Isoleucine dietary intake

The liver plays a central role in the synthesis of nearly all circulating proteins. Plasma contains 60-80 g/L of protein and this is turned over at a rate of approximately 250 g/day. A variety of proteins are constructed in the liver using amino acids (Aa) as their basic building blocks. Amino acids are categorised as essential and non-essential , the former being a requirement of dietary intake as they cannot be constructed in vivo, whereas the latter can be synthesised hepatically. The essential amino acids are further categorised as branched-chain amino acids (BCAA leucine, valine, isoleucine) or aromatic amino acids (AAA phenylalanine, tyrosine, methionine) according to their structure. Table... 

The sulfonylurea herbicides are a new family of chemical compounds, some of which are selectively toxic to weeds but not to crops. The selectivity of the sulfonylureas results from their metabolism to non-toxic compounds by particular crops, but not by weeds. In addition to efficient weed control, the sulfonylurea herbicides provide environmentally desirable properties such as field use rates as low as two grams/hectare and very low toxicity to mammals. The high specificity of the herbicides for their molecular target contributes to both of these properties. In addition, the low toxicity to mammals results from their lack of the target enzyme for the herbicides. Sulfonylureas inhibit the enzyme acetolactate synthase (ALS), also known as acetohydroxyacid synthase (AHAS), which catalyzes the first common step in the biosynthesis of the branched chain amino acids leucine, isoleucine and valine. In mammals these are three of the essential amino acids which must be obtained through dietary intake because the biosynthetic pathway for the branched chain amino acids is not present. The prototype structure of a sulfonylurea herbicide is shown in Figure 1. 

Protein is an essential nutrient for human growth, development, and homeostasis. The nutritive value of dietary proteins depends on its amino acid composition and digestibility. Dietary proteins supply essential amino acids, which are not synthesized in the body. Nonessential amino acids can be synthesized from appropriate precursor substances (Chapter 17). In human adults, essential amino acids are valine, leucine, isoleucine, lysine, methionine, phenylalanine, tryptophan, and threonine. Histidine (and possibly arginine) appears to also be required for support of normal growth in children. In the absence from the diet of an essential amino acid, cellular protein synthesis does not occur. The diet must contain these amino acids in the proper proportions. Thus, quality and quantity of dietary protein consumption and adequate intake of energy (carbohydrates and lipids) are essential. Protein constitutes about 10-15% of the average total energy intake. 

Methylmalonyl-CoA mutase (EC 5.4.99.2). Failure to convert (/ )-methylmalonyl-CoA into succinyl-CoA. Large quantities of methylmalonic acid appear in plasma and urine. Affected children fail to thrive and show pronounced ketoacidosis. Often fatal in early life. Hyperammonemia and intermittent hyperglycinemia are also typical. Restricted protein intake and synthetic diets are helpful, in particular low intakes of leucine, isoleucine, valine, threonine and methionine. A similar condition may arise from a congenital deficiency of methylmalonyl-CoA epimerase (EC 5.1.99.1). Both conditions unresponsive to vitamin Bj2. Another type of methylmalonyl aciduria is thought to result from an hereditary deficiency of deoxyadenosyl transferase (transfers the 5 -deoxyade-nosyl group in cobalamin synthesis), which provides the coenzyme of methylmalonyl-CoA mutase. This condition responds to injection of B,2. Dietary B12 deficiency also results in methylmalonic aciduria. 

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