Circular dichroism of chemically modified human plasma alpha1-antitrypsin. Interaction with porcine elastase

Chemical modifications of human plasma alpha1-antitrypsin with reagents which modify lysyl residues (citraconic anhydride, acetic anhydride, formaldehyde and 2,4,6-trinitrobenzenesulfonic acid) and arginyl residued (1,2-cyclohexanedione) were examined with regard to their effect upon the elastase inhibitory capacity of the glycoprotein. 2,4,6-Trinitrobenzenesulfonic acid was employed to quantitate the remaining free amino groups (epsilon-NH2 groups of lysine) and the extent of modifications. Amino acid analysis was utilized in the same capacity for the guanidino groups of arginyl residues. The elastase inhibitory capacity of alpha1-antitrypsin was destroyed following trinitrophenylation, citraconylation and acetylation. Circular dichroism of the native and modified derivatives revealed major changes in conformation following trinitrophenylation and citraconylation while CD profiles of acetylated and reductively methylated derivatives differed from that of the native profile considerably less. Reductively methylated alpha1-antitrypsin retained its elastatse inhibitory capacity. The reaction of 1,2-cyclohexanedione with alpha1-antitrypsin did not effect in a loss in inhibitory capacity. Gel filtration studies of native and modified alpha1-antitrypsin on Sephadex G-100 demonstrated an increased molecular weight presumably through molecular aggregation, in the citraconylated and trinitrophenylated derivatives, but not in the cases of the other derivatives. Based upon these studies and previous investigations of our laboratory, it was concluded that (1) alpha1-antitrypsin is a lysyl inhibitor type (i.e., the reactive site is a Lys-X bond), (2) its interaction with elastase follows a pattern similar to trypsin and chymotrypsin, and (3) the positively charged epsilon-NH2 group of lysine is essential for the maintenance of elastase inhibitory capacity.