Spectral studies on two genetic forms of the human serum proteinase inhibitor, alpha-1-antitrypsin

alpha-1-antitrypsin, the major inhibitor of proteolytic enzymes in human serum, was isolated from normal individuals (protease inhibitor type MM) and from those with an inherited deficiency (protease inhibitor type ZZ) of circulatory protein. The two proteins were compared by circular dichroism spectroscopy, and by fluorescence quenching experiments using anionic (I-), and neutral (acrylamide) probes. Both proteins share a similar secondary structure, i.e. approximately 45--50% alpha-helix and 15--20% beta-structure. Evidence was accumulated to show that the microenvironment in the vicinity of the three tryptophanyl residues is altered in Z form as compared to the M form as shown by (a) the absence of the positive dichroic band in the region 290--300 nm of the circular dichroism spectra, (b) a greater than 50% increase in quantum yield in the tryptophanyl fluorescence emission spectra, (c) an increased accessibility of tryptophan to quenching by iodide, and (d) acrylamide quenching experiments which indicate that all tryptophanyl residues in the Z protein are quenched equally or that quenching is dominated by a single residue, while in the M protein, heterogeneous quenching occurs. The potential significance of these findings in terms of alpha-1-antitrypsin deficiency state are discussed.