Uptake of free adenosine and adenosine from adenosine monophosphate by human peripheral blood lymphocytes: possible kinetic role for ecto-5'-nucleotidase in the regulation of intracellular adenosine.

The role of 5'-nucleotidase in the uptake of adenosine from AMP was investigated in lymphocytes from normal subjects and patients with common variable hypogammaglobulinaemia (CVH) and chronic lymphatic leukaemia (CLL). At physiological pH, the Km values for the uptake of adenosine and of adenosine from AMP by intact cells were one order of magnitude higher than the Km values for 5'-nucleotidase. The Vmax values for the hydrolysis of AMP by 5'-nucleotidase were two orders of magnitude greater than for the uptake of adenosine itself or the uptake of adenosine from AMP by normal lymphocytes. 5'-Nucleotidase activity is clearly not rate-limiting in normal lymphocytes for uptake of adenosine from AMP in steady state conditions. Patients with common variable hypogammaglobulinaemia showed a low Vmax for 5'-nucleotidase assayed at pH 7.4 in intact cells as compared to values from control subjects. Michaelis constants (Km) for the uptake of free adenosine and adenosine from AMP as well as 5'-nucleotidase were similar compared to those obtained for controls. The uptake of adenosine moiety from AMP in CLL lymphocytes with a low Vmax for 5'-nucleotidase was also reduced, although not to the same extent as the reduction in 5'-nucleotidase activity. One CLL patient with supranormal levels of 5'-nucleotidase activity showed elevated uptake of adenosine moiety from AMP and of free adenosine. These results suggest that 5'-nucleotidase can influence the salvage of purine by lymphocytes from extracellular nucleotides but only when the enzyme activity is greatly reduced.