Amino acid synthesis - Wikipedia
L Lysine - a 100% pure amino acid from Bulk Nutrients
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Substituting one feed having high digestibility with another of similar digestibility will also give similar performance by either the ileal digestible or amino acid system. However, synthetic amino acids are assumed to be 100 percent digested and a proper evaluation of their use requires the description of feed proteins in terms of ileal digestible amino acids. A major role of animal production is to use by-product feeds arising from the processing of foods for human consumption. Where these by-products have distinctly different availabilities, then advantages for the use of ileal digestible instead of total amino acids can be demonstrated. Tanksley and Knabe (1984) demonstrated that 50 percent of soyabean meal could be replaced in a pig diet with meat and bone meal, so long as the latter were supplemented with lysine and tryptophan to supply the same amount of digestible lysine and tryptophan.
Monogastric animals do not have a requirement for protein as such, but they require nine to ten amino acids which the body cannot synthesize, together with a source of amino nitrogen which can be used for the synthesis of the remaining amino acids. The amino acids that cannot be synthesized must be provided by the diet. They are termed indispensable or essential amino acids. In addition, two amino acids, cysteine and tyrosine, can be synthesized in the body but only from indispensable amino acids methionine and phenylalanine respectively. Consequently, they are not indispensable but a dietary supply spares the need for the indispensable parent amino acid. These are termed semi-indispensable or semi-essential. Arginine is an indispensable amino acid for birds and fish but in mammals it is synthesized as part of the urea cycle. However, as most of the synthesized arginine is broken down to release urea, the amount available for protein synthesis may be inadequate and a dietary supply may promote growth in young animals. Similarly glycine and serine may not be synthesised in sufficient quantities in certain situations, such as in young animals and rapidly growing chicks and so are termed conditionally indispensable. Initially, practical trials were carried out to determine the requirement for each of the indispensable amino acids in turn. This was determined from response curves to supplementation of deficient diets with the amino acid under consideration. A typical response curve to lysine supplementation of a deficient diet for chicks is shown in Figure 6.
PROTEIN SOURCES FOR THE ANIMAL FEED INDUSTRY
Consequently, if the requirement for one amino acid is determined by empirical trial in one situation, the requirements for all the others can be estimated by applying the ratio as determined for the ideal protein. Because lysine is normally the first limiting amino acid in most practical diets and therefore the requirements for lysine were the most studied in empirical trials, lysine is used as the reference amino acid and all others are expressed as a ratio to lysine (Table 2). A first approximation to the ideal ratio is the amino acid composition of the whole body, or of the tissue protein gained during growth. This makes the assumption that each absorbed indispensable amino acid is used with the same efficiency for protein synthesis. This is not true since some amino acids e.g. tryptophan and methionine are used for purposes other than protein synthesis, and others such as cystine and threonine have large losses in intestinal mucoproteins. Also as different proteins turn over at different rates, the ideal pattern changes with change in proportions of the different proteins being synthesised at any one time. For example, as the proportion of protein involved in maintenance of the body compared with accretion of new tissue changes with age, so the ideal pattern will change to reflect the different proteins involved. Consequently, the ideal pattern has evolved in recent years as some of these factors have been studied. Given an accurate determination of the lysine requirement in terms of percent of diet or g/MJ ME in any given situation, the requirement for the remaining indispensable amino acids can be calculated.
Mild heating in the presence of reducing sugars or aldehydes results in loss of available lysine with little change in digestibility. Mild to moderate heating causes loss of sulphydryl groups, formation of disulphide cross links, racemisation of L to D-aspartic acid and reduced digestibility of all amino acids. Moisture content during heating is critical in both losses of available lysine and of sulphydryl groups. Mild processing gives best digestibility for monogastric animals and is especially important for young mammals and fish. Ruminant feeds benefit from more severe heat treatment and special processing to reduce protein degradability when amino acid composition is well balanced. Growth under commercial conditions is often less than under good experimental conditions, reflecting challenges to the immune system. Dietary proteins can both cause and affect an immune response. Dietary proteins may need special processing to reduce antigenic factors. The presence of dietary fibre, phytic acid or tannins in protein feeds reduce amino acid digestibility, increase endogenous N loss and the energetic cost of intestinal protein synthesis with consequent reduction in growth rate.
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The amino acid with the lowest score below 100 is the limiting amino acid. Amino acids present in a greater amount relative to the ideal protein than the limiting amino acid, i.e. having a higher score, can only be used in protein synthesis up to the level sustained by the limiting amino acid. The amount in excess will be deaminated and the carbon skeleton used as a source of energy. Consequently, the score for the limiting amino acid becomes the chemical score for the protein. An example of the use of the ideal protein pattern to calculate chemical score of feeds is given in Table 3. For maize, lysine is the first limiting amino acid. For soybean meal, methionine +cystine (M+C) is the first limiting.
This is a semi-essential amino acid, because although it is normally synthesized in sufficient amounts by the body, supplementation is sometimes required (for example, due to inborn errors of urea synthesis, protein malnutrition, excessive lysine intake, burns, peritoneal dialysis, and rapid growth).
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Moughan . (1991) compared the determined growth of pigs fed a barley based grower diet with the response to lysine supplementation of a lysine deficient synthetic diet based on casein. The observed growth was 0.925 of the expected growth based on intake of apparently absorbed lysine. To achieve an equal ME intake, 15.3 percent more dry matter was fed of the test diet than the synthetic diets. Thus the endogenous losses would be less on the synthetic diet leaving more of the supplementary lysine available to support growth. The series of experiments by Batterham . (1990 - 1994) and Beech . (1991) all had the same form; a comparison of cottonseed meal, meat and bone meal and soybean meal as examples of feeds with low, medium and high ileal digestibility. The growth and N retention of pigs fed three diets formulated to supply the same limiting level of ileal lysine, methionine, threonine, tryptophan or isoleucine were measured. The main difference was observed between cottonseed meal and the other two meals, with smaller (lysine, threonine) and non significant differences (methionine, tryptophan), in N retention between meat and bone meal and soybean meal. Diets were fed on a scale to provide the same DE/W 0.75 but the cottonseed diets had 8.3 percent less DE/kg than the soybean diets, with the meat and bone meal diets intermediate. Consequently, the amount of dry matter fed differed and basal endogenous loss would be less for soybean than meat and bone meal or for cottonseed meal allowing more of the absorbed limiting amino acid to be used for growth. The presence of gossypol and raffinose in cottonseed makes this protein particularly susceptible to heat damage by binding, specifically with the epsilon-amino group of lysine (Martinez ., 1961). This may make it unavailable without any major change in digestibility of the protein (see below). Cottonseed meal and products such as dried milk powders where reducing sugars are potentially present, may be special cases where ileal digestibility fails to reflect the full loss of available lysine through early Maillard reactions. For the majority of protein concentrates this is unlikely to be a major factor. Indeed, the Batterham group in a study of isoleucine, where the meals used were cottonseed, lupin seed meal and soya bean meal, ileal digestibility correctly predicted growth performance (Batterham and Andersen, 1994). Correction for the known differences in ileal true digestibility must be an improvement over the use of chemically determined total amino acid content.
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