Evidence for Distinct l -Methionine Catabolic Pathways in the Yeast Geotrichum candidum and the Bacterium Brevibacterium linens

Abstract

ABSTRACT Tracing experiments were carried out to identify volatile and nonvolatile l -methionine degradation intermediates and end products in the yeast Geotrichum candidum and in the bacterium Brevibacterium linens , both of which are present in the surface flora of certain soft cheeses and contribute to the ripening reactions. Since the acid-sensitive bacterium B. linens is known to produce larger amounts and a greater variety of volatile sulfur compounds (VSCs) than the yeast G. candidum produces, we examined whether the l -methionine degradation routes of these microorganisms differ. In both microorganisms, methanethiol and α-ketobutyrate are generated; the former compound is the precursor of other VSCs, and the latter is subsequently degraded to 2,3-pentanedione, which has not been described previously as an end product of l -methionine catabolism. However, the l -methionine degradation pathways differ in the first steps of l -methionine degradation. l -Methionine degradation is initiated by a one-step degradation process in the bacterium B. linens , whereas a two-step degradation pathway with 4-methylthio-2-oxobutyric acid (MOBA) and 4-methylthio-2-hydroxybutyric acid (MHBA) as intermediates is used in the yeast G. candidum . Since G. candidum develops earlier than B. linens during the ripening process, MOBA and MHBA generated by G.candidum could also be used as precursors for VSC production by B. linens .