Metabolic activities in Cyanophora paradoxa and its cyanelles : I. The enzymes of assimilatory nitrate reduction.

The taxonomic affinity of Cyanophora paradoxa and its endosymbiotically living cyanelles has not yet been resolved. In the present communication, the enzymes of assimilatory nitrate reduction are investigated in cell-free preparations from the cyanelles and from the eukaryotic host. Nitrate reductase of Cyanophora is a NADH-dependent, soluble enzyme, occurring only in the protoplasm of the eukaryotic host. In contrast, nitrite reductase is ferredoxin-dependent and bound to the thylakoids of cyanelles. Glutamine synthetase and ferredoxin-dependent glutamate synthase (GOGAT) are present both in cyanelles and the eukaryote. Activity levels of alanine dehydrogenase and glutamic acid dehydrogenase are marginal in Cyanopnora, indicating that ammonia is suggest assimilated by the glutamine synthetase GOGAT pathway. The data also that NH 4 (+) leaves the cyanelles to meet the nitrogen requirements of the eukaryote. It is concluded that the pathway of assimilatory nitrate reduction is similar in Cyanophora and photosynthetic eukaryotic cells and is different from that in byanobacteria.