Sulfhydration of reactive cysteines in target proteins is now recognized as a major route by which H2S mediates signal transduction and regulates various cellular processes. Among the enzymatic systems permitting the formation of cysteine persulfide from non-activated sulfur compounds, 3-mercaptopyruvate sulfurtransferases can be considered as a model of thiolate-based chemistry for carbon-sulfur bond breaking. These ubiquitous enzymes transfer a sulfur atom from 3-mercaptopyruvate (3-MP) to a thiol acceptor via a cysteine-persulfide intermediate, but the mechanistic basis for its formation is still unclear. To address this question, kinetic approaches were developed for studying the reaction catalyzed by the human and Escherichia coli enzymes and the role of several conserved residues was also investigated. We showed that the first step of sulfur transfer that leads to pyruvate release and formation of the persulfide intermediate is very efficient for both enzymes. It critically depends on the electrostatic contribution provided by the CGSGVT catalytic loop while any role of the so-called Ser/His/Asp triad can be excluded. Furthermore, solvent kinetic isotopic effect and proton inventory studies revealed a concerted mechanism in which the water-mediated protonation of the pyruvate enolate and the S0 transfer from the deprotonated 3-MP to the thiolate form of the catalytic cysteine occurs concomitantly.