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Conversion of D-Tryptophan to Nicotinic Acid in the Rat¹

Department of Biochemistry, College of Medicine, University of Iowa, Iowa City, Iowa 52240

Subcutaneous injection of D-tryptophan into rats fed a diet devoid of tryptophan and niacin yielded as much urinary N¹-methylnicotinamide as the injection of L-tryptophan did. L-Kynurenine yielded about half as much, but D-kynurenine produced little. Injection of the L-kynurenine in four equal doses increased the output markedly. Most of the injected D-kynurenine was excreted. The residue yielded more urinary 3-hydroxy-D-kynurenine than the larger residue of unexcreted L-kynurenine did 3-hydroxy-L-kynurenine, but less kynurenic acid and anthranilic acid and markedly less xanthurenic acid. In the tissue tests, liver slices converted D-kynurenine to 3-hydroxy-D-kynurenine but produced no accumulation of 3-hydroxy-L-kynurenine with L-kynurenine. Kidney slices hydroxylated D- and L-kynurenine to their asymmetrically corresponding 3-hydroxykynurenines. Liver and kidney homogenates accumulated neither 3-hydroxykynurenine isomer. All slices and homogenates yielded xanthurenic acid, apparently more readily from L- than from D-kynurenine. Previous tissue tests have shown little inversion of D-kynurenine or 3-hydroxy-D-kynurenine, or cleavage of them by kynureninase. The tissue tests, the limited production of N¹-methylnicotinamide from injected D-kynurenine, and the ready yields of indolepyruvic acid and N¹-methylnicotinamide from injected D-tryptophan all favor the view that the D-tryptophan first underwent inversion to L-tryptophan, which was then metabolized via L-kynurenine, 3-hydroxy-L-kynurenine, and 3-hydroxyanthranilic acid to yield the urinary N¹-methylnicotinamide.

KEY WORDS: • D-tryptophan • D-kynurenine • nicotinic acid synthesis

¹ The data presented are taken from the dissertation submitted by Horace H. Loh in partial fulfillment of requirements for the degree of Doctor of Philosophy in Biochemistry in the Graduate College of the University of Iowa. The financial aid of U. S. Public Health Service Grants AM 01770 and AM 09797 from the National Institute of Arthritis and Metabolic Diseases is gratefully acknowledged. A preliminary report was presented before the American Society of Biological Chemists in Chicago, April 16, 1964 Cf. Loh, H. H., and C. P. Berg 1964 Metabolism of D-tryptophan. Federation Proc. 23: 421 (abstr. 1916).

² To whom inquiries should be addressed.

Manuscript received 8 June 1971.