Arginase I (also designated liver-type arginase), which is expressed almost exclusively in the liver, catalyzes the conversion of arginine to ornithine and urea (1). The human arginase I gene, which maps to chromosome 6q23, encodes a 322 amino acid protein. Arginase I exists as a homotrimeric protein and contains a binuclear manganese cluster (2-4). Arginase II catalyzes the same reaction as arginase I, but differs in its tissue specificity and subcellular location (5,6). Specifically, argin