Fisher & Newsholme (1984) recently reported the kinetic properties of the adenosine kinase purified from rat heart. The enzyme was similar to that isolated from a number of other species and organs, had a Km for adenosine in the range 0.2-0.4 gM and was uncompetitively inhibited by Mg2. Based on these experiments in vitro and on the total tissue content of adenosine, the authors repeated their conclusion (Arch & Newsholme, 1978) that the adenosine kinase is saturated with substrate in the normoxic heart and also in other tissues subjected to basal metabolic loads. They further speculate that the activity of adenosine kinase might be reduced during ATP catabolism and thus enhance the elevation of adenosine concentration seen during perfusion of the heart with hypoxic buffer.

Both these conclusions are at variance with our experiments directly testing these proposals and reported in Newby et al.(1983). In our experi-ments, the activity of adenosine kinase was determined in situ by measuring the rate of incorporation of radiolabelled adenosine into cellular nucleotides. The validity of this measurement, which is a minimum estimate of the activity of the enzyme under conditions of endogenous regulation, was established by inhibiting nucleo-side incorporation with two specific inhibitors of adenosine kinase, 5'-amino-5'-deoxyadenosine and 5-iodotubercidin (Newby, 1981). When the rate of adenosine formation was determined in polymorphonuclear leucocytes or in cultured heart cells it was only a small proportion of the activity of adenosine kinase determined in situ. Thus at the steady state, the rates of adenosine formation and metabolism being equal, the kinase reaction must be far from saturation. Using similar methodology, Bontemps et al.(1983) reached a similar conclusion for isolated hepatocytes.