A low Km nucleoside 3'(2'),5-bisphosphate 3'(2')-phosphohydrolase from rat liver

In the course of an investigation on the occurrence in rat liver of a specific hydrolytic activity on adenosine 2',5'-bisphosphate, a nucleoside 3'(2'),5'-bisphosphate 3'(2')-phosphohydrolase was purified following standard procedures. The enzyme hydrolyzes the phosphate group joined to the 3' or the 2' position of the following nucleotides (relative velocities indicated in brackets): PAdoP (100), PCydP (95), PGuoP (80), PAdo2'P (40), PdAdoP (4), SPAdoP (18). Other nucleotides were not substrates of the reaction: NADP+, PAdoPP, PPGuoP, AdoP, PAdo, GuoP, PGuo, ADP, ATP, cAMP, adenosine(3')phospho(5')adenosine. The Km values determined for PAdoP and PAdo2'P were 10 and 7 microM, respectively. Two isoforms were separated by chromatography on a Mono Q column. Both isoforms were kinetically indistinguishable, presenting a pI value of 5.35, a molecular mass of 38 kDa, pH optimum of 8.0, and strictly required Mg2+ or Mn2+. An enzymatic activity similar to the one described here has already been reported in guinea pig liver [5]. These authors however only obtained 1 enzymatic form with Km values of 3.1 and 1.8 mM for PAdoP and PAdo2'P, respectively. The potential physiological role of this enzyme in the metabolism of sulphate is also considered. The previously registered number EC 3.1.3.7 could be applied to this activity.