Cysteine methionine sparing

The results demonstrate that a near-7,ero N balance occurred with the diet containing O.S g of methionine. Higher levels of methionine resulted in a positive N balance. The iSf balance was dearly negative with the 0.7-g methionine diet. The following statement illustrates the ability of cysteine to spare methionine An unusually small amount of methionine (0.1 g) was able to support a positive N balance when cystine was included in the diet. Cysteine alone, that is, with no methionine, resulted in a negative N balance. Cysteine (or cystine) can replace a large frachon of our requirement for methionine, For this reason, the requirement for methionine is sometimes stated as a combined requirement for methionine plus cysteine. 

The concept of sparing of one nutrient by another was introduced earlier, where it was demonstrated that dietary carbohydrate can spare protein. Similarly, cysteine can spare methionine and tyrosine can spare phenylalanine. A certain proportion of dietary methionine is converted to cysteine. Mediionine normally supplies part of the body s needs for cysteine. With cysteine-free diets, methionine can supply all of the body s needs for cysteine. The methionine catabolic pathway that leads to cysteine production is shown in Figure 8.27. Only the sulfur atom of methionine appears in the molecule of cysteine serine supplies the carbon skeleton of cysteine. a-Ketobutyrate is a byproduct of the pathway. a-Ketobutyrate is further degraded to propionyl-CoA by BCKA dehydrogenase or pyruvate dehydrogenase. Propionyl-CoA is then converted to succinyl-CoA, an intermediate of the Krebs cycle. 

For a long time, it was considered that, unlike the other vitamins, vitamin E had no specific functions rather it was the major Upid-soluble, radicaltrapping antioxidant in membranes. Many of its functions can be met by synthetic antioxidants however, some of the effects of vitamin E deficiency in experimental animals, including testicular atrophy and necrotizing myopathy, do not respond to synthetic antioxidants. The antioxidant roles of vitamin E and the trace element selenium are closely related and, to a great extent, either can compensate for a deficiency of the other. The sulfur amino acids (methionine and cysteine) also have a vitamin E-sparing effect. 

A) cysteine spares methionine—ingestion of cysteine reduces the need for methionine in the diet... 

Cysteine is similar in structure to serine, but has an SH group on the b-carbon, instead of an OH group as in serine. This amino acid is extremely important in protein structure with regard to forming disulfide bonds and potential chelation. Cysteine is not considered an essential amino acid in people because it can be formed from serine and methionine. However, since methionine is an essential amino acid, in some cases improved growth can be obtained by adding cysteine to the diet, because it will spare the amount of methionine required to form the cysteine. Thus, on low-methionine diets, cysteine can be beneficial toward growth. A number of proteins are low in sulfur amino acids and, therefore, this methionine-cysteine relationship may become important. 

An important factor related to the cysteine sparing effect on methionine is the fact that homocysteine has... 

The homocysteine can accept a methyl group from betaine or tetrahydrofolate-C 1 to reform methionine. However, much of the homocysteine combines with serine to form cystathionine. The cleavage of cystathionine forms cysteine and homoserine. Thus, cysteine is not an essential amino acid, but it does have a sparing effect on methionine. 

Two closely related aromatic amino acids are phenylalanine and tyrosine. The metabolism of these two amino acids is of medical interest for two reasons. First, a large number of metabolic diseases is associated with the metabolism of these two amino acids second, a large number of important biological compounds other than protein are formed from these amino acids. Phenylalanine can be converted to tyrosine in a unidirectional, physiologically irreversible reaction. Phenylalanine is an essential amino acid that must be preformed in the diet, whereas tyrosine is not considered an essential amino acid because it can be formed from L-phenylalanine. However, the relationship is analogous to that previously indicated for cysteine and methionine the amount of phenylalanine required in the diet depends on the tyrosine content of the diet, that is, the lower the tyrosine content, the more phenylalanine required. This is referred to as a sparing effect that tyrosine has on the phenylalanine requirement. 

In addition to the sulfur-containing amino acids methionine and cysteine, inorganic sulfate, sulfur dioxide and organic sulfur compounds such glutathione, taurine and N-acetyl-methionine (to a lesser extent) are also important for the sulfur intake by sparing the required amounts of methionine and cysteine in the diet. The natural content of sulfur dioxide of food is far lower than the total sulfur content (Table 5.4). Sulfur dioxide is used for food conservation, and this in turn has led to the proposal of upper limit values for this sulfur compound (see Section 5.7 and Table 5.11). 

Cysteine inhibits cystathionine 3-synthase and, therefore, regulates its own production to adjust for the dietary supply of cysteine. Because cysteine derives its sulfur from the essential amino acid methionine, cysteine becomes essential if the supply of methionine is inadequate for cysteine synthesis. Conversely, an adequate dietary source of cysteine spares methionine that is, it decreases the amount that must be degraded to produce cysteine. 

Homocysteine provides the sulfur atom for the synthesis of cysteine (see Chapter 39). In this case, homocysteine reacts with serine to form cystathionine, which is cleaved, yielding cysteine and a-ketobutyrate. The first reaction in this sequence is inhibited by cysteine. Thus, methionine, via homocysteine, is not used for cysteine synthesis unless the levels of cysteine in the body are lower than required for its metabolic functions. An adequate dietary supply of cysteine, therefore, can spare (or reduce) the dietary requirement for methionine. 

The solubility in organic solvents is not very good because of the polar characteristics of the amino acids. All amino acids are insoluble in ether. Only cysteine and proline are relatively soluble in ethanol (1.5g/100g at 19 °C). Methionine, arginine, leucine (0.0217 g/100 g 25 °C), glutamic acid (0.00035 g/100 g 25 °C), phenylalanine, hydroxy-proline, histidine and tryptophan are sparingly soluble in ethanol. The solubility of isoleucine in hot ethanol is relatively high (0.09 g/100 g at 20 °C 0.13 g/100 g at 78-80 °C). 

The requirements of essential amino acids (g/100 g protein) for children are lysine 5.44 methionine + cysteine 3.52 threonine 4.0 leucine 7.04 phenylalanine + tyrosine 6.08 histidine 1.4 and Uyptophan 0.96. Tyrosine and cystine are not essential amino acids, but they spare the requirement of phenylalanine and methionine, respectively. 

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