3-Mercaptopyruvate, transfer

The first hint of an essential role of molybdenum in metabolism came from the discovery that animals raised on a diet deficient in molybdenum had decreased liver xanthine oxidase activity. There is no evidence that xanthine oxidase is essential for all life, but a human genetic deficiency of sulfite oxidase or of its molybdopterin coenzyme can be lethal.646,646a,b The conversion of molybdate into the molybdopterin cofactor in E. coli depends upon at least five genes.677 In Drosophila the addition of the cyanolyzable sulfur (Eq. 16-64) is the final step in formation of xanthine dehydrogenase.678 It is of interest that sulfur (S°) can be transferred from rhodanese (see Eq. 24-45), or from a related mercaptopyruvate sulfurtransferase679 into the desulfo form of xanthine oxidase to generate an active enzyme.680... 

An important property of 3-mercaptopyruvate arises from electron withdrawal by the carbonyl group. This makes the SH group electrophilic and able to be transferred as SH+, S°, to a variety of nucleophiles (Eq. 24-44). Thus sulfite yields thiosulfate (S2032 + H+, Eq. 24-45, step a), cyanide yields thiocyanate (Eq. 24-45, step b), and cysteine sulfinate yields alanine thiosulfonate.448 461 The reactions are catalyzed by mercaptopyruvate sulfurtransferase, an enzyme very similar to thiosulfate sulfurtransferase. The latter is a liver enzyme often called by the traditional... 

Okoh and Pitt, 1982 Silver et al., 1982). Thus, rat LD50 values for NaCN by the peroral (po) and intraperitoneal (ip) routes are respectively 5.7 and 4.72 mg kg-1, and the corresponding values for NaSCN are 764 and 540 mg kg-1 thus, the conversion of NaCN to NaSCN results in a decrease in acute lethal toxicity by a factor of about 120-fold in the rat (Ballantyne, 1984). SCN is renally excreted with a half-life of 2.7 days in healthy subjects (Schulz et al., 1983). Two enzyme systems are responsible for the transulphuration process (Ballantyne, 1987b Lang, 1933 Sorbo, 1975) thiosulphate-cyanide transulphurase (EC 2.8.1.1 rhodanese) and (3-mercaptopyruvate-cyanide transulphurase (EC 2.8.1.2). Thiosulphate-cyanide transulphurase is a mitochondrial enzyme that catalyzes the transfer of a sulphane sulphur atom from sulphur donors to sulphur acceptors ... 

According to present knowledge transfer of bivalent or sulphane sulphiir from thiosulphate, thiocystine, 3-mercaptopyruvate and some other donors is catalyzed by at least three different enzymes rhodanese /EC 2.8, 1.1/, thiosulphate reductase /no EC number/ and mercapto-pyruvate sulphurtransferase /feC 2.8.1,2/ /for references see Westley, 1973 Koj et al.,1975/ Th mechanism of their action is not uniform rhodanese operates by a double-displacement mechanism /cf. Westley, 1973/ mercaptopyruvate sulphurtransferase by a sequential reaction /Jarabak and Westley,1978/, while the mode of action of thiosulphate reductase is not elucidated. 

In a subsequent reaction, the thiol group of mercaptopyruvate is transferred to a thiol acceptor to form pyruvate and a disulfide. An enzyme supporting the latter reaction (mercaptopyruvate sulfotransferase) which uses dithioerythritol, mercaptoethanol, and mercaptopyruvate as acceptors has been reported in several higher plants, Chlorella, and Synechococcus (Schmidt, 1984). However, information on the formation of mercaptopyruvate in plants is required before... 

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