1. The hepatic concentration of several nucleotides and metabolites was measured during the first few minutes after an intravenous load of fructose to mice. The first changes, observed at 30s, were a decrease in the concentration of Pi and a simultaneous accumulation of fructose 1-phosphate. The decrease in the concentrations of ATP and GTP proceeded more slowly. An increase in the concentration of IMP was detected only after 1 min and could therefore not be considered to be the cause of the accumulation of fructose 1-phosphate. 2. To explain the temporary burst of adenine nucleotide breakdown that occurs after a load of fructose, the kinetics of AMP deaminase (EC from rat liver were reinvestigated at physiological (0.2 mM) concentration of substrate. For this purpose, a new radiochemical-assay procedure was developed. At 0.2mM-AMP a low activity could be measured, which was more than 90% inhibited by 5mM-Pi. ATP (3MM) increased the enzyme activity over 200-fold. Pi alone did not influence the ATP-activated enzyme, but 0.5mM-GTP caused a 60% inhibition. The combined effect of both inhibitors at their physiological concentrations reached 95%. 3. It is proposed that the rapid degradation of adenine nucleotides that occurs after a load of fructose is caused by a decrease in the concentration of both inhibitors, Pi and GTP, soon counteracted by the decrease in the concentration of ATP. 4. Some of the kinetic parameters of liver AMP deaminase were computed in terms of the concerted transition theory of Monod, Wyman & Changeux (1965) (J. Mol. Biol. 12, 88-118).

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