The inhibition of skeletal-muscle fructose 1,6-diphosphatase by adenosine monophosphate.

1. The present study extends the finding of Krebs & Woodford (1965) that muscle fructose diphosphatase is more sensitive to AMP inhibition than liver fructose diphosphatase. 2. Hen breast fructose diphosphatase has a K(i) for AMP of 0.1mum; the plot of percentage inhibition is non-sigmoid and the reciprocal plot of activity against AMP concentration is sometimes linear. 3. Percentage inhibition plots for other muscle fructose diphosphatases are sigmoid curves which exhibit different threshold responses to the AMP concentration. 4. The intracellular content of AMP in all muscles tested exceeds the inhibition concentration range of AMP. 5. The sensitivity of muscle fructose diphosphatase to AMP inhibition is decreased by the presence of Mg(2+) or Mn(2+) ions; in the presence of Mn(2+) the inhibition curve for hen breast fructose diphosphatase becomes sigmoid. 6. From the formation constants for the Mg(2+) and Mn(2+) chelates, the effect of these ions in chelation of AMP can be calculated. Although chelation of AMP can explain the Mg(2+) effect, it cannot explain the marked relief of AMP inhibition by Mn(2+). 7. It is suggested that Mn(2+) has a specific effect on this enzyme which reduces the sensitivity to AMP inhibition.