Transfer of a sulfur ion to cyanide or to other thiol compounds. Also has weak rhodanese activity. Detoxifies cyanide and is required for thiosulfate biosynthesis. Acts as an antioxidant. In combination with cysteine aminotransferase (CAT), contributes to the catabolism of cysteine and is an important producer of hydrogen sulfide in the brain, retina and vascular endothelial cells. Hydrogen sulfide H(2)S is an important synaptic modulator, signaling molecule, smooth muscle contractor and neuroprotectant. Its production by the 3MST/CAT pathway is regulated by calcium ions. 2-oxo-3-sulfanylpropanoate + [thioredoxin]-dithiol = [thioredoxin]-disulfide + H(+) + hydrogen sulfide + pyruvate By oxidative stress, and thioredoxin. Under oxidative stress conditions, the catalytic cysteine site is converted to a sulfenate which inhibits the MPST enzyme activity. Reduced thioredoxin cleaves an intersubunit disulfide bond to turn on the redox switch and reactivate the enzyme. Inhibited by different oxidants, hydrogen peroxide and tetrathionate. Monomer (active form). Homodimer; disulfide-linked (inactive form). Expressed in the brain and retina. In the retina, localized to the inner and outer plexiform layer, the inner and outer nuclear layer and the outer segments of photoreceptors. In the brain, localized to neurons of mitral cell layers, glomerular, and external plexiform layers in the olfactory bulb. Also found in Purkinje cell stomata and proximal dendrites. In the spinal cord, localized to large neurons. In the cerebral cortex, localized to pyramidial neurons in layers II/III and V, and in layers I-VI of neocortical areas. In the hippocampus, found in CA1 and CA3 pyramidal cells. In the developing brain, maintained expression from 16 dpc to postnatal day 14. Levels decrease between postnatal day 28 and postnatal day 52 and increase again with further aging up to 156 days old. Contains two rhodanese domains with different primary structures but with near identical secondary structure conformations suggesting a common evolutionary origin. Only the C-terminal rhodanese domain contains the catalytic cysteine residue (By similarity). Thioredoxin (Trx) or dihydrolipoic acid (DHLA) are required to release hydrogen sulfide from the persulfide intermediate.