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A cytosolic 5′-nucleotidase, acting preferentially on IMP and GMP, has been isolated from human colon carcinoma extracts. This enzyme activity catalyzes also the transfer of the phosphate group of 5′-nucleoside monophosphates (mainly, 5′-IMP, 5′-GMP, and their deoxycounterparts) to nucleosides (preferentially inosine and deoxyinosine, but also nucleoside analogs, such as 8-azaguanosine and 2′,3′-dideoxyinosine). It has been proposed that the enzyme mechanism involves the formation of a phosphorylated enzyme as an intermediate which can transfer the phosphate group either to water or to the nucleoside. The enzyme is activated by some effectors, such as ATP and 2,3-diphosphoglycerate. Results indicate that the effect of these activators is mainly to favor the transfer of the phosphate of the phosphorylated intermediate to the nucleoside (i.e., the nucleoside phosphotransferase activity). This …