AMP deaminases of rat small intestine

Abstract

Phosphocellulose column chromatography revealed the existence of two forms of AMP deaminase both in whole tissue and in the intestinal epothelium. AMP deaminase I, which eluted from the column as a first activity peak, exhibited hyperbolic, nonregulatory kinetics, The substrate half-saturation constants were determined to be 0.3 and 0.7 mM at pH 6.5 and 7.2, respectively, and did not change in the presence of ATP, GTP and P i. AMP deaminase II, which eluted from the column as a second activity peak, was strongly activated by ATP and inhibited by GTP and P i. The S 0.5 constants were 3.5 and 7.1 at pH 6.5 and 7.2, respectively. At pH 7.2 ATP (1 mM) S 0.5 decreased to mM and caused the sigmoidicity to shift to hyperbolic. The ATP half-activation constant was increased 9-fold in the presence of GTP and was not affected by P i. Mg 2+ significantly altered the effects exerted by nucleotides. The S 0.5 value was lowered 10-fold in the presence of MgATP and 5-fold in the presence of MgATP, MgGTP and P i. When MgATP was present, AMP deamise II from rat small intestine was less susceptible to inhibition by GTP and P i. A comparison of the kinetic properties of the enzyme, in particular the greater than 100% increase in V max observed in the presence of MgCI 2 at low (1 mM) substrate concentration, indicates that MgATP is the true physiological activator. Guo PP[NH] P at low concentrations, in contrast to GTP, did not affect the enzyme and even activated it at concentrations above 0.2 mM. We postulate that AMP deaminase II may have a function similar to that of the rat liver enzyme. The significance of the existence of an additional, non-regulatory form of AMP deaminase in rat small intestine is discussed.