18O labeling of chlorophyll d in Acaryochloris marina reveals that chlorophyll a and molecular oxygen are precursors

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Journal Article
Journal of Biological Chemistry, 2010, 285 (37), pp. 28450 - 28456
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The cyanobacterium Acaryochloris marina was cultured in the presence of either H-2 O-18 or O-18(2), and the newly synthesized chlorophylls (Chl a and Chl d) were isolated using high performance liquid chromatography and analyzed by mass spectroscopy. In the presence of H-2 O-18, newly synthesized Chl a and d, both incorporated up to four isotopic O-18 atoms. Time course H-2 O-18 labeling experiments showed incorporation of isotopic O-18 atoms originating from H-2 O-18 into Chl a, with over 90% of Chl a O-18-labeled at 48 h. The incorporation of isotopic O-18 atoms into Chl d upon incubation in H-2 O-18 was slower compared with Chl a with similar to 50% O-18-labeled Chl d at 115 h. The rapid turnover of newly synthesized Chl a suggested that Chl a is the direct biosynthetic precursor of Chl d. In the presence of O-18(2) gas, one isotopic O-18 atom was incorporated into Chl a with approximately the same kinetic incorporation rate observed in the H-2 O-18 labeling experiment, reaching over 90% labeling intensity at 48 h. The incorporation of two isotopic O-18 atoms derived from molecular oxygen (O-18(2)) was observed in the extracted Chl d, and the percentage of double isotopic O-18-labeled Chl d increased in parallel with the decrease of non-isotopic-labeled Chl d. This clearly indicated that the oxygen atom in the C3(1)-formyl group of Chl d is derived from dioxygen via an oxygenase-type reaction mechanism.
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