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Glutamine Synthetase/Glutamine: -Ketoglutarate Aminotransferase in Chloroplasts from the Marine Alga Caulerpa simpliciuscula¹

^a Russell Grimwade School of Biochemistry, University of Melbourne, Parkville, Victoria 3052 Australia

The enzymic capacities for ammonia assimilation into amino acids have been investigated in chloroplasts from the siphonous green alga Caulerpa simpliciuscula (Turner) C. Ag. The results show that these chloroplasts differ from those of higher plants in having present simultaneously the enzymic capacities to permit assimilation of ammonia by two pathways. Glutamine synthetase (EC 6.3.1.2) activity at levels up to 4 µmoles per mg chlorophyll per hour were found in soluble extracts of the chloroplasts. Glutamine(amide):-ketoglutarate aminotransferase (oxidoreductase ferredoxin) (EC 1.4.7.1) activity at levels up to 1.4 µmoles per mg chlorophyll per hour was detected by incubation of photosynthetically active chloroplasts either in light or with reduced ferredoxin. Together these enzymes provide the capacity for the conventional pathway of ammonium assimilation in chloroplasts via glutamine. A similar level of a glutamate dehydrogenase with an unusually low K_m for ammonia which has been described previously in these chloroplasts provides the second potential pathway.