Unusual Cystine Lyase Activity of the Enzyme Alliinase: Direct Formation of Polysulphides

Abstract

Garlic (Allium sativum L.) and related species are highly estimated as foods, spices, and herbal remedies in many parts of the world. Sulphur-containing flavour compounds like allicin (allyl 2-propenethiosulphinate) are responsible for the smell and taste of freshly crunched garlic. These substances are formed by the action of alliinase (EC 4.4.1.4) on cysteine sulphoxides, e. g., alliin [S-(+)-allyl-L-cysteine sulphoxide]. Additionally, alliinase catalyses the C-S lysis of cystine in the manner of a cystine lyase. Ammonium, pyruvate and elementary sulphur but not cysteine could be detected as reaction products. The ratios between cystine, ammonium and pyruvate are 1 : 1.9 : 1.9 suggesting a new type of reaction mechanism. Thiocysteine and disulphine were assumed as intermediates. The pH optimum of the cystine lyase activity was found at pH 7.5 and the temperature optimum was at 44 °C. The K_M value for the homogeneous enzyme was at 2.65 mM and V_max was at 4.12 nkat/mg using cystine as substrate. Moreover, parallel incubation of cystine and alliin gave mainly allyl (poly)sulphides as reaction products instead of allicin. These substances had not been observed as direct enzymatic products until now. Thus, the significance of alliinase and its enzymatic products has to be newly considered in terms of ecological, pharmacological, and biochemical aspects.

Cystine lyase - alliinase - garlic - disulphine - allyl polysulphides - Allium sativum - Alliaceae

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