title: The effect of three alpha-keto acids on 3-mercaptopyruvate sulfurtransferase activity.
abstract: 3-Mercaptopyruvate sulfurtransferase catalyzes the transfer of sulfur from 3-mercaptopyruvate to several possible acceptor molecules, one of which is cyanide. Because the transsulfuration of cyanide is the primary in vivo mechanism of detoxification, 3-mercaptopyruvate sulfurtransferase may function in the enzymatic detoxification of cyanide in vivo. Three alpha-keto acids (alpha-ketobutyrate, alpha-ketoglutarate, and pyruvate) have previously been demonstrated to be cyanide antidotes in vivo, and it has been suggested that this is due to the nonenzymatic binding of cyanide by the alpha-keto acid. However, it has also been proposed that alpha-keto acids may increase the activity of enzymes involved in the transsulfuration of cyanide. Thus, the effect of these three alpha-keto acids on the enzyme 3-mercaptopyruvate sulfurtransferase was examined. All three alpha-keto acids inhibited 3-mercaptopyruvate sulfurtransferase in a concentration-dependent manner and were determined to be uncompetitive inhibitors of MST with respect to 3-mercaptopyruvate. The inhibitor constant Ki was estimated by two methods for each inhibitor and ranged from 4.3 to 6.3 mM. The I50, which is the inhibitor concentration that produces 50% inhibition, was calculated for all three alpha-keto acids and ranged between 9.5 and 13.7 mM. These observations add further support to the hypothesis that the mechanism of the alpha-keto acid antidotes is the nonenzymatic binding of cyanide, not stimulation of enzymes involved in the transsulfuration of cyanide to thiocyanate.
authors.affiliation: Biochemical Pharmacology Branch, U.S. Army Medical Research Institute of Chemical Defense, Aberdeen Proving Ground, MD 21010-5425, USA.
authors.forename: D W
authors.initials: DW
authors.lastname: Porter
authors.forename: S I
authors.initials: SI
authors.lastname: Baskin
mesh_terms: D000931:Antidotes / Q000378:metabolism / Q000494:pharmacology*; D001667:Binding, Competitive; D002087:Butyrates / Q000378:metabolism / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004591:Electrophoresis, Polyacrylamide Gel; D007656:Ketoglutaric Acids / Q000378:metabolism / Q000494:pharmacology*; D007700:Kinetics; D010084:Oxidation-Reduction; D019289:Pyruvic Acid / Q000378:metabolism / Q000494:pharmacology*; D012015:Reference Standards; D013056:Spectrophotometry, Ultraviolet; D013466:Sulfurtransferases / Q000037:antagonists & inhibitors*
chemical_list: D000931:Antidotes; D002087:Butyrates; D007656:Ketoglutaric Acids; C005087:alpha-ketobutyric acid; D019289:Pyruvic Acid; D013466:Sulfurtransferases; C021550:3-mercaptopyruvate sulphurtransferase