Expression, purification, and characterization of human type II arginase

Human type II arginase, which is extrahepatic and mitochondrial in location , catalyzes the hydrolysis of arginine to form ornithine and urea. While ty pe I arginases function in the net production of urea for excretion of exce ss nitrogen, type II arginases are believed to function primarily in the ne t production of ornithine, a precursor of polyamines, glutamate, and prolin e. Type II arginases may also regulate nitric oxide biosynthesis by modulat ing arginine availability for nitric oxide synthase, Recombinant human type II arginase was expressed in Escherichia coli and purified to apparent hom ogeneity. The K-m of arginine for type II arginase is approximately 4.8 mM at physiological pH, Type II arginase exists primarily as a trimer, althoug h higher order oligomers were observed. Borate is a noncompetitive inhibito r of the enzyme, with a K-is of 0.32 mM and a K-ii of 0.3 mM. Ornithine, a product of the reaction catalyzed by arginase and a potent inhibitor of typ e I arginase, is a poor inhibitor of the type II isozyme. The findings pres ented here indicate that isozyme-selectivity exists between type I and type II arginases for binding of substrate and products, as well as inhibitors. Therefore, inhibitors with greater isozyme-selectivity for type II arginas e may be identified and utilized for the therapeutic treatment of smooth mu scle disorders, such as erectile dysfunction. (C) 2001 Academic Press.