Inhibition of yeast cytosine deaminase by 5-bromo-2-pyrimidinone and its covalent hydrate

Yeast cytosine deaminase (EC 3.5.4.1) is inhibited by 5‐bromo‐2‐pyrimidinone. In aqueous solution at neutral pH three forms of this compound (the anion, the parent, and the covalent hydrate) are in equilibrium. Experiments were undertaken in order to determine the relative contributions of these three forms to the observed inhibition. The anion makes little or no contribution. Both the parent and the covalent hydrate inhibit the enzyme, with the Ki for the hydrate being 0.2–0.02 times that of the parent. In the presence of stoichiometric concentrations of the enzyme, the equilibrium between parent and hydrate is displaced towards the hydrate; however, the hydration is not catalyzed by cytosine deaminase.