The intermediary metabolism of tryptophan by cat and rat tissue preparations.

Reports from several laboratories have shown that cats display a peculiar tryptophan metabolism as compared with other mammals. In 1941, Kotake and Nakayama (1) reported that L-kynurenine could be quantitatively converted to anthranilic acid when incubated with cat liver preparations. Gordon, Kaufman, and Jackson (a), on the other hand, found no kynurenic acid in urine of cats even when tryptophan was added to the high protein diet on which the animals were kept. Lepkovsky and Nielsen (3) and Miller and Baumann (4) showed that pyridoxine-deficient rats excreted large amounts of xanthurenic acid; however, Carvalho da Silva, Fried, and de Angelis (5) were unable to find xanthurenic acid in urine of pyridoxine-deficient cats even when supplemental doses of tryptophan were fed in the diet. Furthermore, they showed that L-tryptophan was not effective in relieving the avitaminosis symptoms of niacin-deficient cats in contrast to observations in other species (6). Previous studies in this laboratory (7) have shown that cats fed a high protein diet supplemented with single doses of L-tryptophan did not excrete any appreciable amount of most of the known metabolites of this amino acid. Only 0.07 per cent of the administered dose of tryptophan could be accounted for in the urine as the metabolites measured. Apparently, therefore, the cat has an active kynureninase, but is unable to use tryptophan in place of niacin and does not excrete significant quantities of the usual urinary metabolites of tryptophan. The present work deals with measurements in vitro of the activities of some of the enzymes involved in tryptophan metabolism in an effort to explain the absence of tryptophan metabolites in cat urine. The enzymatic studies of Mason and Berg (8, 9) and of Knox and Mehler (10) served as valuable guides in the development of these studies. The enzymes compared in cats and rats were tryptophan peroxidase, kynureninase, kynure-