The human blood platelet: a cellular model to study the degradation of thymidine and its inhibition.

Intact platelets catabolize extracellular thymidine into thymine. Studies of the concentration dependent degradation of thymidine by intact platelets indicate a Michaelis mechanism with an apparent Km of about 0.12 mM and a Vmax of 2.5 nmoles/min for 3 X 10(8) platelets. This degradation process is inhibited by various nucleosides, pyrimidine bases and C-5 or C-6 substituted uracils. Cytidine, deoxycytidine, adenosine and deoxyadenosine seem to inhibit thymidine degradation by reducing the intracellular transport of thymidine. Uridine inhibits both the thymidine transport and the activity of the phosphorolytic enzyme, thymidine phosphorylase (EC 2.4.2.4). Some substituted uracils are specific inhibitors of thymidine phosphorylase activity. 6-Amino-5-bromouracil, the most active of them, either with acellular extracts or purified thymidine phosphorylase, is also the best inhibitor of thymidine degradation in intact human platelets. Platelets constitute a new model to study the efficiency of specific inhibitors on thymidine catabolism in an 'human intact cell' which contains only one pyrimidine nucleoside phosphorylase, the thymidine phosphorylase.