Cysteine dietary requirement

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. 

Of the 20 amino acids in proteins, the body can readily synthesize eight if an appropriate nitrogen source is available. Two others can be synthesized from other amino acids of the diet tyrosine from phenylalanine and cysteine from methionine. The rest must be provided in the diet (Chapter 17), since the body can synthesize none or an insufficient amount. The dietary requirement depends on several factors. Beside essential amino acids, the diet should provide the nitrogen required for synthesis of the nonessential amino acids. 

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 requirements for the essential amino acids are further complicated by the finding that two non-essential amino acids can only be synthesized in the body if two of the essential amino acids are present in sufficient amounts. Tyrosine is formed directly from phenylalanine so that the requirement for phenylalanine is less when tyrosine is present than when it is absent firom the diet. Similarly the sulphur that is required for the synthesis of cysteine can only be obtained fi-om methionine so that the dietary requirements for the sulphur-containing amino acids should be considered together. If cysteine is present in ample amounts the requirement for methionine will be minimal, but if cysteine is in short supply more methionine is needed. 

Pyrrolysine trait is restricted to several microbes, and only one organism has both Pyl and Sec. Of the 22 standard amino acids, 8 are called essential amino acids because the human body cannot synthesize them from other compounds at the level needed for normal growth, so they must be obtained from food. In addition, cysteine, taurine, tyrosine, histidine and arginine are semiessential amino-acids in children, because the metabolic pathways that synthesize these amino acids are not fully developed. The amounts required also depend on the age and health of the individual, so it is hard to make general statements about the dietary requirement for some amino acids. 

Two amino acids—cysteine and tyrosine—can be synthesized in the body, but only from essential amino acid ptecutsots (cysteine from methionine and tyrosine from phenylalanine). The dietary intakes of cysteine and tytosine thus affect the requirements for methionine and phenylalanine. The remaining 11 amino acids in proteins are considered to be nonessential or dispensable, since they can be synthesized as long as there is enough total protein in the diet—ie, if one of these amino acids is omitted from the diet, nitrogen balance can stiU be maintained. Howevet, only three amino acids—alanine, aspartate, and glutamate—can be considered to be truly dispensable they ate synthesized from common metabolic intetmediates (pyruvate, ox-... 

Tyrosine is formed from phenylalanine by phenylalanine hydroxylase. The reaction requires molecular oxygen and the coen zyme tetrahydrobiopterin, which can be synthesized by the body. One atom of molecular oxygen becomes the hydroxyl group of tyro sine, and the other atom is reduced to water. During the reaction, tetrahydrobiopterin is oxidized to dihydrobiopterin. Tetrahydro biopterin is regenerated from dihydrobiopterin in a separate reaction requiring NADPH. Tyrosine, like cysteine, is formed from an essen tial amino acid and, is therefore, nonessential only in the presence of adequate dietary phenylalanine. 

A quantitatively important pathway of cysteine catabolism in animals is oxidation to cysteine sulfinate (Fig. 24-25, reaction z),450 a two-step hydroxyl-ation requiring 02, NADPH or NADH, and Fe2+. Cysteine sulfinic acid can be further oxidized to cyste-ic acid (cysteine sulfonate),454 which can be decarbox-ylated to taurine. The latter is a component of bile salts (Fig. 22-16) and is one of the most abundant free amino acids in human tissues 455-457 Its concentration is high in excitable tissues, and it may be a neurotransmitter (Chapter 30). Taurine may have a special function in retinal photoreceptor cells. It is an essential dietary amino acid for cats, who may die of heart failure in its absence,458 and under some conditions for humans.459 In many marine invertebrates, teleosts, and amphibians taurine serves as a regulator of osmotic pressure, its concentration decreasing in fresh water and increasing in salt water. A similar role has been suggested for taurine in mammalian hearts. A chronically low concentration of Na+ leads to increased taurine.460 Taurine can be reduced to isethionic acid... 

Our results have been recapitulated with other proteins of varying nutritional value to S. exigua and H. zea they include soy protein, tomato foliar protein, corn gluten and zein. In all cases, more than 2.5% dietary protein was required to alleviate antinutritional effects, because these proteins are less nutritious than casein (Table III). The ability of a protein to alleviate the toxicity of o-quinones is proportional to its nutritional value to the insect (Table III). The proteins ability to function as an alkylatable sink (alleviation of antinutritive effects) is correlated with the relative amounts of alkylatable amino acids (e.g., lysine, cysteine, histidine, methionine Felton and Duffey, unpublished data). 

The influence of cysteine on the methionine requirement is demonstrated by the following study. The subject was a student who was awarded a Ph.U. after conducting the study on himself. The subject consumed diets that were complete except that the concentration of methionine was varied (Table 8.12), The dietary amino acids were supplied in the form of pure amino acids, rather than as protein, to allow full control over the levels of amino adds supplied by the diet. Cysteine was supplied as cy.stine. Cystine is a dimer of cysteine, in which the two cysteine residues are connected via a disulfide bond (R——R). Cystine is readily converted to cysteine in the body. The methionine requirement was assessed by determining the conditions that supported a zero or slightly positive nitrogen balance. 

