Arginase catalyzes the hydrolysis of arginine to ornithine and urea. Polyamine metabolism is regulated by the activity of the enzyme ornithine decarboxylase (ODC). Biosynthesis begins with decarboxylation of ornithine by ornithine decarboxylase (ODC) to produce putrescine. The biosynthesis of hyoscine begins with the decarboxylation of L-ornithine to putrescine by ornithine decarboxylase. The active form of ornithine decarboxylase is a homodimer. The second step facilitated by ornithine transcarbamylase converts carbamoyl phosphate and ornithine into citrulline. It is responsible for converting carbamoyl phosphate and ornithine into citrulline. Citrulline is made from ornithine and carbamoyl phosphate by ornithine carbamoyltransferase. The urea cycle is facilitated by carbamoyl phosphate synthetase I and ornithine transcarbamylase. Using his Warburg manometer, he mixed a slice of liver with purified ornithine and citrulline. In animals, however, the urea cycle derives putrescine from ornithine. There are two distinct active sites formed by the homodimerization of ornithine decarboxylase. The second isozyme, Arginase II, has been implicated in the regulation of intracellular arginine/ornithine levels. The separate pathways of converting ornithine to putrescine in plants and animals have converged. This situation is not unique and may have occurred with ornithine and arginine.