1. The mechanisms that control the oxidative phase of the pentose phosphate cycle in mussel hepatopancreas were investigated. 2. The effects of GSSG (oxidized glutathione) on the inhibition of glucose 6-phosphate dehydrogenase by NADPH [Eggleston & Krebs (1974) Biochem. J. 138, 425-435] extend to 6-phosphogluconate dehydrogenase. 3. The effect of GSSG on both enzymes increases as the [NADP+1]/[NADPH] ratio decreases; greater percentage deinhibition always was obtained for 6-phosphogluconate dehydrogenase. 4. Increasing concentration of GSSG increased the percentage deinhibition. This effect is more pronounced with 6-phosphogluconate dehydrogenase. 5. We confirmed the apparent imbalance between the activities of the two enzymes [sapag-Hagar, Lagunas & Sols (1973) Biochem. Biophys. Res. Commun, 50, 179-185] in the presence of 10mM-Mg2+. 6. The imbalance practically disappears when the substrate concentrations are less than saturating and Mg2+ approaches physiological concentrations. 7. The addition of GSSG at physiological concentrations allows the activities of both enzymes to be measured at high [NADPH]/[NADP+] ratios ratios and the co-operative action of GSSG and Mg2+ on the imbalance between the two enzymes to be verified. 8. The control of the activity of the two enzymes of the pentose cycle could be carried out by deinhibition of the two dehydrogenases and by the intracellular concentrations of substrates and inorganic ions.

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