New roles for cysteine and transsulfuration enzymes: production of H2S, a neuromodulator and smooth muscle relaxant.

The enzymes of the transsulfuration pathway also have the capacity to catalyze the desulfhydration of cysteine. Recent studies demonstrate a role of the transsulfuration enzymes, cystathionine gamma-lyase and cystathionine beta-synthase, in catalyzing the desulfhydration of cysteine in brain and smooth muscle. The H2S produced from cysteine functions as a neuromodulator and smooth muscle relaxant. In glutamatergic neurons, the production of H2S by cystathionine beta-synthase enhances N-methyl-D-aspartate (NMDA) receptor-mediated currents. In smooth muscle cells, H2S produced by cystathionine gamma-lyase enhances the outward flux of potassium by opening potassium channels, leading to hyperpolarization of membrane potential and smooth muscle relaxation.