Biosynthesis of α-acetolactate and its conversion to diacetyl and acetoin in cell-free extracts of Lactobacillus casei

Abstract

Cell-free extracts of Lactobacillus casei 393 required two enzyme systems for production of acetoin and diacetyl from pyruvate. An enzyme closely associated with the particulate fraction obtained after sonic oscillation produced α-acetolactate, required thiamine pyrophosphate and magnesium for optimal activity, and was inhibited by citrate. Acetolactate was converted to diacetyl and acetoin by both enzymatic and nonenzymatic processes. The enzyme responsible for conversion of acetolactate to diacetyl and acetoin was readily solubilized by sonication of cells. This enzyme required thiamine pyrophosphate or pyridoxalamine for optimal activity and its activity was enhanced in cells cultured in media containing citrate. Results obtained suggested that under conditions which exist in cultures, α-acetolactate was decarboxylated to diacetyl primarily by a nonenzymatic process.