Two forms of alpha-l-antitrypsin (α₁-AT), α₁-AT(I), and α_I-AT(II), were separately obtained from human sera which had been pooled and stored for 3 to 6 months at -20°C by a combination of ion-exchange chromatography, gel-filtration, and affinity chromatography.
The N-terminal amino acid sequences of α₁-AT(I) and α_I-AT(II) were tentatively determined as Glu-Asp-Pro-Gln-Gly-Asp-Ala-Ala-Gln-Glu-Thr-Asp-Thr-Ser-His-(His)- and Glu-Thr-Asp -Thr-Ser-Hi s-His-Asp-G In-Asp-Ser-Pro-Thr-Phe-Asn-Lys-Leu-Thr-Pro-Asn-Leu-Ala-Glu-Phe-, respectively. The amino acid sequence of α₁-AT(II) corresponds to the sequence beginning at the tenth glutamic acid residue from the N-terminus of α₁-AT(I). Thus, α₁-AT(II) may arise by cleavage of the Gln-Glu bond in α₁-AT(l) by an enzyme either originally present in the serum or produced by contaminating microorganisms. Based on a combination of the N-terminal sequences of α₁-AT(l) and α₁-AT(II), the sequence of the N-terminal 33 residues of α₁-AT is tentatively proposed.
α₁-AT isolated from fresh human plasma had the same N-terminal amino acid sequence as α₁-AT(I).
The C-terminal amino acid sequences of the three ai AT's were identified as -Gln-Lys by the carboxypeptidase digestion method.
Amino acid analysis of performic acid-oxidized α₁-AT showed that this protein contained one mole of cysteine residue per mole of protein, which was linked to a free cysteine through a disulfide bond.
Most of the α₁-AT preparations inhibited bovine trypsin at a molar ratio of 1:1. However, certain preparations bound more than one mole of trypsin per mole of inhibitor.