Studies on the kynurenine adminotransferase activity in rat liver and kidney

A mathematical model of tryptophan metabolism via the kynurenine pathway provides insights into the effects of vitamin B-6 deficiency, tryptophan loading, and induction of tryptophan 2,3-dioxygenase on tryptophan metabolites

Vitamin B-6 deficiency is associated with impaired tryptophan metabolism because of the coenzyme role of pyridoxal 5'-phosphate (PLP) for kynureninase and kynurenine aminotransferase. To investigate the underlying mechanism, we developed a mathematical model of tryptophan metabolism via the kynurenine pathway. The model includes mammalian data on enzyme kinetics and tryptophan transport from the intestinal lumen to liver, muscle, and brain. Regulatory mechanisms and inhibition of relevant enzymes were included. We simulated the effects of graded reduction in cellular PLP concentration, tryptophan loads and induction of tryptophan 2,3-dioxygenase (TDO) on metabolite profiles and urinary excretion. The model predictions matched experimental data and provided clarification of the response of metabolites in various extents of vitamin B-6 deficiency. We found that moderate deficiency yielded increased 3-hydroxykynurenine and a decrease in kynurenic acid and anthranilic acid. More severe deficiency also yielded an increase in kynurenine and xanthurenic acid and more pronounced effects on the other metabolites. Tryptophan load simulations with and without vitamin B-6 deficiency showed altered metabolite concentrations consistent with published data. Induction of TDO caused an increase in all metabolites, and TDO induction together with a simulated vitamin B-6 deficiency, as has been reported in oral contraceptive users, yielded increases in kynurenine, 3-hydroxykynurenine, and xanthurenic acid and decreases in kynurenic acid and anthranilic acid. These results show that the model successfully simulated tryptophan metabolism via the kynurenine pathway and can be used to complement experimental investigations.

Tryptophan Metabolism Along the Kynurenine Pathway in Rats

Enzyme activities along the kynurenine pathway, liver tryptophan 2,3-dioxygenase, small intestine indole 2,3-dioxygenase, liver and kidney kynurenine 3-monooxygenase, kynureninase, kynurenine-oxoglutarate transaminase, 3-hydroxyanthranilate 3,4-dioxygenase, and aminocarboxymuconate-semialdehyde decarboxylase, involved in the catabolism of tryptophan, were studied in male adult Wistar albino rats. Intestine superoxide dismutase and serum tryptophan were also determined. Hepatic tryptophan 2,3-dioxygenase is present both as holoenzyme and apoenzyme, but the total activity is inferior to that of intestine indole 2,3-dioxygenase which, therefore, actively oxidizes tryptophan in rats. However, this activity is inhibited by scavengers for the superoxide anion, such as superoxide dismutase, which also shows to be active in small intestine of rat. However, the more active enzymes appeared to be kynurenine 3-monooxygenase and 3-hydroxyanthranilate 3,4-dioxygenase. The former is equally active in both liver and kidney, the latter is more active in liver. Kynurenine-oxoglutarate transaminase is much more active in kidney than in liver, and much more active than kynureninase, which shows similar activities in both tissues. In contrast to the high activity of 3-hydroxyanthranilate 3,4-dioxygenase, aminocarboxymuconate-semialdehyde decarboxylase is 30-35 times less active, showing the efficiency of conversion of tryptophan to NAD. These data demonstrate that rat is a useful animal model for studying tryptophan metabolism along the kynurenine pathway. Serum tryptophan appeared to be 90% bound to proteins. Results demonstrate that, in rat, tryptophan is mainly metabolised along the kynurenine pathway. Therefore, rat is a suitable animal model for studying tryptophan metabolism in the pathological field.

Liver and kidney kynurenine aminotransferase activity in different strains of rats

Variations in liver and kidney kynurenine aminotransferase (KAT) activity in Pittsburg-Yoshida, Brown-Norway, albino Wistar, Sprague-Dawley, Long Evans and heterozygous Gunn rats were studied. In liver, values of KAT specific activity, expressed as mumoles of kynurenic acid formed per hour per mg of protein, were different in the groups of Brown-Norway and Pittsburg-Yoshida rats versus Long Evans and Sprague-Dawley rats. The activity expressed as mumoles of kynurenic acid per g of fresh liver showed other differences, being significantly higher in Gunn with respect to other strains of rats and lower in Pittsburg-Yoshida and Brown-Norway rats. In addition, KAT activity was significantly lower in Pittsburg-Yoshida than in Brown-Norway rats. In kidney, the specific activity of kynurenine aminotransferase showed significant differences in the values of Sprague-Dawley and Long Evans rats versus the other strains. The activity expressed per g of fresh tissue was significantly higher in Wistar, Sprague-Dawley, Long Evans and Gunn than in Pittsburg-Yoshida and Brown-Norway rats. No significant differences were found in values between hyperlipidemic Pittsburg-Yoshida and their control Brown-Norway rats. These results high-light the importance of considering various rat strains when inbred animal experimental models are used.

Purification, characterization and identification of rat liver mitochondrial kynurenine aminotransferase with alpha-aminoadipate aminotransferase

Kynurenine aminotransferase (L-kynurenine:2-oxoglutarate aminotransferase (cyclizing), EC 2.6.1.7) was purified 378-fold from rat liver mitochondria by digitonin solubilization, heat treatment, DEAE-Sepharose CL-6B chromatography, Sephadex G-100 gel filtration, hydroxyapatite chromatography and chromatofocusing. Elution patterns of alpha-aminoadipate aminotransferase (EC 2.6.1.39) activity were identical with those of kynurenine aminotransferase activity on all column chromatographies. The ratios of the two specific activities were constant throughout the purification. On polyacrylamide gel electrophoresis both activities were detected at the same position. Both enzymatic activities showed the same inactivation curves upon heat inactivation at various temperatures. alpha-Aminoadipate showed competitive inhibiton against kynurenine or 3-hydroxykynurenine. alpha-Ketoadipate was utilized in the kynurenine aminotransferase reaction as an amino acceptor in place of alpha-ketoglutarate. The Km value for alpha-ketoadipate was 10 microM, lower than for alpha-ketoglutarate. These observations indicate that kynurenine aminotransferase is identical with alpha-aminoadipate aminotransferase. The Km values of purified kynurenine aminotransferase were determined at pH 6.5 as: kynurenine, 4.3 mM; pyridoxal 5'-phosphate, 4.2 microM; alpha-ketoglutarate, 20 microM (kynurenine substrate), and 3-hydroxykynurenine, 5.7 mM; pyridoxal 5'-phosphate, 1.7 microM; alpha-ketoglutarate, 13 microM (3-hydroxy-kynurenine substrate). The enzyme was strongly inhibited by Hg2+ and p-chloromercuribenzoate.