The Sulfur Amino Acids

Although there is a fairly extensive knowledge of the metabolism of the sulfur moieties of these amino acids, which is discussed in the chapter, Metabolism of Sulfur-Containing Compounds, there is little definite information about the fate of their carbon residues. A few inferences can be drawn from the incomplete and scattered data at hand, and these are discussed below. 

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The sulfur amino acid content of soy protein can be enhanced by preparing plasteins from soy protein hydrolysate and sources of methionine or cystine, such as ovalbumin hydrolysate (plastein AB), wool keratin hydrolysate (plastein AC), or L-methionine ethyl ester [3082-77-7] (alkaU saponified plastein) (153). Typical PER values for a 1 2 mixture of plastein AC and soybean, and a 1 3 mixture of alkah-saponified plastein and soybean protein, were 2.86 and 3.38, respectively, as compared with 1.28 for the soy protein hydrolysate and 2.40 for casein. 

Assimilatory sulfate reducers reduce sulfate to the sulfhydryl level where it is incorporated into the sulfur amino acids of protein. Many plants and bacteria can do this. 

The high biological value of soy has been attributed to its sulfur amino acid content. As mentioned previously, the sulfur amino acid ratios may have accounted for the large change in acid excretion in the meat period compared with the soy (Figure 1). 

Haulm (stem) The possibility of using winged bean haulm as a potential raw material for single cell protein production in the tropics has been investigated by Zomer et al. (78). Semi-sol id fermentation of haulm, using various fungi, yielded a product containing 20% true protein, a 30% increase over the native material. The fermentation also increased the sulfur amino acid content by 1.5 to 6 fold. 

None of the amino acids are bound to protein, with exception of the sulfur amino acids cysteine and homocysteine and the heterocyclic amino acid tryptophan. Care should be taken in the deproteinization of biological samples in order to preserve these amino acids. 

The AAA thus has two photometers in series. Since for every eluting amino acid both signals are recorded, the so-called 570/440 absorbance ratio may be of help in the identification process. As an example the simple primary amino acids have a 570/440 ratio of approximately 6, whereas the sulfur amino acids (cystine, sulfocysteine) have much lower ratios, approaching a value of 1. Small peptides and glycyl-amino acids will react with ninhydrin, an important fact for the diagnosis of prolidase deficiency and aspartylglycosaminuria. 

Excessive heat can cause destruction of amino acid residues. The amino acid most susceptible to direct heat destruction is cystine. Although not an essential amino acid, cystine does have a sparing effect on the dietary requirement for methionine. As a result, cystine destruction can be nutritionally important. In addition, many vegetable proteins are limiting in the sulfur amino acids. Cystine destruction would be particularly harmful for these proteins. 

In the chain from soils to plants to humans, inorganic sulfur, or more accurately, the sulfate ion (SO42-), is taken up by plants and converted within the plant to organic compounds (the sulfur amino acids). These amino acids combine with other amino acids to make up plant protein. When the plant is eaten by a human or by livestock animals, die protein is broken down and die amino acids are absorbed from the digestive tract and recombined 111 the proteins of the animal body. The most important feature of sulfur in die food chain is that plants use inorganic sulfur compounds to make sulfur amino acids, whereas animals and humans use the sulfur amino acids for their own processes and excrete inorganic sulfur compounds resulting from the metabolism of the sulfur amino acids. 

Since animals tend to concentrate in their own proteins the sulfur amino acids contained in the plants diey eat, such animal products (meat. eggs, and cheese) are valuable sources of the essential sulfur amino acids in human diets. In regions where die diet is composed almost entirely of foods of plant origin, deficiencies of sulfur amino acids may be critical in human nutrition. Frequently, persons in such areas (also voluntary vegetarians) are also likely to suffer from a number of odier dietary insufficiencies unless supplemental sources are used. 

In addition to their importance as essential amino acids for humans, the quantitative determination of cysteine and methionine seems to be growing in importance in the animal feed industry. The dietary requirements for the sulfur amino acids tend to be very high in many animals. This is presumably due to the magnitude of hair/feather growth and the fact that the structural proteins that comprise hair/feathers often have high cyst(e)ine content. 

Thus, among the sulfur amino acids, hydrogen sulfide is a likely derivative of cysteine, and dimethyl disulfide is a likely derivative of cystine. The formation of dimethyl sulfide and dimethyl disulfide from methionine is readily deduced from expected recombinations of thiomethyl and methyl free radicals. Dimethyl disulfide is the major product. 

Sulfur metabolism of insects. VI. Metabolism of the sulfur amino acids and related compounds in the German cockroach, Blatella Germanica (L.). Ibid., 21, 129 (1961). With S. M. Henry. 

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. 

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. 

Degradation of the sulfur amino acids cysteine and methionine to H2S and other volatile sulfur compounds has been observed under laboratory conditions but their roles under wine fermentation conditions are less clear (Eschenbruch 1974 Jiranek et al. 1995a Moreira et al. 2002 Perpete et al. 2006 Rankine 1963 Vos and... 

XII, there is no strong competition by simple amino acids or by hydroxy-amino acids in acidic media. A more rapid reaction is observed with the basic amino acids, phenylalanine and aspartic acid, and a very rapid consumption of NBS by the sulfur amino acids, tryptophan and histidine. 

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-... 

In recent years, several reports have dealt with the decreased nutritional value of oxidized food proteins (53, 57, 58, 59). Although oxidation per se of the sulfur amino acids is a factor in reduced nutritive... 

In fertilized eggs, the sulfur amino acid called ovothiol (see here) plays a role comparable to that of glutathione. That is, ovothiol protects the egg against oxidative damage by peroxides produced at the egg surface early in fertilization. Oxidized ovothiol is in turn reduced by glutathione. 

Sulfur has an antagonistic effect on several essential trace elements. Excessive amounts of sulfur can induce a secondary deficiency of copper (mainly in animals), cobalt and selenium. Ho vever, not only the sulfur amino acid cysteine but also sulfate eliminates the adverse effects of copper-, cobalt-or selenium-based toxicities (Baker and Czarnecki-Maulden 1987). Sulfate increases the urinary loss of selenate, but not of selenite this explains the assumption that there is a direct antagonism between sulfate and selenate (Schrauzer 1998). 

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