The biosynthesis of ovothiol A ( N 1 ‐methyl‐4‐mercaptohistidine)

Crude extracts of Crithidia fasciculata catalyse the formation of 4‐mercapto‐ l ‐histidine, an intermediate in the biosynthesis of ovothiol A ( N 1 ‐methyl‐4‐mercaptohistidine), in the presence of histidine, cysteine, Fe 2+ and pyridoxal phosphate. This activity was present in a 35–55% ammonium sulfate fraction that was shown to produce a transsulfuration intermediate in the absence of pyridoxal phosphate. The transsulfuration intermediate was isolated and identified as S ‐(4′‐ l ‐histidyl)‐ l ‐cysteine sulfoxide. The synthase activity, partially purified by anion‐exchange chromatography, was shown to require oxygen and could be used to synthesize a number of isotopically labeled S ‐(4′‐ l ‐histidyl)‐ l ‐cysteine sulfoxides. Sulfoxide lyase activity was partially resolved from the synthase by anion‐exchange chromatography. The phenylhydrazone of the product derived from the cysteine moiety of the sulfoxide coeluted with the phenylhydrazone of pyruvate on HPLC, but this assignment could not be confirmed by mass spectral analysis. S ‐(4′‐[ 14 C] l ‐histidyl)‐[U‐ 13 C 3 , 15 N] l ‐cysteine sulfoxide was synthesized and converted to products of the lyase reaction in the presence of lactate dehydrogenase and NADH. The 13 C‐labeled product was identified by 13 C‐NMR spectroscopy as lactate and the primary product of the lyase reaction is therefore pyruvate. With S ‐(4′[ 3 H] l ‐histidyl)‐[ 14 C] l ‐cysteine sulfoxide as the substrate [ 14 C]lactate, [ 14 C]cysteine and [ 3 H]4‐mercaptohistidine could be detected as products of the lyase reaction, but the sum of the two thiol species exceeded the amount of sulfoxide substrate used. Evidence is presented that this anomaly was due to the utilization of sulfur from dithiothreitol for the formation of cysteine.