Biochemistry of Amino Acids - A. Meister 1961

The Role of Amino Acids in Nutrition
Study of animal growth when fed amino acid mixtures

Young rats can grow on a diet containing a mixture of 10 Essential Amino Acids; however, it has been established that their growth rate increases by approximately 25% on a diet including 19 amino acids [32]. The entire group of non-essential Amino acids can be replaced by an equivalent amount (calculated on a nitrogen basis) of ammonium citrate or ammonium acetate [64, 65], as well as partially by urea. Schoenheimer [66] and Bloch [67] found that only a very small amount of dietary urea nitrogen is incorporated into urinary ammonia and Proteins. However, experiments with C¹⁴-urea demonstrated that urea is rapidly converted into carbon dioxide [68, 69]. The breakdown of urea into carbon dioxide and ammonia is catalyzed by Bacteria present in The Stomach, intestines, and other PARTS OF THE body (such as the Upper Respiratory Tract) [69]. Supplementing a diet consisting of 10 essential amino acids with non-essential amino acids, ammonium ions, or urea yields a better effect than merely increasing The amount of the essential amino acids themselves. From this, it can be concluded that essential Amino acids are generally converted more slowly into the metabolic products required for growth [70]; consequently, experimental conditions are possible under which ammonium ions exert a more favorable effect on growth than a mixture of essential amino acids. As mentioned above, Certain amino acids required to support growth and nitrogen balance can be partially replaced by non-essential amino acids. For instance, in young rats, cystine can cover from ¹/₆ to ¹/₃ of the Methionine requirement [30, 31], and Tyrosine can supply about half of the phenylalanine requirement [32]. The possibility of replacing methionine with homocysteine depends on the presence of vitamin B₁₂ and Folic acid or methyl group Donors in the diet. It is possible that conditions will be found under which growth is maintained even in the absence of certain other "essential" amino acids. The results of studies determining growth and nitrogen balance merely indicate that these Functions are not sustained by in vivo synthesis processes.

It is possible that animals possess some capacity to synthesize essential amino acids. When Brain preparations from one-day-old mice were incubated in the presence of uniformly C¹⁴-labeled glucose, the isotopic label was found in Almost all amino acids (including essential ones), with the exception of Proline and Threonine. Turnover rate studies showed that 5 to 12% of the amino acids in brain proteins were replaced using glucose carbon. In similar experiments with Liver and intestinal Tissues, no isotope incorporation was observed; nor was it detected in experiments using the brains of older animals. It is unlikely that essential amino acid molecules can be synthesized de novo in the brain of a newborn animal; in the experiment, about 50% of the radioactivity was found in the a-carboxyl groups of the amino acids. Nevertheless, these data are of considerable interest, pointing to metabolic phenomena that deserve further investigation [71–74]. Studies with amino acid mixtures have also demonstrated that for the optimal utilization of essential amino acids, all of them must be present simultaneously [22, 75, 76]. Cannon and co-workers [77] found that when animals were fed alternately on two diets each containing five different essential amino acids, amino acid utilization was impaired. A single amino acid administered parenterally is utilized only if given no later than a few hours after the animal has been fed [20]. The necessity for the simultaneous delivery of all essential amino acids indicates that amino acids cannot be stored in the body in any significant quantity; they are removed from the Organism through catabolic processes and excretion. The administration of incomplete amino acid mixtures or proteins is accompanied by an increased urinary excretion of amino acids. It follows that the Biological value of a protein depends to a certain extent on the rate at which individual amino acids are liberated during its Digestion. The presence of non-essential amino acids is also of known importance; therefore, their presence in the diet or the rate at which they are synthesized from fats, CARBOHYDRATES, or essential amino acids is highly significant.

It has long been known that animals can maintain a state of nitrogen balance under conditions where the organism experiences a loss of carbon. It has also been established that carbohydrates and fats exert a protein-sparing effect, apparently acting as sources of carbon skeletons for the synthesis of certain non-essential amino acids. Studies by Rose and his associates on human amino acid requirements showed that maintaining nitrogen balance in human subjects receiving an amino acid mixture requires a relatively high caloric intake. In three subjects, it was established that when casein served as the dietary nitrogen source, nitrogen balance was maintained on a diet providing 35 kcal per kg of body weight. However, when using an amino acid mixture equivalent to casein, 45.5 kcal per kg was required to maintain nitrogen balance. These findings are rather difficult to explain at present. The superiority of casein over an equivalent amino acid mixture may depend on The rate of amino acid absorption. Apparently, free amino acids in mixtures are absorbed faster than protein-bound amino acids, and such rapid delivery may be less favorable for the organism. Nevertheless, it is hard to understand why supplying an increased number of calories helps overcome this difficulty. The possibility of direct absorption and utilization of intact protein or Peptides must also be considered [78–83]. Certain non-nitrogenous dietary components may in some way promote the utilization of amino acids in the body.