1. A 5′-nucleotidase with a strong preference for AMP over IMP was characterized in homogenates and subcellular fractions of pigeon heart by using concentrations of ATP, ADP and AMP which mimicked those present in the ischaemic tissue. 2. The AMP-5′-nucleotidase had a neutral pH optimum and an apparent Km in the range 4.6-5.2 mM. It was stimulated by ATP plus ADP, and was inhibited by other nucleoside monophosphates, Pi and p-nitrophenyl phosphate, but not by ribose 5-phosphate or beta-glycerophosphate. The enzyme was not inhibited by [alpha beta-methylene] ADP or by 5′-deoxy-5′-isobutylthioadenosine, an inhibitor of the previously purified IMP-preferring cytosolic 5′-nucleotidase. 3. Subcellular-fractionation studies indicated that the enzyme has access to cytosolic AMP, although it may be associated by weak ionic interactions with an organelle present in the low-speed particulate fraction. 4. A 5′-nucleotidase was detected under similar conditions in homogenates of rat heart. 5. The activity of the pigeon heart AMP-5′-nucleotidase was sufficient to account for previously measured rates of ischaemia-induced adenosine formation. The similar activity in rat heart could, however, account for only part of ischaemia-induced adenosine formation in this tissue.
