Alternative NAD+-dependent formate dehydrogenases in the facultative methylotroph Mycobacterium vaccae 10

Mycobacterium vaccae 10 growing in methanol medium synthesizes two inducible alternative NAD(+)-dependent formate dehydrogenases (FDH). In the presence of molybdenum, the dominating form of the enzyme is FDHI with Mr 440 kDa and Km 0.32 mM for sodium formate. FDHI reduced ferricyanide as well as NAD+, and it was reversibly inactivated by formate. NAD+ stabilized FDHI against this inactivation. Under conditions of artificial molybdenum deficiency (tungsten in the medium), the second enzyme (FDHII) appeared with Mr about 93 kDa and Km 8.3 mM for sodium formate, and no FDHI activity was detected. FDHII did not reduce ferricyanide and was not inactivated by formate. The activity of FDHI was restored in tungsten-grown cells by pulse addition of molybdenum under conditions of blocked protein synthesis, suggesting the pre-existence of inactive apo-FDHI.