A specific adenosine phosphorylase, distinct from purine nucleoside phosphorylase

Abstract

Several enzymes, catalyzing the reversible phosphorolysis of purine nucleosides have been described in eucaryotic cells and in micro-organisms [l-6] . The best known is the ‘purine nucleoside phosphorylase’ (EC 2.4.2.1) acting on the nucleosides of hypoxanthine and guanine. Adenosine is not used as substrate by this enzyme [6-81. Phosphorolysis of adenosine has been reported in Salmonella typhimurium [9], where a single enzyme protein appears to act on inosine, guanosine and adenosine. Adenine was found to be substrate for purine nucleoside phosphorylase of four mammalian sources, but its unfavourable kinetic parameters with respect to those of hypoxanthine and guanine are against the role of adenine as a physiological substrate [lO,l l] . In Mycoplasma both adenosine and inosine phosphorolysis have been observed, but no attempt has been made to correlate the two activities to different proteins [12]. The separation of adenosine phosphorylase from the purine nucleoside phosphorylase has never been reported so far, even though Miech and Coll. have presented indirect evidence that in Schistosoma Mansoni worms adenosine phosphorylase activity is a separate entity from purine nucleoside phosphorylase: this conclusion is based on differences in the pH-activity curves and studies with product and nucleoside analogues inhibitors [ 131. The data presented in this paper give the first direct evidence that at least in B. subtilis the phosphorolysis of adenosine and that of inosine and guanosine are catalyzed by distinct enzyme proteins, which can be