alpha(1)-Protease inhibitor (alpha(1)PI), the most abundant serine protease inhibitor found in human plasma (at 30-60 mu M) is a glycoprotein (53 kDa) having a single cysteine residue at position 232 (Cys(232)). We have found that Cys232 of human alpha(1)PI was readily S-nitrosylated by nitric oxide (NO) without affecting inhibitory activity to trypsin or elastase. S-nitrosylated alpha(1)PI (S-NO-alpha(1)PI) not only retained inhibitory activity against these serine proteases, but also gained thiol protease inhibitory activity against a Streptococcus pyrogenes protease; the parental alpha(1)PI did not have this activity. Furthermore, S-NO-alpha(1)PI exhibited bacteriostatic activity against Salmonella typhimurium at concentrations of 0.1-10 mu M which were 20- to 3000-fold stronger than those of the other NO-generating compounds or S-nitroso compounds such as S-nitrosoalbumin and S-nitrosoglutathione. NO appears to be transferred into the bacterial cells from S-NO-alpha(1)PI via transnitrosylation, as evidenced by electron spin resonance spectroscopy with an NO spin trap. Thus, we conclude that S-NO-alpha(1)PI may be generated from the reaction between alpha(1)PI and NO under inflammatory conditions, in which production Of both is known to increase. As a result, new functions, i.e., antibacterial and thiol protease inhibitory activities of alpha(1)PI, were generated.