Biochemical origins of lactaldehyde and hydroxyacetone in Methanocaldococcus jannaschii

The biochemical routes for the metabolism of methylglyoxal and the formation of lactaldehyde and hydroxyacetone in Methanocaldococcus jannaschii have been established. The addition of methylglyoxal and NADH, NADPH, F420H2, or DTT to a M. jannaschii cell extract stimulated the production of both lactaldehyde and hydroxyacetone. Using appropriately labeled NADH, NADPH, and F420H2, hydride transfer was only observed from F420H2 to lactaldehyde. It was shown that cell extracts of this Archaea readily catalyzed the F420H2-dependent reduction of methylglyoxal to lactaldehyde, a precursor of the lactate found in coenzyme F-420. This conversion was established by measuring the incorporation of deuterium from (5RS)[5-H-2(1)]F420H2 into the C-2 position of the formed lactaldehyde. In vivo generated (5R)[5-H-2(1)]F420H2 was also found to incorporate deuterium into lactaldehyde. The experimental data indicated that the pro-R hydrogen of F420H2 was transferred during the reduction. The stereochemistry of this transfer was opposite from that observed for all other known enzyme-catalyzed hydride-transfer reactions involving F-420. [1,3,3,3-H-2(4)]-Methylglyoxal was incorporated into lactaldehyde and hydroxyacetone as an intact unit during this reduction with the occurrence of some deuterium exchange. The exchange observed during this incorporation into lactaldehyde was substantially more than the exchange observed during the incorporation into the hydroxyacetone. The hydroxyacetone was derived directly from methylglyoxal, with the hydrogen for the reduction being derived from water. Hydroxyacetone was also readily formed by the condensation of pyruvate with formaldehyde. The product of the MJ0663 gene was shown to catalyze this condensation reaction.