Modulation of cysteine metabolism in mice—Effects of propargylglycine and l-cyst(e)ine-degrading enzymes

In an attempt at evaluating the therapeutic effects of cysteine depletion on the growth of cysteine-dependent L1210 leukemia, the effects in vivo of a cysteine- and cystine-degrading enzyme as well as the combined use of an inhibitor of cysteine biosynthesis with cystine-free diets, were measured in mice. The kinetic properties of rat liver γ-cystathionase (EC 4.2.1.13) and Enterobacter cloacae cysteine desulfhydrase (EC 4.4.1.1) toward the catabolism of cystine and cysteine, as well as the inhibitory properties of some possible physiological components, are described. A novel and sensitive assay for the detection of cysteine in biological fluids was used to measure the ability of these enzymes to deplete plasma cysteine levels when injected into normal mice. Cysteine desulfhydrase caused no alteration in mouse plasma cysteine concentrations, presumably due to rapid clearance ( T 1 2 less than 10 min) of the enzyme. γ-Cystathionase caused a 60 per cent drop in plasma cysteine concentrations which returned to normal with the clearance of the enzyme ( T 1 2 = 2 hr ). The properties and limitations of these enzymes are discussed within the context of their ability to deplete plasma cys(e)ine in vivo. When propargylglycine, an effective covalent inhibitor of γ-cystathionase, both in vitro ( K i = 0.1 mM, saturating T 1 2 = 0.5 min ) and in vivo, was combined with diets lacking cystine, no reduction in plasma cysteine or increase in survival of mice bearing L1210 leukemia could be observed. The compound would induce a cystathioninemia and a cystathioninuria when given to mice and was not toxic in cell culture or in animals when tested with an adequate source of cystine, but was highly toxic in the absence of cystine.