Biosynthesis of the 3-acetyl and 131-oxo groups of bacteriochlorophyll a in the facultative aerobic bacterium, Rhodovulum sulfidophilum -: The presence of both oxygenase and hydratase pathways for isocyclic ring formation

Using O-18-labelling and mass spectrometry, we have examined bacteriochlorophyll a formation in Rhodovulum sulfidophilum: formerly known as Rhodobacter sulfidophilus, which forms large amounts of BChl a both aerobically in the dark and anaerobically in the light. R. sulfidophilum, growing under strict anaerobiosis in the light, possesses hydratases which incorporate O-18 label from (H2O)-O-18 into both the 13(1)-ore and 3-acetyl oxygens; in addition, the four carboxyl oxygens at C13(3) and C17(3) were labelled by (H2O)-O-18. Under aerobic conditions in the dark, the labelling of the 13(1)-ore group by (H2O)-O-18 was reduced indicating that O-16 was being incorporated into this group from air. R. sulfidophilum, grown in the dark under an atmosphere initially containing 50% O-18(2) in Ar, possessed an oxygenase which incorporated O-18 label from O-18(2) specifically into the 13(1)-oxo group; under these conditions the acetyl and carboxyl groups remained unlabelled. Thus, both an oxygenase and hydratase operate in R. sulfidophilum to form the 13(1)-oxo group of ring E of BChl a, the 3-acetyl group oxygen, however, arises only from water via a hydratase.