S-nitrosylated human alpha(1)-protease inhibitor.

Alpha(1)-protease inhibitor (alpha(1)PI) is an acute phase plasma protein, and possesses a single cysteine residue at position 232. A single cysteinyl sulfhydryl of human alpha(1)PI is found to be readily S-nitrosylated by nitric oxide (NO) in vitro without affecting the inhibitory capacity against bovine trypsin or elastase, a major target protease of alpha(1)PI in vivo. S-nitroso-alpha(1)PI (S-NO-alpha(1)PI) was also formed by the reaction of alpha(1)PI with NO produced excessively by a murine macrophage cell line (RAW264 cells) upon infection with Salmonella typhimurium and in an ex vivo perfusion system of the liver obtained from lipopolysaccharide-treated rats. S-NO-alpha(1)PI (10(-9)-10(-6) M) induces a dose-dependent relaxation of the ring preparation of rabbit aorta. Also, S-NO-alpha(1)PI but not alpha(1)PI shows a potent inhibitory effect on platelet aggregation. Unprecedented observation is that S-NO-alpha(1)PI showed a potent bacteriostatic effect against a wide range of bacteria at the concentration of 1-10 microM, which was 10-1000-fold stronger than that of NO and other S-nitrosylated compounds including S-nitrosylated albumin and S-nitrosylated glutathione. These results suggest that S-NO-alpha(1)PI is produced as an NO sink under inflammatory conditions, where production of both alpha(1)PI and NO is highly up-regulated, and it may function as a soluble factor which consists of an innate defense system through not only the protease inhibitory activity but also its antibacterial activity and facilitating the peripheral blood flow. Therefore, S-nitrosylation of alpha(1)PI occurring under physiological conditions in vivo should diversify the biological functions contributing to cytoprotective effects of alpha(1)PI.