Amino acids as energy sources

To discuss amino acids as energy sources seems at first sight a rather bizarre notion. It is not that there are any particular difficulties in accepting the concept, but rather that it seems a somewhat extravagant or whimsical idea, rather like a discussion of caviar as a pig food. Nevertheless, the topic does merit discussion. For some animals protein regularly predominates in the diet. In adult humans in the Western world protein intake is normally substantially greater than the requirement for amino acids, so the excess is simply of value as an energy source. In addition, a knowledge of factors that determine the catabolism of amino acids might enable us to minimize these losses when the availability of amino acids is limited. It is proposed to consider the catabolism of amino acids firstly with respect to a supply of dietary amino acids, and secondly with respect to endogenously derived amino acids. Dietary amino acids The general principles underlying the catabolism of the indispensable amino acids are well illustrated by some recent studies of Kang-Lee & Harper (1977; 1978). They examined the effect in rats, of variation in dietary intake of histidine or threonine on the rate of catabolism. With a low intake of these amino acids, the plasma concentrations remained low and little of the (14C-labelled) dietary amino acid appeared as 14C02. When the intake was greater than the dietary requirement, there was a sharp rise in the plasma amino acid concentration and a marked rise in the rate of 14C02 production. These observations are readily explained on biochemical grounds. Enzymes initiating the catabolism of amino acids have Michaelis constants in the milli-molar range (Krebs, 1972) while for enzymes initiating protein synthesis (e.g. t-RNA synthetases of threonine; Allende et al. 1966) and arginine (Allende & Allende, 1964) the values are in the micromolar range. Thus at low concentration, amino acids will be largely bound to the synthetases. Only when the synthetic pathway of protein is saturated will the catabolic pathway assume significant importance. Increasing the dietary intake of threonine or histidine does not induce increased catabolic enzyme activity, but this occurs following a high protein diet. In these circumstances there is still no increased oxidation when the diet is switched to one limiting for threonine or histidine, but when they are present in an amount above requirement, the increased enzyme activity results in an increased rate of oxidation.