Comparison of the structure and aspects of the proteinase-binding properties of cystic fibrotic alpha-macroglobulin with normal alpha 2-macroglobulin.

Considerable attention has been focused recently on alpha 2-macroglobulin (alpha 2M), a major endopeptidase inhibitor in blood plasma, as a possible source of the primary defect in cystic fibrosis (CF). We report here studies designed to compare the structure of CF alpha 2M with normal alpha 2M to determine if there is a difference. The physicochemical properties of purified alpha 2M as revealed by various electrophoretic techniques, covalent proteinase binding properties, and primary structural studies on a variety of partial hydrolyzates of CF alpha 2M and normal alpha 2M are compared. These studies were carried out on eight different individual isolates of CF alpha 2M and three age-matched normal alpha 2M preparations and alpha 2M isolated from fetal cord blood. Three properties of CF alpha 2M were studied by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE): (1) the existence of four identically-sized subunits in the native molecule (10), (2) the cleavage of this subunit into fragments of approximately 100,000 daltons upon interaction with proteinases (10), and (3) the cleavage of an alkaline/heat sensitive bond to produce 120,000 and 60,000 dalton fragments (11). Both CF and normal alpha 2M were cleaved to the extent of 79-87%, CF alpha 2M behaves identically with normal alpha 2M with regard to all these properties. Salvesen and Barrett (24) have demonstrated that varying proportions of several [125I]-labeled proteinases form SDS-stable, non-reducible links to normal alpha 2M. Two of the CF alpha 2M preparations were studied to determine if similar covalent binding of proteinases occurred. The positions of the labeled and % of proteinase bound bands in SDS/reduced PAGE system were identical for normal alpha 2M and CF alpha 2M. These results indicate that CF alpha 2M behaves normally with regard to covalent binding of proteinases. Qualitative comparison of the peptide fragments separated by SDS-PAGE or isoelectric focusing of CF and normal alpha 2M produced by partial proteolysis with trypsin, chymotrypsin or Staphylococcus aureus V-8 proteinase did not reveal any difference unique to CF alpha 2M. The cyanogen bromide fragmentation studies and the cysteine cleavage studies also indicated that no major change in the positions of methionyl residues or cysteinyl/cystinyl residues has occurred in CF alpha 2M. The failure of all these different studies and those reported by others to demonstrate any differences between CF and normal alpha 2M makes it highly unlikely that there is a primary defect in alpha 2M in CF.