The molecular weight of rabbit muscle aldolase and the properties of the subunits in acid solution

Abstract

Re-examination of the weight-average molecular weight of rabbit muscle aldolase in 0.15 m NaCl by the high-speed sedimentation equilibrium and field relaxation methods gave values of 160,000 ± 1500 and 162,500, respectively. When the protein was allowed to dissociate in an acidic solution (pH 2.0, μ = 0.06) the weight-average molecular weight was estimated as 40,000 ± 2000. The results are consistent with a structure composed of four polypeptide chains. Extensive polydispersity was observed after the protein was dialyzed in acidic buffer for more than 3 days in the cold. The thermodynamic parameters calculated for the breakdown of carboxymethylated aldolase under this condition at two temperatures suggested that the subunits are degraded in acid, with the cleavage of one or more acid-labile peptide bonds. O-Phenanthroline oxidized aldolase, containing approximately four disulfide bonds per protein molecule, showed evidence of degradation in acid only after treatment with mercaptoethanol. Intra-subunit disulfide bridges in the oxidized enzyme appear to hold the fragments together.