Sulfinic acid oxidase

A primary experimental fact relating to this reaction is the formation of sulfate (under aerobic conditions) from cysteine sulfinic acid by the action of ground or extracted rat livers (85). This oxidation was carried out by Medes (85) in order to study the existence of a sulfinic acid oxidase responsible for the action. But it is difficult to assume that a single enzyme would have the ability to split the cysteinesulfinic acid into an organic molecule without sulfur and into inorganic sulfur, and then oxidize the latter to sulfate. Numerous explanations have been proposed to resolve this difficulty. According to Pirie (95), the removal of the sulfur from cysteine-sulfenic acid would take place as sulfite, which in turn would be oxidized spontaneously to sulfate (reaction 31). Medes and Floyd (86) have two other theories to explain the sulfate formation. The first of these two theories may be written ... 

Another route of metabolism for cysteine sulfinic acid is transamination to 3-sulfinylpyruvate, a compound that undergoes ready loss of S02 in a reaction analogous to the decarboxylation of oxaloacetate (reaction o, Fig. 24-25). This probably represents one of the major routes by which sulfur is removed from organic compounds in the animal body. However, before being excreted the sulfite must be oxidized to sulfate by the Mo-containing sulfite oxidase. The essentiality of sulfite oxidase is evidenced by the severe neurological defect observed in its absence (Chapter 16). 

A combination of D-amino acid oxidase and L-amino transferase is an example of a deracemization by stereoinversion. The product is an L-amino acid. The reaction catalyzed by amino transferase has an equilibrium constant close to unity, a very unpractical situation leading to uncomplete transformation and to the production of almost inseparable mixtures of amino acids (at least two, the amino acid product and the amino add used as an amino donor). For preparative purposes it is therefore mandatory to shift the equihbrium to the product side. A recent example of a deracemization procedure based on this coupled enzymatic system is the preparation of L-2-naphthyl-alanine 6 as illustrated in Scheme 13.9 [28]. The reaction occurs in one pot with initial oxidation of the D-amino acid catalyzed by D-amino acid oxidase from Rhodotonda gracilis. The hydrogen peroxide that is formed in stoichiometric amounts is decomposed by catalase. The a-keto add is the substrate for L-aspartate amino transferase (L-Asp amino transferase), which is able to use L-cysteine sulfinic acid 7 as an amino donor. 

Figure 14.7. Pathways for the synthesis of taurine from cysteine. Cysteine sulfinate decarboxylase, EC 4.1.1.29 cysteic acid decarboxylase, EC 4.1.1.29 (glutamate decarboxylase, EC 4.1.1.15) cysteine oxidase, EC 1.13.11.20 cysteamine oxygenase, EC 1.13.11.19 and hypotaurine oxidase, EC 1.8.1.3. Relative molecular masses (Mr) cysteine, 121.2 cysteamine, 77.2 cysteine sulfinic acid, 153.2 cysteic acid, 169.2 hypotaurine, 109.1 and taurine, 125.1.

The third reaction, the oxidation of cysteine sulfinate, proceeds differently in Proteus vulgaris and in animal tissues. As shown by Singer and Kearney 18,20) the Proteus enzyme system oxidizes it directly to cysteate. This oxidation requires a pyridine nucleotide coenzyme which appears to be closely related to DPN+ 23). On the other hand, in rat liver mitochondria a soluble enzyme system oxidizes L-cysteine sulfinate in the presence of DPN+ to j8-sulfinylpyruvate and NH3 20). This reaction is analogous to that catalyzed by glutamic dehydrogenase [see Eq. (7)]. There is some indication that kidney n-amino acid oxidase can also oxidize cysteine sulfinate 24). Among these pathways of cysteine sulfinate metabolism the transaminative one is of the greatest importance. 

Naphthylamine D-Amino acid oxidase Rhodotorula gracilis), L-aspartate aminotransferase Esch ichia coli) (rflc)-2-Naphtylamine + cysteine sulfinic acid DE [50]... 

This action is inhibited by hydrogen cyanide, sodium azide, and carbon monoxide. The inhibition by carbon monoxide is suppressed by light (66). It should be pointed out that cytochrome oxidase, with or without cytochrome c, does not exert any oxidizing action on methionine or cysteine sulfinic acid. 

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