Synthesis and Dietary Sources of Amino Acids

Animals are dependent for growth on a source of fixed (i.e., reduced) nitrogen from other animals or plants plants in turn are dependent on bacteria for fixing nitrogen. Humans need fixed nitrogen, which must come from the diet (normally as protein), particularly for protein and nucleic acid synthesis but also for synthesizing many specialized metabolites such as porphyrins and phospholipids. 

Why are plant proteins not as useful as animal protein for dietary purposes  

Cereal proteins are only about 70 percent efficient for dietary replacement purposes. The reason is that cereal proteins are deficient in lysine, an essential amino acid for humans (see Amino Acid Synthesis in this section). Thus a diet based on one source of protein (e.g., corn) can lead to malnutrition. A partial solution to the problem has been the breeding of high-lysine corn. Other plant proteins, particularly those from pod seeds (e.g., peas and beans) are deficient in the sulfur-containing amino acids. A successful vegetarian diet will therefore be balanced in cereals and pod seeds. 

The accepted amount of protein required to maintain nitrogen balance is 28 g per day for a 70-kg person, i.e, about 3,8 g of nitrogen. This is estimated by measuring the N excretion over 6 to 7 days on a protein-free diet. If the protein source is from cereal, then the daily intake would have to be increased to about 40 g per day for a 70-kg person. The difference is due to the variable amounts of essential amino acids found in proteins. The amount required by growing children is larger the accepted figure is about 0.6 g per kilogram per day. 

Eskimos, who have a high protein intake, do have a shorter life span than Europeans, but there is no clear correlation of this with dietary protein. They also have a high intake of saturated fat. 

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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. 

Protein turnover is not completely efficient in the reutilization of amino acids. Some are lost by oxidative catabolism, while others are used in synthesis of non-protein metabolites. For this reason, a dietary source of protein is needed to maintain adequate synthesis of protein. During periods of growth, pregnancy, lactation, or recovery from illness, supplemental dietary protein is required. These processes are affected by energy supply and hormonal factors. An overview of amino acid metabolism is presented in Figure 17-1. 

Amino acids serve a number of functions. Although the most important role of amino acids is the synthesis of proteins, they are also the principal source of the nitrogen atoms required in various synthetic reaction pathways. In addition, the nonnitrogen parts of amino acids (referred to as carbon skeletons) are a source of energy, as well as precursors, in several reaction pathways. Therefore an adequate intake of amino acids, in the form of dietary protein, is essential for an animal s proper growth and development. 

Cysteine inhibits cystathionine 3-synthase and, therefore, regulates its own production to adjust for the dietary supply of cysteine. Because cysteine derives its sulfur from the essential amino acid methionine, cysteine becomes essential if the supply of methionine is inadequate for cysteine synthesis. Conversely, an adequate dietary source of cysteine spares methionine that is, it decreases the amount that must be degraded to produce cysteine. 

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). 

It is well established that carbohydrates are utihzed in the synthesis of certain amino acids in the body and that the oxidation of amino acids is increased when the dietary source of carbohydrate or fat is inadequate to meet the caloric needs. Therefore, the differences between carbohydrates in promoting the efficient utilization of protein are of interest. 

An adult has a requirement for a dietary intake of protein because there is continual oxidation of amino acids as a source of metabolic fuel and for gluconeogenesis in the fasting state. In the fed state, amino acids in excess of immediate requirements for protein synthesis are oxidized. Overall, for an adult in nitrogen balance, the total amount of amino acids being metabolized will be equal to the total intake of amino acids in dietary proteins. 

The dietary proteins are broken down into amino acids during digestion. They are then absorbed and distributed by the bloodstream to the body cells, which rebuild these amino acids into body proteins. Although the primary use of amino acids in the body is synthesis, amino acids can be catabo-lized as a source of energy. 

The commercial availability of stable isotope ( C, N, H)-labelled compounds and highly accurate mass spectrometers has made it possible to probe the details of metabolic pathways involved in macronutrient catabolism and neogenesis. This paper highlights aspects of animal nutrition and metabolism in which uniformly C-labelled [U- C] substrates and C-massisotopomer distribution approaches have been applied to investigations of amino acid and carbohydrate synthesis and catabolism. We will focus on the application of [U- C] substrates as tracers in chickens, fish, sheep, and cells in culture to quantify rates of macronutrient synthesis, identification of the sources of dietary nutrients that serve as substrates for neogenesis of macronutrients, and investigations of the intercormectivity of the pathways of macronutrient metabolism with those of the Krebs cycle to preserve metabolic flexibility via anaplerotic and cataplerotic sequences. 

The amino acid L-tryptophan is the precursor for the synthesis of 5-HT. The synthesis and primary metabolic pathways of 5-HT are shown in Figure 13-5. The initial step in the synthesis of serotonin is the facilitated transport of the amino acid L-tryptophan from blood into brain. The primary source of tryptophan is dietary protein. Other neutral amino acids, such as phenylalanine, leucine and methionine, are transported by the same carrier into the brain. Therefore, the entry of tryptophan into brain is not only related to its concentration in blood but is also a function of its concentration in relation to the concentrations of other neutral amino acids. Consequently, lowering the dietary intake of tryptophan while raising the intake of the amino acids with which it competes for transport into brain lowers the content of 5-HT in brain and changes certain behaviors associated with 5-HT function. This strategy for lowering the brain content of 5-HT has been used clinically to evaluate the importance of brain 5-HT in the mechanism of action of psychotherapeutic drugs. 

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. 

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