γ-Glutamyl- S-allylmercaptocysteine (GSAMC), a putative precursor compound of S-allylmercaptocysteine (SAMC), was isolated and identified from aged garlic extract (AGE). We analyzed the change of their contents in AGE during the aging process, chronologically from 1 to 22 months. The formation of these compounds occurred mostly during the early stage of the aging period: the SAMC content reached a maximum at approximately 4 months, whereas the GSAMC content reached a maximum at 1 month and then decreased during the subsequent aging period. To assess the possible relationship between the change of the two compounds during the aging process, we set up the model reactions with the hypothesis that GSAMC is produced from γ-glutamyl- S-allylcysteine (GSAC)/γ-gultamyl- S-1-propenylcysteine (GS1PC) and that SAMC is produced from GSAMC by endogenous γ-glutamyl transpeptidase (GGT) in garlic during the early stage of the aging process. We found that, in the model reactions, SAMC was produced from GSAMC by the garlic protein fraction having GGT activity and its production was suppressed by a GGT inhibitor. Furthermore, the production of GSAMC from allicin and GSAC/GS1PC was found in another model reaction. The reaction between allicin and GS1PC was faster than that between allicin and GSAC and, thus, may be involved in the production of GSAMC in the early stage of the aging process.
Keywords: GGT; S-allylmercaptocystine; aged garlic extract; allicin; γ-glutamyl-S-allylmercaptocystine.