Methionine association with cystine

Amino Acid Composition. Values vary substantially with differences of species, body location, and technique of isolation, purification, or analysis. However, distinctive patterns are associated with some membrane and fibrous protein isolates (Table 1). High proportions of proline and cystine are consistent with observed chemical and mechanical resistance of membranes (32, 68, 71). Similarly, increased methionine and... 

Animal experiments have shown that faulty nutrition, i.e. > 90% fat, < 10% protein and < 2 mg choline per day, leads to pronounced fatty fiver and even fatty cirrhosis within a few weeks. The same changes could be observed when the protein intake remained more or less normal, while extremely little methionine and choline was offered. With a partial surplus of certain foodstuffs, the special nature of the excessive nutritional components is also of considerable importance. The term partial malnutrition may, for example, be associated with a pronounced protein deficiency (and thus possibly inadequate production of lipoproteins) or a lack of lipotropic substances (such as methionine, choline, cystine, glycocoUbetaine, pyridoxine, casein and various N- or S-methylated substances). Protein deficiency has particularly severe consequences when toxic substances are absorbed at the same time or when the organism has to fight bacterial or parasitic infections. A diseased liver reacts to both a serious deficiency in and an excessive supply of different nutrients (e.g. proteins, certain kinds of amino acids, various lipids, trace elements) with unfavourable or even complicative developments during the course of disease. 

RNase A Representative Enzyme. Both H and OH radicals in fluid, aqueous solutions react with RNase in ways that lead to inactivation. Mee and coworkers (30) have found that H leads to aggregation, and that cystine, methionine, and tyrosine are mainly affected. Adams and coworkers (31), investigating the specificity of free radical reaction with RNase, found that OH and Br2 are effective inactivators because they rapidly react with histidine, which is associated with the active site. 

Requirements for essential amino acids in poultry and pigs have been devised, and some of these are presented in Appendix 2, Tables A2.9 and A2.10. However, there are considerable complications associated with defining amino add requirements because of interactions between the essential amino acids, between essential and non-essential amino acids, and between amino acids and other nutrients. In chicks, the requirement for glycine is increased by low concentrations of methionine, arginine or B vitamins. Similarly, one amino acid may be converted to another. For example, if cystine or its metabolically active form cysteine is deficient in the diet, it can be synthesised by the animal from methionine. The requirement for methionine is therefore partly dependent on the cystine (or cysteine) content of the diet, and the two amino acids are often considered together (i.e. the requirement is stated for cystine -I- methionine). It should be noted, however, that the two are not mutually interconvertible methionine is not synthesised from cystine and therefore part of the total requirement must always be met by methionine. Phenylalanine and tyrosine have a similar relationship, and in the chick glycine and serine are interconvertible. 

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