Metabolism of Isomers of 3-Hydroxykynurenine-C 14 to Quinolinic Acid,Niacin Metabolites and Carbon Dioxide. ∗

Summary

DL-hydroxykynurenine-keto-C¹⁴ was synthesized and resolved into the D- and L-isomers. These were administered to rats and mice by intraperitoneal injection and urinary and respiratory radioactivity were measured. L-hydroxykynurenine was rapidly converted to CO₂. Urinary quinolinic acid, nicotinic acid and N-methylnicotinamide were isolated by carrier procedures and the radioactivity measured. The L-hydroxykynurenine was a more effective precursor of quinolinic acid than was the D-isomer; however, label in nicotinic acid was about the same from both isomers. D-hydroxykynurenine was not converted to CO₂ appreciably but was excreted probably unchanged in the urine. The metabolism of DL-hydroxykynurenine was compared in the rat, mouse, dog and cat and found to be generally the same with regard to CO₂ production, and urinary quinolinic acid production. No differences were observed between the cat and rats or dogs which would explain the inability of cats to utilize tryptophan as a significant source of niacin.