Feedback Inhibition of Thymidine Kinase by Thymidine T&phosphate*

That the regulation of cell division and deoxyribonucleic acid synthesis is subject to some kind of chemical control in higher organisms is widely accepted (2-5). Davidson (2) has suggested that deoxyribonucleic acid synthesis takes place at critical concentrations of the deoxyribonucleoside triphosphates and that these compounds are components of a homeostatic mechanism. There are now at least five negative feedback controls that have been demonstrated in the formation of the pyrimidine deoxyriboside triphosphates (Fig. 1). The first was reported by Reichard, Canellakis, and Canellakis (6), who reported that the conversion of cytidine diphosphate to deoxycytidine diphosphate through unknown intermediates (7) is strongly inhibited by deoxyguanosine triphosphate and deoxyadenosine triphosphate and to a lesser extent by thymidine triphosphate but not by deoxycytidine triphosphate (labeled nfb1 in Fig. I). This feedback suggests an explanation for the inhibition of tissue cultures by high levels of thymidine and also explains how deoxycytidine could relieve the inhibition (8).l The Reichard mechanism would not prevent dTTP from accumulating if large amounts of thymidine were available, and it was thus of interest to find a second feedback mechanism (nfbZ in Fig. 1) in which dTTP prevents its own synthesis by inhibiting thymidine kinase (1). This inhibition has been confirmed by Maley and Maley (9), who in addition reported inhibition of deoxyuridine kinase by dTTP, deoxycytidine kinase inhibition by dCTP, and deoxycytidylate deaminase inhibition by dTTP (nfbS, nfb4, and qfb5 in Fig. 1). The available evidence indicates that deoxycytidine kinase is inhibited only by its distal product dCTP and not by other deoxyriboside triphosphates (9), and similarly, that thymidine kinase is inhibited only by its distal product dTTP and not by other deoxyriboside triphosphates, as reported below. The inhibition of thymidine kinase by dTTP, called nfb2 in Fig. 1, is the subject of this paper. Evidence is presented that the degree of inhibition is strongly modulated by the concentrations of the substrates, thymidine and adenosine triphosphate. Fig. 1 indicates the well known alternative precursors of dTTP: CDP (7), deoxycytidine, deoxyuridine, and thymidine. In addi-