Comparison of Tryptophan Metabolism in Vivo and in Isolated Hepatocytes from Vitamin B6 Deficient Mice
Bender, D. A.
Danielian, P. S.
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Tryptophan dioxygenase meets the traditional criteria for the rate-limiting enzyme of the kynurenine pathway of tryptophan metabolism. From the concentrations of substrates in the liver and published values of Km and Vmax (Bender et al., 1975; Bender and McCreanor, 1985; Takikawa et al., 1986), the steady-state rates of activity can be calculated. As shown in Table 1, such calculations show that tryptophan dioxygenase has the lowest activity of the pathway under basal conditions. Furthermore, it is regulated by a variety of mechanisms, including: induction of mRNA synthesis by glucocorticoid hormones; increased translation of mRNA by glucagon; stabilization of the enzyme protein against catabolism by both tryptophan and the heme cofactor; and feedback inhibition and repression by the reduced nicotinamide nucleotide coenzymes NADH and NADPH.