Transport and metabolism of adenosine in human erythrocytes: effect of transport inhibitors and regulation by phosphate

Rapid kinetic techniques were applied to determine the effect of transport inhibitors on the transport and metabolism of adenosine in human red cells. Dipyridamole inhibited the equilibrium exchange of 500 microM adenosine by deoxycoformycin-treated cells in a similar concentration dependent manner as the equilibrium exchange and zero-trans influx of uridine with 50% inhibition being observed at about 20 nM. Intracellular phosphorylation of adenosine at an extracellular concentration of 5 microM was inhibited only by dipyridamole concentrations greater than or equal to 100 nM, which inhibited transport about 95%. Lower concentrations of dipyridamole actually stimulated adenosine phosphorylation, because the reduced influx of adenosine lessened substrate inhibition of adenosine kinase. When the cells were not treated with deoxycoformycin, greater than 95% of the adenosine entering the cells at a concentration of 100 microM became deaminated. A 95-98% inhibition of adenosine transport by treatment with dipyridamole, dilazep, or nitrobenzylthioinosine inhibited its deamination practically completely, whereas adenosine phosphorylation was inhibited only 50-85%. Whether adenosine entering the cells is phosphorylated or deaminated is strictly based on the kinetic properties of the responsible enzymes, substrate inhibition of adenosine kinase, and the absolute intracellular steady state concentration of adenosine attained. The latter approaches the extracellular concentration of adenosine, since transport is not rate limiting, except when modulated by transport inhibitors. In spite of the extensive adenosine deamination in cells incubated with 100 microM adenosine, little IMP accumulated intracellularly when the medium phosphate concentration was 1 mM, but IMP formation increased progressively with increase in phosphate concentration to 80 mM. The intracellular phosphoribosylation of adenine and hypoxanthine were similarly dependent on phosphate concentration. The results indicate that adenosine is the main purine source for erythrocytes and is very efficiently taken up and converted to nucleotides under physiological conditions, whereas hypoxanthine and adenine are not significantly salvaged. Hypoxanthine resulting from nucleotide turnover in these cells is expected to be primarily released from the cells. Adenosine was also dephosphorylated in human red cells presumably by 5'-methylthioadenosine phosphorylase, but this reaction seems without physiological significance as it occurs only at high adenosine and phosphate concentrations and if deamination is inhibited.