The Effects of Thymidine Administration on Thymidylate Kinase Activity and on DNA Synthesis in Mammalian Tissues

Abstract

An increase in enzyme activity following substrate administration has been observed in a number of mammalian systems. Although this phenomenon has often been called “induction,” the applicability of the latter term as defined by Jacob and Monod (1961)— “the formation of enzyme electively provoked by a substrate”—remains to be demonstrated in most instances, and alternative mechanisms have been implicated in several. For example, Kenney (1960) has presented evidence which suggests that the increase in tyrosine-α-ketoglutarate transaminase activity observed in rats injected with tyrosine results from the activation of a pre-existing protein by the substrate. Similarly, the increase in tryptophane pyrrolase activity noted in rats following tryptophane administration is at least in part attributable to increased saturation of the enzyme by an activator (Greengard and Feigelson, 1961). We should like to present evidence for another mechanism of an increase in the activity of a mammalian enzyme. The studies to be presented...