Inhibitory effect of phosphate on nucleoside phosphotransferase from chicken intestinal mucosa

Nucleoside phosphotransferase purified from chicken intestinal mucosa is a multisubunit protein of high molecular weight regulated by many nucleotides. The adsorption of a nucleotide effector to regulatory sites favors the conversion of the enzyme to a stable form and induces a modification of the substrate site, increasing its affinity for substrate donors. We studied the effects exerted by orthophosphate on the reaction and on the stability of the enzyme. P(i) inhibits competitively nucleoside phosphotransferase activity, whichever nucleoside monophosphate is employed as donor. Nucleotide effector (d-TDP) removes entirely the inhibitory effect exerted by different concentrations of P(i). The sigmoidal kinetic response to activation by d-TDP is increased by 100 μM P(i), which raises the interaction coefficient from 2.25 to 3.54. P(i), like nucleotide effectors, stabilizes nucleoside phosphotransferase against heat and enzyme dilution. Although we do not exclude the possibility that P(i) at higher concentrations can also compete effectively with phosphate donors at the catalytic site, we were inclined to think that the findings are compatible with competition between phosphate and nucleotides at regulatory sites.