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High specific activity of uridine kinase was found in cultured peritubular cells (3.0 nmol/min per mg protein) which was more than 3-fold higher than that found in cultured Sertoli cells (0.79 nmol/min per mg protein). In the various classes of germ cells a decrease in specific uridine kinase activity was associated with increased maturity of the cells, primary spermatocytes, round spermatids and spermatozoa showing 1.3, 0.65 and 0.16 nmol/min per mg protein, respectively. A relationship between uridine kinase activity and the rate of RNA synthesis in these cells is suggested. A decrease in specific uridine kinase activity in testis with increasing age supports the finding of lower uridine kinase in mature germ cells than in earlier germ cells and somatic cells. This finding is further supported by the observation that cryptorchidism, which is associated with a time-dependent depletion of germ cells, resulted in an increase in specific uridine kinase activity. The results indicate that pyrimidine salvage is important in earlier germ cells, as well as in somatic cells in the testis, to produce substrates for nucleic acid synthesis.

Cellular distribution, developmental changes and effects of cryptorchidism on uridine kinase in the rat testis