Human erythrocyte pyrimidine 5'-nucleotidase, PN-I.

Erythrocyte maturation is accompanied by RNA degradation and release of mononucleotides. Pyrimidine 5'-nucleotidase, PN-I, has been purified and characterized. The molecular and enzymatic properties determined for the enzyme shows a 36-kDa and 5.1 pI monomeric protein with no disulfide bridges and no phosphate content. The activity is dependent on Mg(2+), while it is inactivated by heavy metals and by thiol-reactive reagents. PN-I is specific for pyrimidine nucleoside monophosphates, including the antineoplastic agents 5'-AZTMP and 5'-Ara-CMP. PN-I possess phosphotransferase activity able to exchange phosphate between pyrimidine nucleoside monophosphates and pyrimidine nucleosides, including AZT and Ara-Cyd. Amino acid sequence has been obtained from tryptic and CNBr peptides. PN-I cDNA sequence, coding for a 286-residue protein, has been retrieved from tag database, amplified by PCR, and expressed in Escherichia coli. The recombinant protein was fully active and showed identical properties with respect to PN-I. Substantial identity has been revealed with the partial sequences reported for p36, an alpha-interferon-induced protein. The significance of this identity is discussed.