Novel Functions of Human α1-Protease Inhibitor after S-Nitrosylation: Inhibition of Cysteine Protease and Antibacterial Activity

α1-Protease inhibitor (α1PI), the most abundant serine protease inhibitor found in human plasma (at 30–60 μM), is a glycoprotein (53 kDa) having a single cysteine residue at position 232 (Cys232). We have found that Cys232 of human α1PI was readily S-nitrosylated by nitric oxide (NO) without affecting inhibitory activity to trypsin or elastase. S-nitrosylated α1PI (S-NO-α1PI) not only retained inhibitory activity against these serine proteases, but also gained thiol protease inhibitory activity against a Streptococcus pyogenes protease; the parental α1PI did not have this activity. Furthermore, S-NO-α1PI exhibited bacteriostatic activity against Salmonella typhimurium at concentrations of 0.1–10 μM, 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-α1PI via transnitrosylation, as evidenced by electron spin resonance spectroscopy with an NO spin trap. Thus, we conclude that S-NO-α1PI may be generated from the reaction between α1PI 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 α1PI, were generated.