DIFFERENTIAL HYDROLYSIS OF ALK(EN)YL CYSTEINE SULFOXIDES BY ALLIINASEIN ONION MACERATES - FLAVOR IMPLICATIONS

Intact cells of Allium spp have no odour, but when cells are disrupted , the enzyme alliinase (EC 4.4.1.4) hydrolyses the S-alk(en)yl sulphox ides (ACSOs) to produce pyruvate, ammonia and the many volatile sulphu r compounds associated with flavour and odour. In onions, there are th ree main sulphoxides: methyl, propyl and 1-propenyl, which gives rise to onion's tear-producing effect. The relative proportions of these ar e a factor in determining the subsequent flavour. gamma-Glutamyl pepti des, which are biosynthetic intermediates to ACSOs, are also present i n significant amounts, but their contribution to flavour is not known. There is little work on the reaction of alliinase in vivo. The hydrol ysis of ACSOs by alliinase was studied in rapidly macerated bulbs at i ntervals between 5 s and 2 h. ACSO and gamma-glutamyl peptide levels w ere measured by HPLC, and pyruvate levels were measured spectrophotome trically. Although the hydrolysis of propenyl cysteine sulphoxide (Pre nCSO) was immediate and almost 100% between 5 and 20 s after bulb mace ration, the hydrolysis of propyl cysteine sulphoxide (PCSO) and methyl cysteine sulphoxide (MCSO) was incomplete. After 5 s maceration, abou t 50% PCSO and MCSO remained and thereafter no further hydrolysis occu rred. The levels of ACSOs and the extent to which they were hydrolysed were dependent on the cultivar and sulphur environment in which it wa s grown. The addition of pyridoxal phosphate cofactor enhanced the ext ent of MCSO and PCSO hydrolysis. Additional purified alliinase per sc had no effect. Substantial hydrolysis of gamma-glutamyl peptides also occurred after maceration, and this was unexpected. Levels of pyruvate product were between 15 and 25% of the expected amount from ACSO hydr olysis assuming a stoichiometric relationship. The incomplete hydrolys is of MCSO and PCSO, enhancement of activity by additional pyridoxal p hosphate and non-stoichiometric production of pyruvate are evidenced t hat reaction inhibition of alliinase may be occurring in onion macerat es. (C) 1998 Society of Chemical Industry.