Effect of body weight and sex on activity of enzymes involved in amino acid metabolism

The total body activities of four transaminases and two oxidases concerned with aromatic amino acid metabolism were studied in five species of mammals of varying size to test for correlations between total body enzyme activity and body weight. It was found that as a general rule the following expression was obeyed: enzyme x weight k = constant (the weight parity expression), where k varied numerically from –0.25 to –0.73 depending on the enzyme. The two oxidases had values which varied from –0.71 to –0.73 which are very close to the value –0.73 established for total oxygen utilization as a function of body weight. The use of the weight parity expression permitted the analysis within a given species of enzyme activities in animals with different body weights. Such an analysis proved useful in distinguishing an enzyme, tyrosine-α-ketoglutarate transaminase, whose level showed a true sex difference from others whose levels showed apparent sex differences which were actually referable to the smaller body weights of the females. Direct experimentation involving castration and hormone treatment substantiated this conclusion and identified testosterone as the important controlling factor. This sex difference was explicable by the known effects of adrenal cortical hormones on the level of this enzyme and by the reported differences in the rate of hepatic metabolism of adrenal cortical hormone in male and female animals.