Biosynthesis of benzylglucosinolate, cyanogenic glucosides and phenylpropanoids in Carica papaya

Abstract

Benzylglucosinolate was detected in all of the tissues of Carica papaya (pawpaw). No other glucosinolates were detected in any tissue of C. papaya. Previous suggestions that indolyl-3-methylglucosinolate might be present could not be confirmed. The highest concentrations of benzylglucosinolate were found in the youngest leaves, but the compound was also detected in leaf stalks, stem internodes and roots. The presence of benzylglucosinolate in shoots was developmentally regulated-high concentrations in young tissues, declining as they matured. The exception was the stem internodes which maintained relatively constant concentrations. Tap roots had higher glucosinolate content than young roots. Cyanide, specifically released from cyanogenic glucosides, was detected in leaves and roots of C. papaya. Cyanide was not detected in comparable glucosinolate-containing tissues from Brassica napus (oilseed rape). The cyanide concentrations were highest in the tap roots and young leaves of C. papaya, suggesting that cyanogenic glucoside accumulation was also developmentally regulated. NADPH-dependent L- phenylalanine monooxygenase activity was detected in leaves of C. papaya catalysing the oxidative decarboxylation of L-phenylalanine. This monooxygenase activity was restricted to leaves, and could not be detected in any other tissues. No other monooxygenase activities were detected, in any tissues, active with any of the amino acids tested. Activity was highest in the young leaves and declined as leaves expanded and matured. This enzyme was significantly inhibited by several cytochrome P450 inhibitors, and to a lesser extent by the flavoprotein-specific inhibitor diphenylene iodonium. No other aromatic amino acids tested were either substrates or inhibitors of this enzyme, suggesting a high degree of substrate specificity. Two other key enzymes involved in the metabolism of L-Phe and L-Phe-derived compounds, phenylalanine-ammonia lyase and peroxidase, were found to be similarly developmentally regulated in tissues of C. papaya. Activities were highest in young tissues and declined as the tissues matured.