MPST and homocystinuria: Of the three enzymes, cystathionine-γ-lyase (CSE), cystathionine-β-synthase (CBS), and 3-mercaptopyruvate sulfurtransferase (3-MST) can utilize L-cysteine as a substrate to produce H2S. Deficiency of H2S-producing enzymes results in some disorders such as homocystinuria, which is characterized by mental retardation, skeletal abnormalities, increased urine homocysteine, increased risks of thromboembolism, and early onset of atherosclerosis [9–11].