(a) Nondenaturing PAGE (7.5-15% wt/vol) showing that a mixture of IZ α₁-antitrypsin loses protein from the monomeric band and forms high molecular mass polymers more readily than I α₁-antitrypsin alone. The proteins were incubated at 2 mg/ml and 41°C in 50 mM Tris, 50 mM KCl (pH 7.4). All lanes contain 10 μg protein. Top: I α₁-antitrypsin; bottom: IZ α₁-antitrypsin. Lane 1, time 0; lane 2, 1 day; lane 3, 2 days; lane 4, 3 days; lane 5, 6 days; lane 6, 12 days. (b) Rate of polymerization of M, I, S, and Z α₁-antitrypsin mutants at 0.1 mg/ml and 45°C determined from the measurement of intrinsic tryptophan fluorescence. The values for the rate of polymerization (Table 1) were obtained from fitting the data to Equation 1 and are the weighted mean and standard error of three (I, S, and Z α₁-antitrypsin) or four (M α₁-antitrypsin) experiments. The rate of polymer formation of mixtures of IZ and MZ α₁-antitrypsin were calculated using a 1:1 mixture of the variants at 45°C. The concentration of each variant in the mixture was half that shown in b in order to keep the final protein concentration at 0.1 mg/ml. The curves for each component of the reaction were dissected from the final profile, and the rate of each component in the mixture was obtained by fitting the data to Equation 1. The results shown in Table 2 are the weighted mean and SE of two kinetic experiments.