Comparison of Responses of Plasma Amino Acid Concentrations to Changes of Dietary Amino Acids, and Threonine Metabolism in Chickens and Rats

Although energy requirements of all animals are formulated as 3/4 power of body weight, there is no formula to express the amino acid requirements of all animals. The amino acid requirements are affected by many factors. It is very laborious and time consuming to determine amino acid requirements by traditional methods. Therefore. it is impossible to determine all amino acid requirements. The plasma amino acid concentration is one of the methods to determine amino acid requirements within a short period using the small number of animals. In this study, in order to ascertain the availability of plasma amino acid concentration as a parameter to determine amino acid requirements of chickens and rats within a short period, following five experiments were conducted. In Chapter II, the speed of response of plasma amino acid concentration to changes of dietary levels of them and the stability of the response of plasma amino acid concentration were studied using the same rats, repeatedly. Plasma concentration of most amino acids responded to changes of dietary levels of them within 1 day and the responses were kept constant for 4 days. If the metabolically unrelated amino acids were combined, it was possible to distinguish the response of multiple amino acids in one trial. In Chapter III, the effects of feeding condition and species of animals on the lowest plasma concentration of amino acids were studied. The lowest plasma amino acid concentrations were not constant, and fluctuated by dietary condition and animal species. Because it was found remarkable difference in the response of plasma concentration of serine to the dietary excess threonine in chickens and rats, the tissue amino acid concentrations and threonine metabolic enzyme activities were investigated in Chapters II and IV. From the determination of enzyme activities related to threonine metabolism, it is suggested that the difference of the conversion from serine to glycine, which mainly caused by the action of serine hydroxymethyltransferase, was one of the reasons why plasma concentration of serine of chickens decreases and that of rats increases, when the dietary threonine was higher than their requirements.