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. (C) 2000 Academic Press.