In an attempt to differentiate between thymidine kinase (EC. induced by herpesvirus hominis type 1 (TK 1) and type 2 (TK 2), the different susceptibilities to the modifying effects of some thymidine analogues proved to be useful criteria: (1) 2′-deoxythymidine-5′-triphosphate (dThd-5′-PPP) inhibits TK 2 at a concentration of 0.125 m by 90%, whereas TK 1 is inhibited at 4.03 m by 50%. (2) 2′-deoxythymidine-5′-monophosphate (dThd-5′-P) competitively inhibits TK 2 at all concentrations tested. On the other hand, the direction of its effect on TK 1 is concentration dependent: at 500 µ it stimulates and at 8 m inhibits TK 1 activity. During enzyme kinetic studies, TK 1 displays substrate inhibition which is reversed by dThd-5′-P. This result explains the stimulating effect of dThd-5′-P at 500 µ. This phenomenon suggests the existence on the enzyme molecule of a second binding site for dThd which mediates substrate inhibition and which can be occupied also by dThd-5′-P. After polyacrylamide gel electrophoresis of TK 1, the stimulation by dThd-5′-P disappears, suggesting the separation of the second binding site from the catalytic centre.


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