Anodic peroxidase isoenzymes and polyphenol oxidase activity from cucumber fruit: Tissue and substrate specificity

Abstract

Peroxidase activity in fresh cucumber fruit was highest in the skin; followed by pericarp then carpel tissues, respectively. Polyphenol oxidase activity, by contrast, was present only in the skin. In crude extracts, both activities had pH optima near 7.0, but exhibited different temperature optima and thermostability. Using hydrogen peroxide with either benzidine, guaiacol, p-phenylenediamine, o-phenylenediamine, o-dianisidine, or 3-amino-9-ethylcarbazole as substrates, more anodic peroxidase isoenzymes were observed in the skin than the pericarp or carpel. The reactivity of the peroxidase isoenzymes toward these stains varied qualitatively and quantitatively, e.g. benzidine stained more and different isoenzymes with less relative intensity than did p-phenylenediamine. Further, no staining occurred with the above substrates in the absence of hydrogen peroxide, or with Fast Blue BB base, caffeic acid, syringaldazine or polyphenol oxidase substrates (catechol, hydroquinone, L-DOPA), regardless of hydrogen peroxide availability. These data show that cucumber fruit anodic peroxidase isoenzymes vary widely in substrate specificity. Further, the high thermostability of cucumber peroxidase and polyphenol oxidase activity suggests that these enzymes may play a role in the darkening of processed cucumber products.