Metabolism of Isomers of 3-Hydroxykynurenine-C14 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.