Some Effects of Dietary Threonine,Tryptophan,and Choline on Liver Enzymes and Fat. ∗

Summary

1. Several liver enzyme systems were studied in rats fed diets known to produce fatty livers in order to determine whether changes in liver fat deposition could be associated with specific enzyme changes. The activities of the soluble cytoplasmic enzymes, xanthine oxidase, and tyrosine oxidase, as well as the rate of endogenous oxidation and decarboxylation, were found to be markedly increased when supplemental threonine was included in a 9% casein ration containing added methionine and tryptophan with or without choline. Several mitochondrial enzyme systems, choline oxidase, succinic oxidase, and pyruvic oxidase showed a sufficiently great variation from experiment to experiment to prevent any differences in the activities of these enzymes from being considered significant. This variation has been attributed to an inverse relationship between the activities of these enzymes and the absolute rate of endogenous oxidation in the liver. 2. When supplemental tryptophan was removed from the ration, all of the differences in enzyme activity between threonine-deficient and threonine-supplemented animals disappeared completely, although the changes in liver fat deposition produced by adding or omitting threonine were unaltered under these conditions. It is suggested that the balance of amino acids in the diet is the primary factor responsible for the enzyme changes observed and that the accumulation of fat in the livers bears no direct relationship to those enzyme changes.