Bioactivation of chemopreventive selenocysteine Se-conjugates and related amino acids by amino acid oxidases novel route of metabolism of selenoaminoacids

Several selenocysteine Se-conjugates have been shown to possess potent chem opreventive activity in animal models for chemical carcinogenesis. As a mec hanism of action, beta -elimination reactions to form chemopreventive selen ols, ammonia, and pyruvate has been proposed. The enzymes involved in these beta -elimination reactions, however, have been partially elucidated. Next to cysteine conjugate beta -lyases, as yet unidentified non-pyridoxal-5 ' -phosphate-dependent enzymes also appear to be involved in cytosolic beta - elimination reactions. In the present study, it was investigated whether am ino acid oxidases contribute to the bioactivation of selenocysteine Se-conj ugates. Using purified L-amino acid oxidase from Crotalus adamanteus as a m odel enzyme, significant beta -elimination activities were indeed observed upon incubation with Semethylselenocysteine (K-m, 195, muM; k(cat), 48 min( -1)), Se-allylselenocysteine (K-m, 608 muM; k(cat), 34 min(-1)), Se-phenyls elenocysteine (K-m, 107 muM; k(cat), 57 min(-1)) and Se-benzylselenocystein e (K-m, 59,muM; k(cat), 13 min(-1)). For all selenocysteine Se-conjugates t ested, the rate of pyruvate formation was comparable to that of hydrogen pe roxide, one of the products of oxidative deamination. The fact that additio n of catalase did not alter pyruvate formation indicated that the beta -eli mination reaction observed was not mediated by selenoxidation/syn-eliminati on due to the hydrogen peroxide formed via the oxidative deamination pathwa y. Using D-amino acid oxidase from porcine kidney and D-SeCys conjugates si milar results were obtained. To delineate whether mammalian L-amino acid ox idases are also able to catalyze beta -elimination of selenocysteine Se-con jugates, rat renal cytosol was fractionated and screened for beta -eliminat ion and oxidative deamination activities. One of the fractions isolated dis played oxidative deamination activity with several amino acids and cysteine S-conjugates. With selenocysteine Se-conjugates as substrates, however, th is fraction displayed both oxidative deamination and beta -elimination acti vities, when incubated in the presence of aminoxyacetic acid to block contr ibution of pyridoxal-5 ' -phosphate-dependent enzymes. The potential signif icance of this novel bioactivation route for the chemopreventive activity o f selenocysteine Se-conjugates is discussed.