α-methyltryptophan and hepatic glycogenesis

Studies of the effect of α-methyltryptophan on carbohydrate metabolism have shown that this amine acid analogue stimulates hepatic glycogenesis in the rat, in contrast to tryptophan which inhibits this process. Other differences in the actions of the two amino acids have been found: whereas tryptophan interferes with the conversion of the exogenous pyruvate into glycogen, α-methyltryptophan does not. Tryptophan promotes an increase in the assayable level of phosphoenolpyruvate carboxykinase in liver, but α-methyltryptophan causes a decrease. α-Methyltryptophan acts only in the presence of glucocorticoids. Furthermore, α-methyltryptophan accelerates in vivo the oxidation of tryptophan, an essential amino acid, thus probably creating an amine acid imbalance. Decreased utilization of amino acids for protein synthesis, together with the availability of endogenous or exogenous glucocorticoids could account for the glycogenetic property of α-methyltryptophan. Some metabolites of tryptophan were also examined. Only 3-hydroxyanthranilic acid mimicked the effect of tryptophan with respect to inhibition of glycogenesis and enhancement of phosphoenolpyruvate carboxykinase activity