FUNCTION OF A GLUTATHIONE-DEPENDENT FORMALDEHYDE DEHYDROGENASE IN RHODOBACTER-SPHAEROIDES FORMALDEHYDE OXIDATION AND ASSIMILATION

Despite its reactivity with many biological molecules, formaldehyde ca n be commonly encountered by virtually all cells. The widespread exist ence of glutathione-dependent formaldehyde dehydrogenases (GSH-FDH) in procaryotes and eucaryotes suggests this enzyme plays a central and u niversal role in biological formaldehyde oxidation. This work sought t o determine the role of GSH-FDH in the facultative phototrophic bacter ium Rhodobacter sphaeroides. Growth phenotypes of wild type and mutant cells, changes in enzyme specific activities, and the pattern of C-13 -labeled compounds detected by NMR spectroscopy cumulatively suggest t hat R. sphaeroides GSH-FDH can play a critical role in formaldehyde me tabolism under both photosynthetic and aerobic respiratory conditions. In photosynthetic cells, the data indicate that GSH-FDH generates red ucing power, in the form of NADH, and one-carbon skeletons that are ox idized to carbon dioxide for subsequent assimilation by the Calvin-Ben son-Bassham cycle. For example, use of methanol as a sole photosynthet ic carbon source increases the specific activities of GSH-FDH, an NAD- dependent formate dehydrogenase, and the key Calvin-Benson-Bassham cyc le enzyme, ribulose-1,5-bisphosphate carboxylase. This role of GSH-FDH is also supported by the pattern of [C-13]formaldehyde oxidation prod ucts that accumulate in photosynthetic cells and the inability of defi ned GSH-FDH or Calvin cycle mutants to use methanol as a sole carbon s ource. Our data also suggest that GSH-FDH acts in formaldehyde dissimi lation when aerobic respiratory cultures cometabolize methanol and suc cinate.