| Abstract: | Fructose-6-phosphate (F6P)-saturation curves (up to 5 mM F6P) for phosphofructokinase (PFK) have been studied at physiological pH (7.1) and inhibitory (1.5 mM) or non-inhibitory (0.25 mM) ATP levels, in rat erythrocytes and reticulocytes. The addition of 300 microM cAMP to control samples activates the enzyme and displaces F6P-saturation curve towards the left, while the addition of cGMP inhibits the enzyme and shifts the curve to the right. The cAMP positive allosteric effect is more evident at inhibitory ATP levels, while the inhibitory effect of cGMP is very similar at both ATP levels. This antagonistic effect is exerted at the same regulatory site, since cAMP also activates the enzyme when cGMP is previously present in the reaction mixture. The physiological significance of this antagonism is not yet clear. |
