The Relationship between Arginine, Citrulline and Ornithine in Tetrahymena pyriformis*

SYNOPSIS. Studies on the metabolism or arginine, citrulline, and ornithine by Tetrahymena pyriformis confirmed that these compounds do not participate in a urea cycle in this organism. No evidence for the enzymes of the cycle or for the presence of urea or urease was found. However, conversion of arginine to ornithine takes place. This system consists of two enzymes, arginine desimidase and a citrulline‐hydrolyzing enzyme. This is different from the arginine dihydrolase enzymes reported for bacteria and yeast, since no labile phosphate is produced in the conversion of citrulline to ornithine. The enzyme responsible for the latter conversion was purified 44‐fold. The pH optimum of the enzyme, its substrate specificity, and the effect of inhibitors on its activity were investigated. The enzyme appears entirely hydrolytic in nature.