Regulation of uridine kinase. Evidence for a regulatory site

Uridine kinase from mouse Ehrlich ascites tumor cells may exist at 4 degrees C in multiple aggregation states that only slowly equilibrate with one another. Increasing the temperature leads to dissociation, and the appearance of a single predominant species: at 22 degrees C the enzyme exists as a tetramer. There is also a break in the dependence of enzyme activity on temperature as measured in an Arrhenius plot. The feedback inhibitors CTP and UTP cause the enzyme to dissociate to the monomer, whereas the substrate ATP reverses this process. Kinetic studies show that the monomer has little or no activity. Studies of the reaction mechanism show that binding of substrates is ordered, leading to a ternary complex, and release of products is ordered: uridine is the first substrate bound, ADP the first product released. Except for the inhibitors UTP and CTP, all other nucleoside triphosphates, whether purine or pyrimidine, or containing ribose or deoxyribose, act as phosphate donor. Especially interesting are the opposite effects of CTP and dCTP on uridine kinase: unlike CTP, dCTP does not dissociate the enzyme and is competent as a phosphate donor. We propose that the various effects of different ligands are best explained by the existence of a regulatory site (with more stringent specificity than the catalytic site) that controls dissociation of uridine kinase to the inactive monomer.