Aminotransferase, L-amino acid oxidase and P-lyase reactions involving L-cysteine S-conjugates found in allium extracts - Relevance to biological activity?

Several cysteine S-conjugates that occur in extracts of garlic and other plants of the allium family possess anti-oxidant properties, and many, including S-allyl-L-cysteine (SAC) and S-allylmercapto-L-cysteine (SAMC), are promising anti-cancer agents. To understand possible biochemical mechanisms contributing to the protective effects, the ability of selected allium-derived L-cysteine S-conjugates to undergo various enzyme-catalyzed transformations was investigated. SAC, SAMC, S-propylmercapto-L-cysteine and S-penta-1,3-dienylmercapto-L-cysteine were shown to be substrates of: (a) highly purified rat kidney glutamine transaminase K (GTK); (b) purified snake venom L-amino acid oxidase; and (c) a cysteine S-conjugate beta-lyase present in rat liver cytosol. S-Methylmercapto-L-cysteine was shown to be a substrate of GTK and L-ammo acid oxidase, but not of the cysteine S-conjugate P-lyase. Evidence is presented that a major enzyme responsible for the cysteine S-conjugate P-lyase reactions in the rat liver cytosol is gamma-cystathionase. The possible role of gamma-cystathionase in generating sulfane sulfur from the disulfide-containing cysteine S-conjugates present in allium extracts, and the possible role of this sulfane sulfur in enzyme regulation, targeting of cancer cells and detoxification reactions is discussed.