C being the optimal. Liquid chromatography/mass spectrometry analysis of a POD-treated p-coumaric acid solution showed the formation of two major products, which were identified as a p-coumaric acid dimer and a decarboxylated dehydrotetramer. The putative oxidation pathway proposed shares common features with those reported for other hydroxycinnamates.