Amino Acids

Investigations in Federal and State agencies, educational institutions, and industrial laboratories made many valuable contributions to that end. Our knowledge of the composition and properties of proteins was extended. New methods for determining the quantities of amino acids in foods were perfected; they are a way of evaluating the nutritive value and supplementary relationships of their proteins. We found out much more about the relative nutritive values of plant and animal proteins, and how they best can be used in combinations and help conserve the scarcer and more expensive protein foods. Scientists demonstrated the variations in the requirements of different species of animals for individual amino acids, and proved that too little protein in the diet means lowered resistance to infections. A lack of certain amino acids in the diet of experimental animals was found to lead to specific physiological injury. More information was obtained on the effect of heat, during commercial processing of foods, on the nutritional properties of their proteins.

Proteins can be discussed only in the light of the amino acids of which they are composed. The nutritive value of proteins, their chemical and physical properties, their relation to health and disease, their effective use in therapy, their commercial applications, all depend on the kind and proportions of the amino acids they contain.

We know that most food proteins are made up of 18 different amino acids combined in different order and manner. Eight of these are nutritionally essential for our well-being: Isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophane, and valine.

The other ten are : Alanine, arginine, aspartic acid, cystine, glutamic acid, glycine, histidine, proline, serine, and tyrosine.

Other amino acids present in some proteins are hydroxyproline, norleucine, and thyroxine. Others have been reported, but not confirmed.

During digestion in the alimentary tract, proteins are broken down into their constituent amino acids, which are then assimiliated and recombined to form new proteins needed for the growth of body tissues and for the formation of compounds essential for various body requirements.

The animal body cannot make all the amino acids it needs. It can produce some of them, but the others must be supplied by the proteins in food. Each of the large number of different proteins that enter into the composition of tissue cells requires a different, but a very definite, assortment of amino acids. A lack of any one that is essential for the formation of any particular protein constitutes a missing link in the chain. That protein cannot be synthesized, no matter how great a surplus there may be of the other amino acids.

The amino acids that cannot be manufactured within the body but must be supplied by the food are generally termed "nutritionally essential." This expression, however, does not imply that the other amino acids are of negligible importance. They are also "building blocks" of tissue proteins, and are necessary, whether or not they are supplied in the diet or whether they have to be manufactured within the body.

Experiments first conducted with young rats showed that ten of the amino acids are indispensable constituents of their diet : Arginine, histidine, lysine, leucine, isoleucine, methionine, phenylalanine, threonine, tryptophane, and valine. Nine are necessary for growth and maintenance. The tenth, arginine, is not necessary for satisfactory growth, but is required for maximum increases in weight. These ten were therefore classed as nutritionally essential.

But later we found that the needs for amino acids vary with different species. Adult humans need neither arginine nor histidine for nitrogen equilibrium. Scientists, therefore, now generally believe that only eight amino acids are nutritionally essential for man. Glycine, an amino acid that at is not essential for the rat, is indispensable for young chicks, otherwise they are subject to poor growth, weakness, and imperfect feather formation. Further investigations will probably reveal other such variations. Little is known about the requirements of the larger farm animals.

Poor growth is not the only noticeable result of a lack of amino acids in the diet of young animals. A lack of certain ones may lead to very specific consequences. Investigators report that a deficiency of tryptophane causes lesions in the eyes and the development of cataracts.

Recently, two new amino acids have been discovered. Lanthionine, which contains sulfur, was obtained from acid hydrolysates of proteins that had been first subjected to mild alkali treatment. Lanthionine is not believed to be a naturally occurring amino acid constituent of proteins, but is derived from cystine in the protein by action of dilute alkali. It was isolated in two forms, one of which is capable of replacing cystine and methionine in the diet of animals. Another amino acid, containing sulfur and selenium, was isolated as a naturally occurring constitutent of a plant grown on soil containing selenium. Plants and grains grown in certain areas where the soil contains selenium are known to have toxic properties, and have caused great losses to feeders of farm animals.

Great progress has been made in developing new methods for determining the amounts of amino acids in proteins. At first long and complicated processes were used. The protein was broken down into its amino-acid constituents by boiling with strong mineral acids. From the mixture thus obtained, most of the amino acids were separated, crystallized, and finally weighed in pure form. The entire procedure required many weeks; the final values in most cases were far too low; unavoidable losses occurred during their separation and purification. A number of highly accurate and rapid methods are now available.

One of the newest is called the microbiological method. It involves the use of certain bacterial organisms that require culture media containing specific amino acids for their maximum growth. Their rate of growth in media containing definitely known quantities of protein hydrolysates is used as a basis for estimating the amount of the amino acid sought in the protein analyzed.

Microbiological methods have been developed for estimating a number of the amino acids which seem to be as accurate as those obtained by chemical methods. The equipment required is found in most laboratories. Assays can be made on extremely small samples of material. The methods are applicable both to isolated proteins and staple foods, and can be carried out in much shorter time than by chemical methods.