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Intestinal Pyrrolase and Formamidase Activities in the Rabbit¹

Department of Biochemistry, College of Medicine, University of Iowa, Iowa City, Iowa 52242

Centrifugation of homogenates of the wall of the lower third of the small intestine of the rabbit at 22,000 x g, followed by dialysis, produced a supernatant solution which converted D-tryptophan to D-kynurenine and L-tryptophan to L-kynurenine. Its capacity to yield kynurenine decreased with storage. Spectral absorption verified the hemoprotein nature of its pyrrolase component, which was apparently effective only in the ferrous state. Several agents known to increase or decrease the activity of the liver enzyme system affected similarly the activity of the intestinal preparation. Purified rat liver formamidase hydrolyzed formyl-DL-kynurenine more readily than formyl-D-kynurenine. The formamidase activity in the intestinal preparation was limited, but stereospecifically similar. Spectrophotometric assays at 321 mµ and 365 mµ showed that the pyrrolase in the rabbit preparation converted D-tryptophan to formyl-D-kynurenine more rapidly than the formamidase present could hydrolyze it. Hence the formyl-D-kynurenine accumulated. Addition of purified formamidase produced a sharp increase at 365 mµ and a sharp decrease at 321 mµ. Analogous tests with L-tryptophan showed little or no accumulation of formyl-L-kynurenine, probably because its slower rate of formation by the intestinal pyrrolase was equalled or exceeded by its more rapid hydrolysis by the intestinal formamidase.

KEY WORDS: • intestinal pyrrolase activity • intestinal formamidase activity • D-tryptophan • formyl-D-kynurenine • D-kynurenine

¹ The report presented is based largely on data from a dissertation submitted by Horace H. Loh in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Biochemistry in the Graduate College of the Universitly of Iowa. The work was aided by U. S. Public Health Grants AM 01770 and AM 09797 from the National Institute of Arthritis and Metabolic Diseases.

Manuscript received 18 August 1972.