γ-Glutamyl transpeptidase-catalyzed synthesis of naturally occurring flavor enhancers

γ-Glutamyl derivatives of S-substituted cysteines are naturally occurring flavor enhancers found in garlic and in other plants of the genus Allium. In this paper we describe a straightforward enzymatic synthesis of γ-glutamyl-S-allylcysteine, γ-glutamyl-S-methylcysteine and γ-glutamylmethionine based on a transpeptidation reaction involving glutathione as the γ-glutamyl donor and catalyzed by a commercially available γ-glutamyl transpeptidase (EC 2.3.2.2). Reaction conditions have been optimized with respect to pH, temperature, reactants molar ratio and enzyme concentration using a method based on TLC technique and computer-assisted image analysis for the quantitative evaluation of the reaction mixtures composition. Preparative enzymatic syntheses of γ-glutamyl-S-allylcysteine, γ-glutamyl-S-methylcysteine and γ-glutamylmethionine were carried out at 1 mmol scale. The method is suited for the laboratory scale synthesis of reference compounds for quality control assay, in that it avoids laborious and low-yielding isolation procedures from natural sources or complicated protection and deprotection steps required for chemical synthesis.