Arginase is a binuclear manganese metalloenzyme that catalyzes the hydrolysis of l-arginine to form l-ornithine and urea. The X-ray crystal structure of a fully active, truncated form of human arginase II complexed with a boronic acid transition state analogue inhibitor has been determined at 2.7 A resolution. This structure is consistent with the hydrolysis of l-arginine through a metal-activated hydroxide mechanism. Given that human arginase II appears to play a role in regulating l-arginine bioavailability to NO synthase in human penile corpus cavernosum smooth muscle, the inhibition of human arginase II is a potential new strategy for the treatment of erectile dysfunction [Kim, N. N., Cox, J. D., Baggio, R. F., Emig, F. A., Mistry, S., Harper, S. L., Speicher, D. W., Morris, S. M., Ash, D. E., Traish, A. M., and Christianson, D. W. (2001) Biochemistry 40, 2678-2688]. Since NO synthase is found in human clitoral corpus cavernosum and vagina, we hypothesized that human arginase II is similarly present in these tissues and functions to regulate l-arginine bioavailability to NO synthase. Accordingly, hemodynamic studies conducted with a boronic acid arginase inhibitor in vivo are summarized, suggesting that the extrahepatic arginase plays a role in both male and female sexual arousal. Therefore, arginase II is a potential target for the treatment of male and female sexual arousal disorders.

Thrombin is the final protease in the blood coagulation cascade and serves both pro-and anticoagulant functions through the cleavage of several targets. The ability of thrombin to specifically recognize a wide range of substrates derives from interactions that occur outside of the active site of thrombin. Thrombin possesses two anion binding exosites, which mediate many of its interactions with cofactors and substrates, and although many structures of thrombin have been solved, few such interactions have been described in molecular detail. Glycosaminoglycan binding to exosite II of thrombin plays a major role in switching off the procoagulant functions of thrombin by mediating its irreversible inhibition by circulating serpins and by its binding to the endothelial cell surface receptor thrombomodulin. Here we report the 1.85-Å structure of human ␣-thrombin bound to a heparin fragment of eight monosaccharide units in length. The asymmetric unit is composed of two thrombin dimers, each sharing a single heparin octasaccharide chain. The observed interactions are fully consistent with previous mutagenesis studies and illustrate on a molecular level the cofactor interaction that is critical for the restriction of clotting to the site of blood vessel injury.Thrombin is the ultimate protease in the blood clotting cascade and is uniquely able to cleave soluble fibrinogen into polymerogenic fibrin, resulting in the formation of the fibrin clot. Thrombin also serves a critical role in stimulating its own generation from the zymogen prothrombin by cleavage activation of upstream coagulation factors and platelet receptors through positive feedback loops (for reviews, see Refs. 2-5). The procoagulant functions of thrombin are counterbalanced by the plasma serpins antithrombin (AT) 1 and heparin cofactor II (HCII) (for reviews, see Refs. 6 and 7) and by the integral membrane protein thrombomodulin (TM) (for review, see Refs. 8 and 9). AT and HCII inhibit thrombin in an irreversible fashion common to all members of the serpin family, whereas TM binding serves to alter the specificity of thrombin away from the prothrombotic substrates in favor of the anticoagulant protein C. Thus thrombin specificity is of paramount importance in determining the hemostatic balance between clotting and bleeding.Close examination of the peptide sequences comprising the natural substrates of thrombin provides little insight into how thrombin specificity is determined, other than to permit drawing the conclusion that determinants of thrombin substrate specificity lie elsewhere (3, 10). The solution of the crystallographic structure of thrombin revealed two cationic patches on the surface of thrombin denoted anion binding exosites I and II (1). Exosite I, on the eastern face of thrombin in the standard orientation (11), is adjacent to, and contiguous with, the active site cleft and is known to be the fibrinogen recognition exosite. Exosite II, on the northwestern face of thrombin, is the more basic of the two exosites and was identified as the putat...