Adenosine uptake, transport, and metabolism in human erythrocytes

Abstract

Using rapid kinetic techniques, we have determined the kinetics of zero-trans influx and equilibrium exchange of adenosine, and its uptake and in situ phosphorylation at 25°C in human erythrocytes which were pretreated with 2′-deoxycoformycin to inhibit deamination of adenosine. Both the K_m and V_max for adenosine transport were about 300 times higher than those for the in situ phosphorylation of adenosine (K_m about 0.2 μM), so that the first order rate constants for both processes were about the same. In contrast, the first order rate constant for adenosine deamination by untreated, intact cells was about 20% of that of adenosine transport or phosphorylation. These kinetic properties of the various steps, in combination with substrate inhibition of adenosine phosphorylation above 1 μM adenosine, assure that, at extracellular concentrations of physiological relevance ( < 1 μM), adenosine is very rapidly and efficiently salvaged by the erythrocytes and converted to ATP, whereas at extracellular concentrations of 10 μM or higher, practically all adenosine transported into the cells is deaminated. When the concentration of adenosine was 0.1 μM, a 10% (v/v) suspension of erythrocytes depleted the extracellular fluid of adenosine within 1 min of incubation at 25°C.