Activities of purine nucleotide metabolizing enzymes in human thymocytes and peripheral blood T lymphocytes: in vitro effect of thymic hormones.

Adenosine deaminase (ADA, E.C. 3.5.4.4) and purine nucleoside phosphorylase (PNP, E.C. 2.4.2.1) activities are essential for the normal development and function of T lymphocytes. A comparison of the specific activity of these two enzymes and of ecto-5'-nucleotidase (5'-N, E.C. 3.1.3.5) of human thymocytes with that of peripheral T lymphocytes of children and young adults shows that significant differences exist between the activities of ADA and 5'-N in thymocytes and peripheral T cells. ADA activity is six-fold higher in thymocytes than in peripheral T cells whereas 5'-N activity is approximately one-fourth lower in thymocytes than in T lymphocytes. PNP activity is two-fold higher in T cells. The apparent Km and Vmax values for the three enzymes in thymocytes and peripheral T lymphocytes have been determined. The observed differences in enzyme activity are probably not entirely due to differences in the affinity of the enzymes for their substrates. The enzyme activities of a thymoma removed from a patient with myasthenia gravis had intermediate enzyme levels for ADA and PNP, but 5'-N was similar to peripheral T cells. Exposure of thymocytes in short-term culture to the thymic hormones thymopoietin (the pentapeptide TP5), thymosin fraction V or serum thymic factor (FTS) had no significant stimulatory or inhibitory effect on the activities of the three enzymes under our experimental conditions.