Hydrogen sulfide (H2S) produced endogenously has been established as the third gaseous signaling molecule, a smooth muscle relaxant and a neuroprotectant (Kimura 2011a, 2011b). Three human enzyme systems produce H2S in the brain, retina and vascular endothelial cells. 3-mercaptopyruvate sulphurtransferase (MPST, aka 3MST) in conjunction with cysteine (aspartate) aminotransferase (CAT, aka GOT2) is decribed here. The first step is the reversible transamination between L-cysteine (L-Cys) and 2-oxoglutarate (2OG, aka alpha-ketoglutarate) to form 3-methylpyruvate (3MPYR) and glutamate (Glu) catalysed by GOT2. Two forms of human aspartate aminotransferase (GOT) enzymes exist; cytosolic (GOT1) and mitochondrial (GOT2). Both are dimeric proteins requiring pyridoxal phosphate for activity. Human GOT2 (Zhou et al. 1998) possesses the same catalytic activity as its rat counterpart (Ubuka et al. 1978).