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Although several pathways contribute to the catabolism of L-cysteine, the products formed are few--taurine + CO2 and pyruvate + ammonia + sulfate. L-Cysteinesulfinate is a key intermediate that is either decarboxylated to ultimately yield taurine or transaminated to yield pyruvate. There is strong evidence that pyruvate is also formed by several cysteinesulfinate-independent pathways collectively referred to as "cysteine desulfhydrase." The quantitative importance of cysteinesulfinate-independent pathways of taurine synthesis is less clear, but it has been suggested that taurine synthesis from the cysteamine released during phosphopantetheine and CoASH turnover accounts for the high taurine content of tissues with very low levels of cysteinesulfinate decarboxylase activity (e.g. skeletal muscle and heart). In the present studies, the metabolic flux through each of these pathways was quantitated in vivo by monitoring the formation of respiratory 14CO2 in mice administered L-[1-14C]- or L-[3-14C]cyst(e)ine. Mice given 0.05 mmol/kg of L-cystine or 0.5 or 2.5 mmol/kg of L-cysteine catabolize 35, 51, and 72% of the dose, respectively, in 6 h; the relative contribution of taurine synthesis to total catabolism decreases from 63 to 51 to 42% as the L-cyst(e)ine dose is increased. To evaluate the role of L-cysteinesulfinate in taurine synthesis, D-cysteinesulfinate was characterized and used as a metabolism-resistant, potent, and specific inhibitor of cysteinesulfinate decarboxylase. Studies with L-[1-14C]- and L-[3-14C]cysteine in the presence of inhibitor indicate that 85-93% of taurine synthesis occurs from L-cysteinesulfinate: the calculated contribution of the phosphopantetheine pathway is small and may approximate zero. L-Cysteinesulfinate transmamination accounts for 25% of pyruvate synthesis from L-[14C]cystine (0.05 mmol/kg) but only 11% of pyruvate synthesis from L-[14C]cysteine (2.5 mmol/kg). Cysteine desulfhydrase reactions account for most of the pyruvate synthesis. |
