Cysteine oxidative deamination

Flavin-containing mitochondrial MAO-A and MAO-B catalyze the oxidative deamination of neurotransmitters, such as dopamine, serotonin, and norepinephrine in the central nervous system and peripheral tissues. The enzymes share 73% sequence homology and follow the same kinetic and chemical mechanism but have different substrate and inhibitor specificities. Chemical modification experiments provide evidence that a histidine residue is essential for the catalysis. There is also strong evidence that two cysteine residues are present in the active site of MAO. 

The Strecker degradation involves the oxidative deamination and decarboxylation of a a-amino acid in the presence of a dicarbonyl compound. The products formed from this reaction are an aldehyde containing one less carbon atom than the original amino acid and an a-aminoketone (Table 9.2). The Strecker degradation of methionine and cystein is a source of sulfur-containing intermediates (e.g hydrogen sulfide and 2-methylthiopropanal = methional) [48]. 

A. Rinaldi, M.B. Fadda and C. De Marco, Oxidative deamination of carboxyethyl-cysteine and carboxymethyl-homocysteine. Physiol. Chem. Phys., 1978, 10 47. 

The direct oxidation of amino acids (oxidative deamination) is insignificant compared to transamination. Decarboxylation is important only for some special metabolic processes. Serine, e.g., is transformed to ethanolamine, one of the major components of the phosphatides. Similarly, cysteine can become /3-mercaptoethyl-amine, a constituent of coenzyme A. Decarboxylation also plays a role in the formation of certain hormones (cf. Chapt. VIII-5). 

Like methionine, cysteine as an a-amino acid can undergo oxidative deamination, giving rise to jS-thiolpyruvic acid. This reaction takes places through the action of the same enzymes, the d-amino acid oxidase, and the /-amino acid oxidases, which act on methionine and which have been discussed on page 374. No facts are available on the fate of the keto acid in question which apparently does not seem to play an important role in the formation of sulfate or taurine. 

Cystine undergoes oxidative deamination according to mechanisms analogous to those discussed above for methionine and cysteine. This oxidative deamination leads evidently to the formation of dithiopyruvic acid, the metabolism of which is poorly understood and does not seem to play any important role in animal metabolism. 

Other than water, protein is the major constituent of meat averaging nearly 21% in heef or chicken meat, with fat varying fiom 4.6 to 11.0% in beef and fiom 2.7 to 12.6% in chickoi. The principal radiolytic reactions of aqueous solutions of aliphatic amino acids are reductive deamination and decarboxylation. Alanine yields NH3, pyruvic add, acetaldehyde, propionic acid, CO2, H2, and ethylamine (6). Sulfur-containing amino adds are espedally sensitive to ionizing radiation. Cysteine can be oxidized to cystine by the hydroxyl radical or it can react with the hydrated electron and produce... 

When present in excess methionine is toxic and must be removed. Transamination to the corresponding 2-oxoacid (Fig. 24-16, step c) occurs in both animals and plants. Oxidative decarboxylation of this oxoacid initiates a major catabolic pathway,305 which probably involves (3 oxidation of the resulting acyl-CoA. In bacteria another catabolic reaction of methionine is y-elimination of methanethiol and deamination to 2-oxobutyrate (reaction d, Fig. 24-16 Fig. 14-7).306 Conversion to homocysteine, via the transmethylation pathway, is also a major catabolic route which is especially important because of the toxicity of excess homocysteine. A hereditary deficiency of cystathionine (3-synthase is associated with greatly elevated homocysteine concentrations in blood and urine and often disastrous early cardiovascular disease.299,307 309b About 5-7% of the general population has an increased level of homocysteine and is also at increased risk of artery disease. An adequate intake of vitamin B6 and especially of folic acid, which is needed for recycling of homocysteine to methionine, is helpful. However, if methionine is in excess it must be removed via the previously discussed transsulfuration pathway (Fig. 24-16, steps h and z ).310 The products are cysteine and 2-oxobutyrate. The latter can be oxidatively decarboxylated to propionyl-CoA and further metabolized, or it can be converted into leucine (Fig. 24-17) and cysteine may be converted to glutathione.2993... 

Recently, the chemical synthesis of 5/i-3-phosphatidylsulfocholine has been reported (Tremblay and Kates, 1979). Biosynthesis of the sulfocholine moiety in Nitzschia alba has been suggested to occur via the deamination, oxidative decarboxylation, and methylation of methionine. In addition, the 1-deoxyceramide-1-sulfonate compound was proposed to be synthesized in a manner analogous to sphingosine, with cysteine replacing serine (Anderson et a/., 1979). 

Lipid oxidation products can interact with proteins and amino acids, and can affect the flavor deterioration and nutritive value of food proteins. Peroxyl radicals are very reactive with labile amino acids (tryptophane, histidine, cysteine, cystine, methionine, lysine and tyrosine), undergoing decarboxylation, decarbonylation and deamination. Methionine is oxidized to a sulfoxide combined cysteine is converted to cystine to form combined thiosulfinate (Figure 11.4). Aldehydes, dialdehydes and epoxides derived from the decomposition of hydroperoxides react with amines to produce imino Schiff bases (R-CH=N-R ). Schiff bases polymerize by aldol condensation producing dimers... 

L-Cysteine is transformed to L-cysteine sulfinic acid and L-cysteic acid. Cysteamine (D 11) yields hypotaurine and taurine (Fig. 189). The latter compounds may be transformed to other secondary products by deamination, thiolation, guanylation, and methylation. L-Cysteine sulfinic acid may be degraded to L-alanine and sulfurous acid, which is oxidized to sulfuric acid. 

Sulfitolysis is another means by which the disulfide bond is broken by some fungi. This reaction occurs in the presence of sulfite and under alkaline conditions. It cleaves the disulfide in cystine to S-sulfocysteine and cysteine [74, 75]. This causes the keratin chains to denature by releasing them from one another (chaini-S-S-chain2-I- SO3" chaini-S-S03 -1- S-chain2). The fungal oxidation of thiols in the cysteine residues yields sulfide for this reaction, and the liberation of ammonium from the deamination of the amino acids creates the alkaline conditions to help drive this reaction [76]. 

Gruen (757) found that the presence of most of the amino acids tested during hydrolysis does not affect the tryptophan recovery. Of the hydroxyamino acids, threonine had no effect at all, while tyrosine has a small effect. On the other hand serine was found to reduce the recovery of tryptophan by about 10% due to a deamination reaction of serine. This produces pyruvic acid which, as a ketoacid, interacts with the indole nucleus of tryptophan (287). Whereas cysteine had no effect, cystine caused a substantial loss of tryptophan (about 40%), and was itself recovered on the analyzer partly in the reduced cysteine form. These results would indicate that a major factor in the loss of tryptophan during acid hydrolysis of a protein is degradative oxidation by cystine. 

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