Racemase activity effected by two dehydrogenases in sulfolactate degradation by Chromohalobacter salexigens: purification of (S)-sulfolactate dehydrogenase

Chromohalobacter salexigens DSM 3043, whose genome has been sequenced, is known to degrade (R,S)-sulfolactate as a sole carbon and energy source for growth. Utilization of the compound(s) was shown to be quantitative, and an eight-gene cluster (Csal_1764-Csal_1771) was hypothesized to encode the enzymes in the degradative pathway. It comprised a transcriptional regulator (SuyR), a Tripartite Tricarboxylate Transporter-family uptake system for sulfolactate (SlcHFG), two sulfolactate dehydrogenases of opposite sulfonate stereochemistry, namely novel SlcC and ComC [(R)-sulfolactate dehydrogenase] [EC 1.1.1.272] and desulfonative sulfolactate sulfo-lyase (SuyAB) [EC 4.4.1.24]. Inducible reduction of 3-sulfopyruvate, inducible SuyAB activity and induction of an unknown protein were detected. Separation of the soluble proteins from induced cells on an anion-exchange column yielded four relevant fractions. Two different fractions reduced sulfopyruvate with NAD(P)H, a third yielded SuyAB activity, and the fourth contained the unknown protein. The latter was identified by peptide-mass fingerprinting as SlcH, the candidate periplasmic binding protein of the transport system. Separated SuyB was also identified by peptide-mass fingerprinting. ComC was partially purified and identified by peptide-mass fingerprinting. The (R)-sulfolactate that ComC produced from sulfopyruvate was a substrate for SuyAB, which showed that SuyAB is (R)-sulfolactate sulfo-lyase. SlcC was purified to homogeneity. This enzyme also formed sulfolactate from sulfopyruvate, but the latter enantiomer was not a substrate for SuyAB. SlcC was obviously ( S)-sulfolactate dehydrogenase.