Is novel signal transducer sulfur oxide involved in the redox cycle of persulfide at the catalytic site cysteine in a stable reaction intermediate of mercaptopyruvate sulfurtransferase?

Abstract In transsulfuration reaction catalyzed by rat mercaptopyruvate sulfurtransferase (MST), a stable persulfide is formed at the catalytic site cysteine Cys(247) as a reaction intermediate. The outer sulfur atom is donated by the substrate, thiosulfate, or by mercaptopyruvate. MST serves as a thioredoxin-dependent antioxidant possessing self-regulated enzymatic activity. After oxidation of persulfurated MST by treatment with hydrogen peroxide, mass spectrometric analysis showed that the outer sulfur atom of the persulfide is oxidized to form Cys-thiosulfenate (Cys-Sγ-SO(-)), Cys-thiosulfinate (Cys-Sγ-SO(2)(-)), and Cys-thiosulfonate (Cys-Sγ-SO(3)(-)). Next, sulfur acceptor substrates including reduced thioredoxin convert all modified cysteines to nonmodified cysteines. Another sulfur acceptor substrate, cyanide, also converted these cysteines via cyanolysis. Thus, sulfur oxides are suggested to release in the redox cycle of persulfide of MST.

Thioredoxin-dependent enzymatic activation of mercaptopyruvate sulfurtransferase. An intersubunit disulfide bond serves as a redox switch for activation.

Rat 3-mercaptopyruvate sulfurtransferase (MST) contains three exposed cysteines as follows: a catalytic site cysteine, Cys(247), in the active site and Cys(154) and Cys(263) on the surface of MST. The corresponding cysteine to Cys(263) is conserved in mammalian MSTs, and Cys(154) is a unique cysteine. MST has monomer-dimer equilibrium with the assistance of oxidants and reductants. The monomer to dimer ratio is maintained at approximately 92:8 in 0.2 m potassium phosphate buffer containing no reductants under air-saturated conditions; the dimer might be symmetrical via an intersubunit disulfide bond between Cys(154) and Cys(154) and between Cys(263) and Cys(263), or asymmetrical via an intersubunit disulfide bond between Cys(154) and Cys(263). Escherichia coli reduced thioredoxin (Trx) cleaved the intersubunit disulfide bond to activate MST to 2.3- and 4.9-fold the levels of activation of dithiothreitol (DTT)-treated and DTT-untreated MST, respectively. Rat Trx also activated MST. On the other hand, reduced glutathione did not affect MST activity. E. coli C35S Trx, in which Cys(35) was replaced with Ser, formed some adducts with MST and activated MST after treatment with DTT. Thus, Cys(32) of E. coli Trx reacted with the redox-active cysteines, Cys(154) and Cys(263), by forming an intersubunit disulfide bond and a sulfenyl Cys(247). A consecutively formed disulfide bond between Trx and MST must be cleaved for the activation. E. coli C32S Trx, however, did not activate MST. Reduced Trx turns on a redox switch for the enzymatic activation of MST, which contributes to the maintenance of cellular redox homeostasis.