In Ehrlich ascites tumour (EAT) cells the main route for dTTP required for DNA synthesis is closely related to thymidylate synthesis activity via the de novo pathway. However, more than 10-time of thymidylate (dTMP) is synthesised by cytosolic thymidine kinase (TK1) via the salvage pathway than needed for DNA synthesis in this cells. Therefore, this study focus to determine if a substrate cycle exists between thymidine (dTdR) and dTMP in the EAT cells. Results show that the ratio of K'eq/Q for the TK1 reaction is 264.2 and for the thymidylate 5'-phosphatase (dTMPase) reaction is 110.9 in the exponentially growing cells, respectively. Since the apparent ratios of K'eq/Q for both reactions are different from equality (ñ1) by two orders, it appears as a non-equilibrium reaction. This indicates that when TK1 and dTMPase are simultaneously active in the exponentially growing cells, a substrate cycle results. The regulation of the excess of non-essential products of dTdR/dTMP for DNA synthesis is involved in a substrate cycle for maintaining a balanced nucleotide pool, hence ensuring a balanced supply of the DNA precursor in the exponentially growing cells. As the tumours continue to grow, cells reached the stationary phase. The ratio of K'eq/Q for TK1 reaction is 7.7 and for the dTMPase reaction is 81.1, showing less than the equilibrium of two orders of magnitude. In this case, dTMPase could not act with TK1 together to form a pair of reaction, leading to an elevated concentration of intracellular dTMP and a dramatically excretion of dTdR into the ascites fluid.
Copyright 2002 S. Karger AG, Basel