Determination of mammalian arginase activity

Of all arginine catabolic enzymes, the arginases and nitric oxide (NO) synthases are the ones that are of greatest interest to many investigators. Mammalian arginases catalyze the hydrolysis of arginine to ornithine and urea and are composed of two distinct isozymes: arginase I, located within the cytosol, and arginase II, located within mitochondria. The arginases not only can inhibit NO synthesis by reducing arginine availability, but also can promote the synthesis of polyamines or proline via production of the common precursor ornithine. Because of their inducibility in many cell types and to their potential impact on multiple biochemical pathways in health and disease, there is growing interest in assays of arginase activity. Although arginase activity may be determined by either spectrophotometric or radiochemical assays, radiochemical assays afford greater sensitivity and do not require correction for any ornithine or urea that may be present in the samples. Part of the arginase assay protocol described in this chapter also can be used for radiochemical assays of enzymes that catalyze decarboxylation reactions. No activity assay currently available is capable of distinguishing the arginase isozymes.