Cystathionine synthetase, a pyridoxal phosphate (vitamin Be) enzyme, catalyzes the condensation of serine and homocysteine to form cystathionine. A deficiency of this enzyme leads to a buildup of homocysteine, which oxidizes to form homocystine. This may result in mental retardation, but sometimes causes dislocated lenses and a tall, asthenic build reminiscent of Marfan s syndrome. Patients with homocystinuria also have a clotting diathesis, requiring care to avoid dehydration during anesthesia. Their cysteine deficiency must be made up from dietary sources. In some cases, dietary intake... 

Folic acid/cobalamin/pyridoxine hydrochloride are nutritional combinations. Folic acid and cobalamin reduce homocysteine by metabolizing it to methionine. Pyridox-ine facilitates breakdown of homocysteine to cysteine and other by-products. They are indicated for nutritional requirement of patients with end-stage renal failure, dialysis, hyperhomocysteinemia, homocystinuria, nutrient malabsorption or inadequate dietary intake, particularly for patients with or at risk for cardiovascular disease, cerebrovascular disease, peripheral vascular disease, arteriosclerotic... 

Among sheep with dietary insufficiencies, the minerals copper and zinc, when supplemented to the diet, have been shown to be important to wool fiber growth. Their effectiveness is attributed to the important roles these minerals play in sulfur amino acid metabolism copper serves to catalyze the oxidation of cysteine to cystine during fiber synthesis [109]. Zinc is required for cell division to occur, and it also appears to play a role in protein metabolism [110]. 

Most of the inorganic sulfate assimilated and reduced by plants appears ultimately in cysteine and methionine. These amino acids contain about 90% of the total sulfur in most plants (Allaway and Thompson, 1966). Nearly all of the cysteine and methionine is in protein. The typical dominance of protein cysteine and protein methionine in the total organic sulfur is illustrated in Table I by analyses of the sulfur components of a lower plant (Chlorella) and a higher plant (Lemna). Thede novo synthesis of cysteine and methionine is one of the key reactions in biology, comparable in importance to the reduction of carbon in photosynthesis (Allaway, 1970). This is so because all nonruminant animals studied require a dietary source of methionine or its precursor, homocysteine. Animals metabolize methionine via cysteine to inorganic sulfate. Plants complete the cycle of sulfur by reduction of inorganic sulfate back to cysteine and methionine, and are thus the ultimate source of the methionine in most animal diets (Siegel, 1975). 

For individuals who do not respond to Bg, a methionine-restricted diet is necessary (Box 14.4). Synthetic methionine-free amino acid medical foods are commercially available. The natural requirement for methionine is met by dietary intake of regular foods or, in infancy, standard infant formulas. The diet is restrictive and requires the use of low-protein products to be truly successful. Cystine is prescribed as necessary to obtain normal cysteine concentrations. Small amounts of vitamin B12 may aid in the remethyl-ation of homocysteine to methionine due to its use as a cofactor by methionine synthase [10,40]. 

This is so because non-ruminant animals require a dietary source of homocysteine, which is normally provided in the form of methionine. ibiimals metabolize these sulfur amino acids eventually to inorganic sulfate. Plants complete the cycle of sulfur by reductive assimilation of inorganic sulfate to methionine (and cysteine) (Siegel, 1975), and are thus the ultimate source of methionine in most animal diets. 

The essentiality of cysteine for the fetus and newborn may underlie their low or nonexistent ability to convert cysteine to taurine, another low molecular weight sulfur containing confound apparently required in large amounts by developing brain (Sturman et al., 1978). The supply of cysteine may all be required for protein synthesis and none spared for taurine formation. Cystelnesulflnic acid decarboxylase (EC 4.1.1.12), the enzyme chiefly responsible for taurine biosynthesis in mammals, develops slowly after birth and reaches maximum activity in mature brain (Agrawal et al.,1971 Pasantes-Morales et al.,1976 Rassin et al., 1979), although the concentration of taurine decreases over this same period (Fig. 4). Cystelnesulflnic acid decarboxylase also uses pyridoxal 5 -phosphate as coenzyme, and is extremely sensitive to a dietary deficiency of vitamin Bg (Hope, 1955 Rassin and Sturman, 1975). 

The determination of arsenic in urine samples to determine exposure requires the hyphenation of hydride generation with ICP-MS. This is because total arsenic analysis, which has been vastly improved with the introduction of CCT and the removal of the ArCl interference, still includes some arsenic species that are present in urine as a result of seafood contributions. The reduction of As +, DMA and MMA (dimethylarsinic acid and monomethylarsonic acid) to As " " with L-cysteine and hydrochloric acid and subsequent hydride generation by mixing with sodium borohydride will measure all the arsenic species except arsenobetaine and arsenocholine. The hydride gas line can be simply connected to the spray chamber (replacing the nebuliser gas) and arsenic is measured using a dry plasma. The hydride generator system removes both the chloride interference (because only the AsHs gas enters the plasma) and the dietary component of exposure (because AB and AC are not reduced to As +). 

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