Peroxidases and the metabolism of capsaicin in Capsicum annuum L.

Capsaicinoids are acid amides of C₉ - C₁₁ branched-chain fatty acids and vanillylamine. These compounds are responsible for the pungency of the Capsicum species and of cultivars regarded as hot peppers. Moreover, it has been suggested that these compounds play an ecological role in seed dispersal. Because they are used in the pharmacological, food and pesticide industries, much attention has been paid on knowing how their accumulation is controlled, both in the fruit and in cell cultures. Such control involves the processes of biosynthesis, conjugation and catabolism. Recent progress has been made on the biosynthetic pathway, and several of the genes coding for biosynthetic enzymes have been cloned and expression studies performed. With regard to catabolism, cumulative evidence supports that capsaicinoids are oxidized in the pepper by peroxidases. Peroxidases are efficient in catalyzing in vitro oxidation of both capsaicin and dihydrocapsaicin. These enzymes are mainly located in placental and the outermost epidermal cell layers of pepper fruits, as occurs with capsaicinoids, and some peroxidases are present in the organelle of capsaicinoid accumulation, that is, the vacuole. Hence, peroxidases are in the right place for this function. The products of capsaicin oxidation by peroxidases have been characterized in vitro, and some of them have been found to appear in vivo in the Capsicum fruit. Details on the kinetics and catalytic cycle for capsaicin oxidation by peroxidases are also discussed.