. The Biochemistry of B vitamins. Vitamins; Vitamin B complex. METABOLISM OF THE B VITAMINS 355 In tracer studies with DL-tryptophan-/3-C 14, the labelled atom may be found in kynurenic acid, but not in niacin. -CH2CH(NH2)COOH r^N-CO—CH2CH(NH2)COOH "* L J-NH2 H. This indicates clearly that the conversion does not involve the pyridine ring of kynurenic acid as a ring precursor of When, however, DL-tryptophan-3-C 14 is employed, the tracer atom is found in the car- boxylic acid group of niacin,107 strongly suggesting that it is the benzene ring of tryptophan which eventually beco
. The Biochemistry of B vitamins. Vitamins; Vitamin B complex. METABOLISM OF THE B VITAMINS 355 In tracer studies with DL-tryptophan-/3-C 14, the labelled atom may be found in kynurenic acid, but not in niacin. -CH2CH(NH2)COOH r^N-CO—CH2CH(NH2)COOH "* L J-NH2 H. This indicates clearly that the conversion does not involve the pyridine ring of kynurenic acid as a ring precursor of When, however, DL-tryptophan-3-C 14 is employed, the tracer atom is found in the car- boxylic acid group of niacin,107 strongly suggesting that it is the benzene ring of tryptophan which eventually becomes the pyridine ring of nicotinic acid. CH2CH(NH2)COOH ^ VCOCH2CH(NH2)COOH V ^^-NH2 OH -COOH f VCOOH -NH2 ~^ l^N J The ring rearrangement presumably involved in the final step of the process is of a type not entirely unknown to organic chemistry. Very recently Henderson and co-workers 107a-c have shown that a probable intermediate in this step is quinolinic acid. Rats on a 9% casein diet containing little niacin but added DL-tryptophan excrete 6-11. mg of this acid per clay. The injection of 1. mM of tryptophan or 3- hydroxyanthranilic acid into such rats increases their niacin excretion tenfold, their F2 excretion twenty-five to one hundredfold, and their quinolinic acid excretion one hundred to three hundredfold. Quinolinic acid is slightly active for rats in replacing dietary niacin, is active in some Neurospora strains, and accumulates in the culture medium of other strains. Rat liver slices or homogenates are able to convert 3- hydroxyanthranilic acid to quinolinic acid, and quinolinic acid is con- verted by weak acid to a substance having niacin activity for Lactobacil- lus Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Williams, Roger John, 1893-. New York : Reinhold Pub. Corp.
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