Summary: Glucose-6-phosphate dehydrogenase (G6PDH) is a key enzyme involved in fixed carbon dissimilation in photosynthetic micro-organisms; in heterocystous cyanobacteria it may also be implicated in the supply of reductant to nitrogenase. In crude cell-free extracts of the N-fixing cyanobacterium G6PDH activity was reversibly deactivated by the thiol agent dithiothreitol in the presence of a low molecular weight protein (12000 mol. wt). Glucose 6-phosphate reversed deactivation when added at high concentration, or prevented deactivation if added with the thiol. NADP, which, like glucose 6-phosphate, is a G6PDH substrate, also deactivated the enzyme; deactivation was reversed or prevented by adding glucose 6-phosphate or glutamine. Purified thioredoxin from , at very low concentrations (2 n), deactivated purified G6PDH in a manner identical to that observed when crude extracts were used in the presence of dithiothreitol. Glutathione did not affect the enzyme.


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