THE DERIVATION OF THE OXYGEN-ATOMS OF THE 13(1)-OXO AND 3-ACETYL GROUPS OF BACTERIOCHLOROPHYLL A FROM WATER IN RHODOBACTER-SPHAEROIDES CELLS ADAPTING FROM RESPIRATORY TO PHOTOSYNTHETIC CONDITIONS - EVIDENCE FOR AN ANAEROBIC PATHWAY FOR THE FORMATION OF ISOCYCLIC RING-E

Using mass spectrometry, we have demonstrated O-18-labelling of both the 13(1)-oxo and 3-acetyl groups of newly-formed bacteriochlorophyll alpha synthesized by Rhodobacter sphaeroides cells during adaptation from respiratory to photosynthetic conditions in the presence of (H2O)-O-18. This derivation of the 13(1)-oxo group of bacteriochlorophyll alpha from water provides a stark contrast with that of chlorophylls in higher plants where ring Epsilon formation is an aerobic process in which the 13(1)-oxo group arises from molecular oxygen via an oxygenase activity, The formation of the 3-acetyl group of bacteriochlorophyll alpha, however, is consistent with the enzymic hydration of the 3-vinyl group of a derivative of chlorophyll alpha.