Sources of amino acids

There are four sources of amino acids that enter the free amino acid pool in the body proteins in food proteins secreted into the stomach and intestine by the digestive glands endogenous proteins and microorganisms that die and release their protein in the colon. 

A summary of the contributions of various processes to protein intake, protein loss and whole-body protein turnover is presented in Table 8.3. 

Protein turnover in an adult is about 4 to 5 g per kg body wt, equivalent to about 250 to 350 g of protein hydrolysed and resynthesised every day in the tissues of an adult human. This represents considerably more protein than is ingested in food. The rates of protein turnover vary enormously, depending on the nature of the protein, the condition of the subject and the tissue (Table 8.3). Proteins (mainly enzymes) in the liver are replaced every few hours or days whereas structural proteins (e.g. collagen, contractile proteins) are stable for several months. Contractile proteins can be degraded relatively rapidly in some conditions (see below). 

There are at least two answers to question (i). First, abnormal proteins can arise in cells due to spontaneous denaturation, errors in protein synthesis, errors in post-translational processing, failure of the correct folding of the protein or damage by free radicals. They are then degraded and replaced by newly synthesised proteins. Secondly, turnover helps to maintain concentrations of free amino acids both within cells and in the blood. This is important to satisfy the requirements for synthesis of essential proteins and peptides (e.g. hormones) and some small nitrogen-containing compounds that play key roles in metabolism (see Table 8.4). 

In answer to question (iii), there are three pathways for protein degradation  

Arteriovenous (AV) difference studies and mammary blood flow measurements (Chapter 1) have shown that in both ruminants and non-ruminants, amino acids for milk protein synthesis are obtained ultimately from blood plasma but that some interconversions occur. The amino acids can be divided into two major groups  

those for which uptake from blood is adequate to supply the requirements for milk protein synthesis and which correspond roughly to the essential amino acids (EAA) and 

those for which uptake is inadequate, i.e. the non-essential amino acids (NEAA). 

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All 20 of the amino acids present in proteins are essential for health. While comparatively rare in the Western world, amino acid deficiency states are endemic in certain regions of West Africa where the diet relies heavily on grains that are poor sources of amino acids such as tryptophan and lysine. These disorders include kwashiorkor, which results when a child is weaned onto a starchy diet poor in protein and marasmus, in which both caloric intake and specific amino acids are deficient. 

The major role of skeletal muscle is movement, which is described and discussed in Chapter 13). Nevertheless, since muscle comprises 40% of the body it is large enough to play a part in control of the blood concentrations of the major fuels glucose, fatty acids, triacylglycerol and some amino acids. Skeletal muscle contains the largest quantity of protein in the body, which is used as a source of amino acids under various conditions (e.g. starvation, trauma, cancer see above). It plays an important part in the metabolism, in particular, of branched-chain amino acids, glutamine and alanine, which are important in the overall metabolism of amino acids in the body (discussed below). 

Storage proteins serve as a which are proteins that fight infections, source of amino acids in milk. 

Every muscle in your body is a source of amino acids for yourself (as well as for any organisms that might end up eating you). In times of starvation, your body will access this source of amino acids, resulting in a decrease in your muscle mass. 

The term protein usually refers to crude protein (CP measured as N content x 6.25) in requirement tables. Protein is required in the diet as a source of amino acids (AAs), which can be regarded as the building blocks for the formation of skin, muscle tissue, feathers, eggs, etc. Body proteins are in a dynamic state with synthesis and degradation occurring continuously therefore, a constant, adequate intake of dietary AAs is required. An inadequate intake of dietary protein (AAs) results in a reduction or cessation of growth or productivity and an interference with essential body functions. 

Cyanobacteria are a rich source of //-amino acids (Table 1.5.2). A multitude of peptides and depsipeptides, with substructures ranging from simple aliphatic... 

There are a number of media available which are not based on a detailed investigation of growth requirements, but rather include crude mixtures of nutrients added to promote cell growth. These include lactalbumin hydrolysate (Appendix 1 Table 9) or yeast extract (Appendix 4) to provide an inexpensive source of amino acids or vitamins. Thus Melnick s monkey kidney media A and B (Melnick, 1955) contain lactalbumin hydrolysate and calf serum in Hanks and Earle s BSS, respectively. Chick embryo extract and tryptose phosphate broth (Appendix 1, Tables 11 and 12) are also used occasionally and their use is referred to where appropriate throughout the book. Mitsuhashi and Maramorosch mosquito cell medium contains lactalbumin hydrolysate, yeast extract and foetal calf serum in a specially developed saline (Mitsuhashi and Maramorosch, 1964 Singh, 1967). 

Nutritional effects (bioavailability of iron, source of amino acids). 

Nutritional significance. As one of the major whey proteins in human milk and also relatively abundant in bovine colostrum, LF is of interest as a dietary source of amino acids as well as for the bioavailability of iron. LF has an... 

One example of the adaptation of bacteria to an unfavourable environment is their response to amino acid starvation. In an environment rich in amino acids, cells do not produce enzymes of amino acid synthesis. However, in the case of a nutritional downshift in the environment, cells must use their own proteins as sources of amino acids for building enzymes required for amino acid biosynthesis pathways (Gottesman and Maurizi, 2001). 

Source of Amino Acid at Position Myoglobin Required... 

It appears that FTT reactions can account reasonably well for most features of organic matter in meteorites. The only alternative process, the Miller-Urey synthesis, fails to account for the aliphatic and aromatic hydrocarbons, nitrogen heterocyclics, many oxygen compounds, the polymer, and carbon isotope fractionations, though it remains an alternative and perhaps superior source of amino acids and may, in an extended sense, be responsible for the deuterium enrichments. 

Dietary protein is a vital source of amino acids. Proteins ingested in the diet are digested into amino acids or small peptides that can be absorbed by the intestine and transported in the blood. Another source of amino acids is the degradation of defective or unneeded cellular proteins. 

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