Synthesis and Catabolism of Cysteine

Synthesis of Arginine and Ornithine and the 1407 1. Synthesis and Catabolism of Cysteine... 

Synthesis and Catabolism of Pro line 1406. .. G. Cysteine and Sulfur Metabolism... 

The rates of synthesis and catabolism of some pyridoxal phosphate-dependent enzymes tire tdtered in deficiency. For example, within a few days of feeding a vittunin Be-free diet to animals, there is a fail in the activity of cysteine sulfinate decarhoxyltise in liver after 2 weeks, the ttmount of the enzyme protein htis fallen to extremely low levels. It is likely that these enzymes are sacrificed to release pyridoxed phosphate for other, more essentied enzymes. Other enzymes show the opposite response - appttrent induction of the apoenzyme in vittunin Be deficiency, presumably in tm attempt to trap as much of the avtiilable pyridoxal phosphate as possible. Sato emd coworkers (1996) demonstrated increased catabolism of apocystathionase in viteunin Be deficiency, but no decrease in the tunount of immunoreactive protein in the liver, as a result of increased transcription. 

The enzymes are widely distributed in microorganisms, plants, and animals. " Three Mo-MPT enzymes have been found in mammals (1) xanthine dehydrogenase see Dehydrogenase) has many, varied roles in purine catabolism, drug metabolism, and oxidative stress response, (2) aldehyde oxidase is important in drug metabolism and the synthesis of retinoic acid from retinal, and (3) sulfite oxidase plays a cmcial role in the detoxification of sulfite produced in the degradation of cysteine and methionine. Genetic Mo-MPT deficiency in... 

FIGURE a.27 Pathway for methionine cataboLsm and cysteine synthesis. Methionine is the source of the sulfur atom of cysteine. Serine is the source of the carbon skeleton of serine. In methionine catabolism, the carbon skeleton of methionine is converted to propionyl-CoA, which eventually enters the Krebs cycle at the point of succinyl-CoA. BCAA dehydrogenase catalyzes the oxidation of a ketobutyrate to propionyXloA-... 

Sulfite oxidase catalyzes one of the final stops in the oxidation of the sulfur amino acids. The catabolism of methionine can result in the appearance of its sulfur atom in cysteine, as shown in Chapter 8. Cysteine can be oxidized to cysteine sulfonate, as shciwn in the section on taurine in Chapter 2, and then degraded to pyruvate. Daily, an average of 25 mmol of sulfite is produced in the body. This amount is large compared with the dally intake of fo< sulfite, which is about 2.5 mmol- The point at which sulfite oxidase occurs in the cysteine catabolic pathway is shown in Figure 10,53, Sulfate (SO ") is required for the synthesis of su I fated polypeptides and polysaccharides. It is thought that sulfate is not required in the dict-... 

Changes in plasma free amino acid (AA) kinetics reflect modification of AAmetabolism in different metabolic states (Melchior e/a/., 2(X)4). Cysteine (CYS) is considered to be conditionally essential during immune system stimulation (ISS) due to its increased utilization for synthesis of protein and metabolites such as albumin, glutathione (GSH), defensins. (Jahoor et cd., 1999 Malmezat et al, 2000 Tang and Selsted, 1993). Plasma CYS flux, GSH level and rates of appearance of sulfate (SO4, end product of CYS catabolism) and taurine (TAU) were determined to evaluate the impact of ISS on CYS kinetics. 

Fig. 3. Regulation of the bound pathway for the assimilation of sulfate into cysteine and associated processes. Carrier refers to an endogenous thiol of uncertain identity in higher plants. Enzymes associated with the sulfate assimilation pathway and the synthesis of O-acetylseiine are (1) high-ailinity sulfate uptake mechanism, (2) ATP-sulfurylase, (3) adenosine S -phosphosulfate (APS) sulfotransferase, (4) organic thiosulfate reductase, (5) cysteine synthase, and (6) serine transacetylase. Cysteine sulfhydrase (7), an enzyme of cysteine catabolism, and nitrate reductase (8), the first enzyme of the nitrate assimilation pathway, are also shown. Inhibitory control of the pathways is shown by discontinuous lines (----) and enhancement by continuous lines (------).

Taurine was discovered in 1827 in ox hUe, where it is conjugated with the bile acids. It was later shown to be a major excretory product of the sulfur amino acids methionine and cysteine. Until about 1976, it was assumed that it was a metabolic end-product whose only function was the conjugation of bile acids. In the rat, taurine synthesis accounts for 70% to 85% of total cysteine catabolism. 

The diet of industrialised countries is rich in proteins and provides the physiological amount of SAA needed for the turnover and synthesis of the proteins in the organism. The absorption of L-Cysteine/L-Cystine by the intestines, originated by a normal or supplemented diet, is practically total. The excess of Cys is known to be quickly catabolized [3] (Figure 2). 

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