Loop-sheet polymerization: the structural basis of Z alpha 1-antitrypsin accumulation in the liver

1. The Z deficiency variant of alpha 1-antitrypsin predisposes the homozygote to early-onset panlobular emphysema and results in the accumulation of antitrypsin within the hepatocyte, which leads to hepatocellular damage and cirrhosis. The mechanism of this accumulation has been shown to be due to the Z mutation (Glu-342-->Lys) perturbing the structure of the protein, allowing a unique interaction between the reactive-centre loop of one molecule and the A sheet of a second. This loop-sheet polymerization occurs spontaneously at 37 degrees C in purified plasma Z but not M antitrypsin. The rate of polymerization is greatly accelerated at 41 degrees C and is blocked by the insertion of a specific peptide into the A sheet of the antitrypsin molecule. Electron microscopy and circular dichroic spectral analysis confirm that the Z antitrypsin polymers formed in vitro have structural identity with those isolated from the liver of a Z homozygote. 2. That loop-sheet polymerization is a more general phenomenon was shown by the examination of a second deficiency variant, antitrypsin Siiyama (Ser-53-->Phe), which is also associated with liver inclusions. Electron microscopy confirmed that isolated antitrypsin Siiyama from the plasma of a homozygote was present as long chains of polymers identical with those of Z antitrypsin.