Short communication: empirical and mechanistic evidence for the role of pyridoxal-5'-phosphate in the generation of methanethiol from methionine.

The catabolism of the sulfur-containing AA Met to form flavor-active volatile sulfur compounds (VSC) is an important mechanism in flavor development during cheese maturation. Numerous enzymes catalyzing AA catabolism require the presence of the cofactor pyri-doxal-5'-phosphate (PLP). In fact, reports have shown that some of these reactions can be catalyzed by PLP alone, albeit at a reduced rate. We hypothesized that, as a specific application in cheese flavor reactions, PLP can react directly with free Met to generate a specific VSC, methanethiol (MTH). In this study, the ability of PLP to catalyze MTH generation from Met was examined under "cheeselike" conditions of salt and pH. Methionine and varying concentrations of PLP were incubated in a buffer (pH 5.2 + 4.0% NaCl) analogous to the aqueous phase of aged Cheddar cheese. Samples were analyzed using headspace solid-phase microextraction, and relative concentrations of VSC were determined by gas chromatography-mass spectrometry. Results showed MTH, dimethyl disulfide, and dimethyl trisulfide production when Met and PLP were incubated together at 7 degrees C (cheese-aging temperature). These results indicate that the production of VSC from Met can occur nonenzymatically as catalyzed by free PLP.