Specificity studies of 3-Mercaptopyruvate sulfurtransferase

3-Mercaptopyruvate sulfurtransferase (E.C. 2.8.1.2; MST) is an enzyme believed to function in the endogenous cyanide (CN) detoxification system because it is capable of transferring sulfur from 3-mercaptopyruvate (3-MP) to CN, forming the less toxic thiocyanate (SCN). To date, 3-MP is the only known sulfur-donor substrate for MST. In an effort to increase the understanding of what chemical properties of 3-MP affect its utilization as a substrate, in vitro enzyme kinetic studies of MST were conducted using two mercaptic acids that are structurally related to 3-MP. Neither of these compounds was able to serve as a sulfur-donor substrate for MST. Inhibitor studies determined that 3-mercaptopropionic acid did not affect the Km of MST for 3-MP but did decrease Vmax and, thus, was determined to be a noncompetitive inhibitor. Alternatively, 2-mercaptopropionic acid 2-MPA decreased Km and Vmax and was determined to be an uncompetitive inhibitor of MST with respect to 3-MP. These data indicate that the alpha-keto group of 3-MP is necessary for its utilization as a substrate, and the inhibitor studies suggest that the position of the sulfur may also affect the binding of these compounds to the enzyme. These observations increase the understanding of what factors can affect the utilization of a compound as a sulfur-donor substrate for MST and may aid in the development of alternative sulfur-donor substrates for MST.