Carbamyl Phosphate Synthesis in a Land Snail, Strophocheilus oblongus

Abstract

Abstract A carbamyl phosphate synthetase has been detected and partially characterized in hepatopancreas tissue of the land snail Strophocheilus oblongus. The enzyme is localized in mitochondria, utilizes l-glutamine, and shows an absolute requirement for N-acetyl-l-glutamate. This unique combination of properties distinguishes the snail enzyme from either of the previously described animal carbamyl phosphate synthetases, carbamyl phosphate synthetase I and carbamyl phosphate synthetase II. The snail enzyme was measured as the fixation of 14C-bicarbonate into citrulline in the presence of excess l-ornithine and ornithine transcarbamylase. ATP and Mg2+ are also required in the reaction; optimal activity is obtained when the Mg2+:ATP ratio is 2. When this ratio is maintained, saturation with ATP occurs at approximately 5 mm. The Km of the enzyme for l-glutamine is estimated to be 2.5 mm and for N-acetylglutamate, 0.3 mm. The major portion of carbamyl phosphate synthetase activity is localized in the mitochondrial fraction of the snail tissue and, since this is also the fraction in which ornithine transcarbamylase occurs, the enzyme is presumed to function in arginine biosynthesis. Aspartate transcarbamylase occurs in the cytosol of the tissue but the amount of carbamyl phosphate synthetase that can be measured in this fraction is negligible. The snail carbamyl phosphate synthetase may thus also function in pyrimidine biosynthesis.