Cysteine metabolism

There are numerous abnormalities of cysteine metabolism. Cystine, lysine, arginine, and ornithine are excreted in cystine-lysinuria (cystinuria), a defect in renal reabsorption. Apart from cystine calculi, cystinuria is benign. The mixed disulfide of L-cysteine and L-homocysteine (Figure 30-9) excreted by cystinuric patients is more soluble than cystine and reduces formation of cystine calculi. Several metabolic defects result in vitamin Bg-responsive or -unresponsive ho-mocystinurias. Defective carrier-mediated transport of cystine results in cystinosis (cystine storage disease) with deposition of cystine crystals in tissues and early mortality from acute renal failure. Despite... 

Methionine metabolism Cysteine metabolism Valine, leucine, and isoleucine degradation... 

Cysteine not only is an essential constituent of proteins but also lies on the major route of incorporation of inorganic sulfur into organic compounds.443 Autotrophic organisms carry out the stepwise reduction of sulfate to sulfite and sulfide (H2S). These reduced sulfur compounds are the ones that are incorporated into organic substances. Animals make use of the organic sulfur compounds formed by the autotrophs and have an active oxidative metabolism by which the compounds can be decomposed and the sulfur reoxidized to sulfate. Several aspects of cysteine metabolism are summarized in Fig. 24-25. Some of the chemistry of inorganic sulfur metabolism has been discussed in earlier chapters. Sulfate is reduced to H2S by sulfate-reducing bacteria (Chapter 18). The initial step in assimilative sulfate reduction, used by... 

Cysteine is considered a nonessential nutrient because it can be synthesized from methionine via the transsulfuration pathway (Figs. 21-1 and 21-2). Production of cysteine is metabolically important because it serves as a source of sulfur for incorporation into proteins and detoxification reactions. A lack of cysteine needed for incorporation into the structural protein collagen may be responsible for the musculoskeletal abnormalities seen in patients with CBS deficiency. A major metabolic use of cysteine is in the production of glutathionine (y-glutamylcysteinylglycine), an important antioxidant. Another important pathway for cysteine metabolism is its oxidation to cysteinesulfinate, which serves as a precursor for taurine, an amino acid that stabilizes cell membranes in the brain. 

Taurine is a dietary essential in the cat, which is an obligate carnivore with a limited capacity for taurine synthesis from cysteine. On a taurine-free diet, neither supplementary methionine nor cysteine will maintain normal plasma concentrations of taurine, because cats have an alternative pathway of cysteine metabolism reaction with mevalonic acid to yield felinine (3-hydroxy-1,1-dimethylpropyl-cysteine), which is excreted in the urine. The activity of cysteine sulfinic acid decarboxylase in cat liver is very low. 

K. Daniels and M. Stipanuk, The Effect on Dietary Cysteine Level on Cysteine Metabolism in Rats, Journal of Nutrition 112 (1982) 2130-2141. 

C. Weinstein, R. Haschemejer and O. Griffith, In Vivo Studies of Cysteine Metabolism, J Biolog Chem 263 (1988) 16568-16579. 

K. Yamaguchi, S. Sakakibaka, J. Asamizu and I. Ueda, Induction and Activation of Cysteine Oxidase on Rat Liver II. The Measurement on Cysteine Metabolism in Vivo and die Activation of in Vivo Activity on Cysteine Oxidase, Biochim Biophys Acta 29 (1973) 48-59. 

Barbara Campanini graduated cum laude in pharmaceutical chemistry and technologies, University of Parma in 1998. In 2002, she received her Ph.D. in molecular biology and pathology, University of Parma defending a dissertation on Structural determinants of the stability of the pyridoxal 5 -phosphate-dependent enzyme 0-acetylserine sulfhydrylase. Since 2006, she works as a research scientist at the University of Parma. Her research interests include the functional characterization of PLP-dependent enzymes involved in cysteine metabolism and the preparation of variants of the green fluorescent protein for structural and spectroscopic studies. 

Taurine is a by-product of cysteine metabolism. Taurine is conjugated to cholic acid to form taurocholate, a bile salt. 

A number of enzymes involved in cysteine metabolism have been described in the tapeworm Hymenolepis diminuta (48). In addition to a high cystathionine /3-synthase activity it has low y-cystathionase activity, and contains cysteine aminotransferase, cysteine dioxygenase and cysteine sulphinate aminotransferase the latter two could be involved in the oxidation of cysteine. 

Gomez-Bautista, M. and Barrett, J. (1988) Cysteine metabolism in the cestode Hymenolepsis diminuta. Parasitology 97 149-159. 

The fact is, there have been no systematic studies of the effects of nutrients like vitamins on the structure or rate of wool or hair growth. However, there are some indications that in dietary insufficiencies, supplements of folic acid (a B complex vitamin) or pyridoxine (a B complex vitamin. Be) could be helpful to hair growth. The logic behind these indications is that these vitamins play a role in cysteine metabolism [106]. On the other hand, panthenol, the precursor to pantothenic acid (another B complex vitamin) has never been demonstrated in any published scientific study to affect the nutrition or growth of hair. In a review on nutrition and hair, Flesch [107] reported, There is no objective evidence available to support the assumption that pantothenic acid has a biochemical role in the production of hair. ... 

A metabolism study was conducted in male Wistar rats to determine the fate of the salt of 3-mercapto-2-oxopropionic acid, the intermediate product in the transamination pathway of L-cysteine metabolism. Results showed that the salt of 3-mercapto-2-oxopropionic acid is metabolized by reduction and frans-sulfuration to yield 3-mercaptolactate cysteine mixed disulfide (S-(2-hydroxy-2-carboxyethylthio) cysteine) and inorganic sulfate, respectively. The reduction is catalysed by lactate dehydrogenase, as indicated by the use of anti-lactate dehydrogenase antiserum. 

Reaction 4 which leads to the formation of cysteine sulfinic acid has never been reported in higher plants. Therefore none of the sequences above apply to cysteine metabolism in higher plants. The following reactions have been identified in higher plants. 

When glutathione synthesis was inhibited by bu-thionine sulfoximine so that were was a 50 % depletion of glutathione, the immortalised rat mesencephalic cell line CSM14.1.4 showed an enhanced synergistic toxicity of sulphite and peroxynitrite (Marshall et al. 1999). Because sulphite is present normally in the brain as a product of cysteine metabolism, and because increased peroxynitrite formation has been reported in Parkinson s disease, these events might contribute to neuronal death. 

It remains to be ascertained to what extent the oxidation of cysteine to the disulfide form contributes to the pathway of cysteine metabolism. 

In Eq. (11) X = Cl, Br, I R = benzene, naphthalene, etc. Substances which form mercapturic acids probably interfere with the physiological pathway of cysteine metabolism. It is conceivable that the process itself reflects an unrecognized role of cysteine. One is reminded of the process of phenolsulfuric acid formation, which was until quite recently an apparently useless curiosity. The pathway of naphthalenemercapturic acid formation is reconstructed as follows ... 

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