Reversible inactivation of serpins at acidic pH

The inhibitory activity of the serpins α1-proteinase inhibitor, α1-antichymotrypsin, α2-antiplasmin, antithrombin and C1-esterase inactivator is rapidly lost at pH 3 but slowly recovers at pH 7.4 with variable first-order rates (t1/2 = 1.4–19.2 min). All except α1-antichymotrypsin undergo a variation in intrinsic fluorescence intensity upon acidification (midpoint ca. 4.5) with a slow bi-exponential return to the initial intensity at pH 7.4 (mean t1/2 = 2.3–23 min). No correlation was found between the time of fluorescence recovery and that of reactivation. The acid-treated serpins are proteolyzed at neutral pH by their target proteinases. α1-Proteinase inhibitor was studied in more detail. Its acidification at pH 3 has a mild effect on its secondary structure, strongly disorders its tertiary structure, changes the microenvironment of Cys232 and causes a very fast change in ellipticity at 225 nm (t1/2 = 1.6 s). Neutralization of the acid-treated α1-proteinase inhibitor is an exothermic phenomenon. It leads to a much faster recovery of activity (t1/2 = 4 ± 1 min) than of fluorescence intensity (t1/2 = 23 ± 19 min), ellipticity (t1/2 = 32 ± 4 min) and change in total energy, indicating that the inhibitory activity of α1-proteinase inhibitor does not require a fully native structure.