Biosynthesis of dTTP during differentation of chick embryo myoblasts in culture: Changes in thymidine kinase, thymidilate kinase and thymidilate synthetase activities

Thymidine kinase, thymidilate kinase and thymidilate synthetase activities have been studied during differentation in culture of muscle cells from chick embryo both in dplicating myoblasts and in post-mitotic myotubes. Thymidine kinase activity starts decreasing long before fusion, when myoblasts are still actively engaged in DNA synthesis and it reaches a minimum value at fusion (approx. 70 h of culture). Also thymidilate synthetase undergoes a precocious alteration of its activity which is low at 24 h culture, it increases up to 48 h and drops to a very low level after fusion. Finally, thymidilate kinase activity remains constant during the duplicative period and declines only after fusion. Pulse-labelling experiments with [³H]thymidine and [³H]deoxycytidine show that the incorporation of these precursors into the deoxynucleotide pool and into DNA parallels the patterns of the enzymatic activities measured in vitro.