Contribution of cysteine aminotransferase and mercaptopyruvate sulfurtransferase to hydrogen sulfide production in peripheral neurons

Hydrogen sulfide (H2S) is a gaseous neuromodulator produced from L-cysteine. H2S is generated by three distinct enzymatic pathways mediated by cystathionine -lyase (CSE), cystathionine -synthase (CBS), and mercaptopyruvate sulfurtransferase (MPST) coupled with cysteine aminotransferase (CAT). This study investigated the relative contributions of these three pathways to H2S production in PC12 cells (rat pheochromocytoma-derived cells) and the rat dorsal root ganglion. CBS, CAT, and MPST, but not CSE, were expressed in the cells and tissues, and appreciable amounts of H2S were produced from L-cysteine in the presence of -ketoglutarate, together with dithiothreitol. The production of H2S was inhibited by a CAT inhibitor (aminooxyacetic acid), competitive CAT substrates (L-aspartate and oxaloacetate), and RNA interference (RNAi) against MPST. Immunocytochemistry revealed a mitochondrial localization of MPST in PC12 cells and dorsal root ganglion neurons, and the amount of H2S produced by CAT/MPST at pH 8.0, a physiological mitochondrial matrix pH, was comparable to that produced by CSE and CBS in the liver and the brain, respectively. Furthermore, H2S production was markedly increased by alkalization. These results indicate that CAT and MPST are primarily responsible for H2S production in peripheral neurons, and that the regulation of mitochondrial metabolism may influence neuronal H2S generation. In the peripheral nervous system, hydrogen sulfide (H2S) has been implicated in neurogenic pain or hyperalgesia. This study provides evidence that H2S is synthesized in peripheral neurons through two mitochondrial enzymes, cysteine aminotransferase (CAT) and mercaptopyruvate sulfurtransferase (MPST). We propose that mitochondrial metabolism plays key roles in the physiology and pathophysiology of the peripheral nervous system via regulation of neuronal H2S production.