Irreversible Inhibition of Lysyl Oxidase by Homocysteine Thiolactone and Its Selenium and Oxygen Analogues

Homocysteine thiolactone, selenohomocysteine lactone, and homoserine lactone were found to be competitive, irreversible inhibitors of lysyl oxidase, with KI values of 21 6 3 mM, 8.3 6 2.2 mM, and 420 6 56 mM, respectively. The first order rate constants for inactivation (k2) of the enzyme varied over a much smaller range, ranging from 0.12 to 0.18 to 0.28 min for the Se-, thio-, and O-lactones, respectively. Mutually exclusive labeling of the enzyme by [1-C]b-aminopropionitrile, [U-C]phenylhydrazine, or [S]homocysteine thiolactone was observed. These labeling results, together with the closely similar perturbations of the near UV-visible spectra of lysyl oxidase and of a model of its lysine tyrosylquinone cofactor by the thiolactone, indicate that the lactones likely derivatize and reduce the active site carbonyl cofactor. Substitution with deuterium at the a-carbon of the thiolactone caused a deuterium kinetic isotope effect on k2 of 3.2 6 0.2, consistent with the involvement of rate-limiting a-proton abstraction during lactone-induced inactivation of the enzyme. The activities of plasma amine oxidase and diamine oxidase were only minimally reduced at concentrations of the sulfur or selenium lactones that fully inhibited lysyl oxidase. Thus, these lactones constitute a new category of mechanism-based inactivators selective for lysyl oxidase. Further, these results may relate to the development of connective tissue defects seen in homocystinuria.