Short Communication: Empirical and Mechanistic Evidence for the Role of Pyridoxal-5′-Phosphate in the Generation of Methanethiol from Methionine

Enzymatic and chemical conversion of Met to volatile sulfur compounds (adapted from Smit et al., 2005; reproduced with permission from Blackwell Publishing, 2006). KMBA = α-keto-4-(methylthio)-butanoic acid; MTH = methanethiol; DMDS = dimethyl disulfide; DMTS = dimethyl trisulfide.

Chemical interconversions of methanethiol (adapted from Rankin et al., 2006). MTH = methanethiol; DMDS = dimethyl disulfide; DMTS = dimethyl trisulfide.

Phosphorylated forms of vitamin B₆. The charge structures shown predominate at pH 5.2 (Kallen, et al. 1985). PLP = pyridoxal-5′-phosphate; PMP = pyridoxamine-5′-phosphate; PNP = pyridoxine-5′-phosphate.

Generation of volatile sulfur compounds (VSC) from Met, in the presence of pyridoxal-5′-phosphate (PLP). Aluminum foil-wrapped, 8-mL stoppered serum vials contained 5mL of 50mM phosphate buffer, pH 5.2, 4% (wt/vol) NaCl, 10mM Met, and either 1 or 2μM PLP. Vials were incubated at 7°C for the time periods indicated, then analyzed for VSC by gas chromatography–mass spectrometry. Data points are averages of triplicate samples. DMDS = dimethyl disulfide. Error bars represent 1 standard deviation.

Pyridoxal-5′-phosphate (PLP)–catalyzed degradation of Met. The reaction sequence shown is for a lyase-type mechanism. MTH = methanethiol.