The purification and properties of nucleoside phosphotransferase from mucosa of chicken intestine

Bovine cytosolic 5'-nucleotidase acts through the formation of an aspartate 52-phosphoenzyme intermediate

Cytosolic 5'-nucleotidase/phosphotransferase (cN-II), specific for purine monophosphates and their deoxyderivatives, acts through the formation of a phosphoenzyme intermediate. Phosphate may either be released leading to 5'-mononucleotide hydrolysis or be transferred to an appropriate nucleoside acceptor, giving rise to a mononucleotide interconversion. Chemical reagents specifically modifying aspartate and glutamate residues inhibit the enzyme, and this inhibition is partially prevented by cN-II substrates and physiological inhibitors. Peptide mapping experiments with the phosphoenzyme previously treated with tritiated borohydride allowed isolation of a radiolabeled peptide. Sequence analysis demonstrated that radioactivity was associated with a hydroxymethyl derivative that resulted from reduction of the Asp-52-phosphate intermediate. Site-directed mutagenesis experiments confirmed the essential role of Asp-52 in the catalytic machinery of the enzyme and suggested also that Asp-54 assists in the formation of the acyl phosphate species. From sequence alignments we conclude that cytosolic 5'-nucleotidase, along with other nucleotidases, belong to a large superfamily of hydrolases with different substrate specificities and functional roles.

Mechanism of the reaction catalysed by cytosolic 5'-nucleotidase/phosphotransferase: formation of a phosphorylated intermediate

Cytosolic 5'-nucleotidase preferentially catalysing the hydrolysis of IMP, GMP and their deoxy derivatives, and endowed with phosphotransferase activity, was purified from calf thymus and its reaction mechanism was studied. In the presence of [32P]IMP, ATP and MgCl2, a covalent enzyme-phosphate intermediate was trapped by mixing with an SDS solution. Heart or acid treatment of the enzyme before incubation with radiolabelled substrate prevented formation of the intermediate. Furthermore, on the basis of studies on the kinetic parameters of the enzyme as function of pH, and of experiments on thiol oxidation and photo-oxidation, we suggest the involvement of cysteine and histidine residue(s) in the reaction mechanism.

Nucleoside phosphotransferase in animal tissues. Tissue distribution and kinetic properties

Amphibian, avian and mammal tissues contain a nucleoside phosphotransferase clearly different from those previously described in vegetables and bacteria. Whatever the animal source, the enzyme showed many similar characteristics as far as substrate specificity, dependence upon Mg2+, instability at 37 degrees C, and the protecting effect of nucleotides were concerned. Moreover, when submitted to gel filtration, the enzyme behaved in all cases as a dissociable high molecular weight protein, whose degree of association was controlled by nucleotides. In amphibian and avian tissues multiple forms of the enzyme seem to be present which differ for the substrate concentration at half-maximal velocity (S0.5); the concentration of nucleotide effector which affords half-maximal protection at 37 degrees C (P0.5); and the Hill coefficient for monophosphate donor. Within each single species, the higher the interaction coefficient was, the lower S0.5 and P0.5 values were. In mammalian tissues one form of nucleoside phosphotransferase seems to prevail where cooperative interactions are almost absent and whose S0.5 as well as P0.5 values do not vary significantly from one tissue to another.