Summary: Glutamate synthase (GOGAT) activities in cell-free extracts of isolated heterocysts and whole filaments of the cyanobacterium Kütz. (ATCC 29413) were determined by measuring [C]glutamate production from 2-oxo[C]glutarate or from [C]glutamine, both of which are GOGAT substrates. There was negligible [C]glutamate production from 2-oxo-[C]glutarate in the presence of aminooxyacetate, which inhibits aminotransferase activity (<3% of that detectable in whole filament extracts). In experiments using [C]glutamine, [C]glutamate production by heterocyst extracts accounted for about 17% of that detected in whole filament extracts, even in the absence of 2-oxoglutarate, ferredoxin and sodium dithionite. It is concluded that virtually all the [C]glutamate formed from [C]glutamine in heterocyst extracts is independent of GOGAT activity.


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