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

Abstract

The enzymic capacities for ammonia assimilation into amino acids were investigated in chloroplasts from the siphonous green alga C. simpliciuscula (Turner) C. Ag. These chloroplasts differ from those of higher plants in having present simultaneously the enzymic capacities to permit assimilation of ammonia by 2 pathways. Glutamine synthetase (EC 6.3.1.2) activity at levels up to 4 .mu.mol/mg chlorophyll per h were found in soluble extracts of the chloroplasts. Glutamine(amide):.alpha.-ketoglutarate aminotransferase (oxidoreductase ferredoxin) (EC 1.4.7.1) activity at levels up to 1.4 .mu.mol/mg chlorophyll per h 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 Km for ammonia which was described previously in these chloroplasts provides the second potential pathway.