Large scale purification and molecular characterization of human recombinant α1-proteinase inhibitor produced in yeasts

The reactivity and oxidation pathway of cysteine 232 in recombinant human alpha 1-antitrypsin

Oxidative damage to the sulfur-containing amino acids, methionine and cysteine, is a major concern in biotechnology and medicine. alpha1-Antitrypsin, which is a metastable and conformationally flexible protein that belongs to the serpin family of protease inhibitors, contains nine methionines and a single cysteine in its primary sequence. Although it is known that methionine oxidation in the protein active site results in a loss of biological activity, there is little specific knowledge regarding the reactivity of its unpaired thiol, Cys-232. In this study, the thiol-modifying reagent NBD-Cl (7-chloro-4-nitrobenz-2-oxa-1,3-diazole) was used to label peroxide-modified alpha1-antitrypsin and demonstrate that the Cys-232 in vitro oxidation pathway begins with a stable sulfenic acid intermediate and is followed by the formation of sulfinic and cysteic acid in successive steps. pH-dependent reactivity with hydrogen peroxide showed that Cys-232 has a pK(a) of 6.86 +/- 0.05, a value that is more than 1.5 pH units lower than that of a typical protein thiol. pH-induced conformational changes in the region surrounding Cys-232 were also examined and indicate that mildly acidic conditions induce a conformation that enhances Cys-232 reactivity. In summary, this work provides new insights into alpha1-antitrypsin reactivity in oxidizing environments and shows that a unique structural environment renders its unpaired thiol, Cys-232, its most reactive amino acid.

Relationship between protein structure and methionine oxidation in recombinant human alpha 1-antitrypsin

alpha 1-Antitrypsin is a metastable and conformationally flexible protein that belongs to the serpin family of protease inhibitors. Although it is known that methionine oxidation in the protein's active site results in a loss of biological activity, there is little specific knowledge regarding the reactivity of each of the protein's methionine residues. In this study, we have used peptide mapping to study the oxidation kinetics of each of alpha 1-antitrypsin's methionines in alpha 1-AT((C232S)) as well as M351L and M358V mutants. These kinetic studies establish that Met1, Met226, Met242, Met351, and Met358 are reactive with hydrogen peroxide at neutral pH and that each reactive methionine is oxidized in a bimolecular, rather than coupled, mechanism. Analysis of Met226, Met351, and Met358 oxidation provides insights regarding the structure of alpha 1-antitrypsin's active site that allow us to relate conformation to experimentally observed reactivity. The relationship between solution pH and methionine oxidation was also examined to evaluate methionine reactivity under conditions that perturb the native structure. Methionine oxidation data show that at pH 5, global conformational changes occur that alter the oxidation susceptibility of each of alpha 1-antitrypsin's 10 methionine residues. Between pH 6 and 9, however, more localized conformational changes occur that affect primarily the reactivity of Met242. In sum, this work provides a detailed analysis of methionine oxidation in alpha 1-antitrypsin and offers new insights into the protein's solution structure.

Expression of human α1 antitrypsin in transgenic sheep

We have recently described the production of large amounts (< or = 65 grams per litre) of enzymatically active human alpha 1 antitrypsin in the milk of transgenic sheep (Wright et al., 1991). Here, we describe in more detail the expression of the human protein in the milk of these animals throughout the lactation period. Human alpha 1 antitrypsin is also found at much lower levels in the plasma of transgenic ewes before, during and after lactation. It is also detected in male plasma at very low levels. We have previously shown human alpha 1 antitrypsin purified from transgenic sheep milk to be indistinguishable from commercially available human plasma derived alpha 1 antitrypsin in terms of gross sugar content and in vitro activity. Here we extend this comparison to more detailed analyses of glycosylation state, amino-terminal sequence, pI value, and molecular weight determination by mass spectrometry.

Purification and biochemical characterization of recombinant alpha 1-antitrypsin variants expressed in Escherichia coli

Site-directed variants of alpha 1-antitrypsin (alpha 1AT) expressed in a recombinant strain of Escherichia coli have been isolated with an overall process yield of 50% following tangential flow ultrafiltration, anion-exchange, immobilized metal affinity, and hydrophobic interaction chromatography. The primary structure of the purified variants including the integrity of the N- and C-termini has been verified by electrospray mass spectrometry of the intact molecules (44 kDa) for two of the variants (alpha 1AT Leu-358 and alpha 1AT Ala-357, Arg-358). Complementary classical peptide mapping and automated amino acid sequencing have verified 75% of the primary sequence of alpha 1AT Ala-357, Arg-358. Isoelectric focusing in an immobilized pH gradient revealed some microheterogeneity which proved to be reproducible from one purification batch to another. The isolated variants of alpha 1AT did not show any signs of proteolytic degradation during the purification process and proved to be fully active against their target proteases. The described process also allowed the complete removal of endotoxins from the preparations, opening the possibility to evaluate these novel protease inhibitors for their in vivo efficacy in different animal models of human disease.