Effect of amino acid metabolites on lysosomal protein degradation. A regulatory role for kynurenine?

Possible derivatives of the amino acids tryptophan, tyrosine and histidine were examined as to their effect on protein metabolism in isolated rat hepatocytes. One of the substances tested, kynurenine (a main product of the catabolism of tryptophan), might be a physiological regulator of the lysosomal degradation of endogenous protein, because of the following. (a) Kynurenine decreased the lysosomal (i.e. methylamine-sensitive) pathway of degradation to a much greater extent than its parent amino acid, without interfering with the non-lysosomal pathway. (b) Kynurenine did not appreciably reduce the (lysosomal) degradation of the endocytosed protein asialo-fetuin, or the rate of protein synthesis, indicating a specificity of action. (c) Electron micrographs revealed a reduction in secondary lysosomes due to kynurenine.