Studies on the structure and function of muscle aldolase. IV. The action of dilute alkali on primary structure and its effect on the determination of subunit molecular weight.

In 0.05 M potassium phosphate buffer (pH 7.0, ZO”), native rabbit muscle aldolase has been found to exhibit a molecular weight (mn N g, pZ) close to 160,000, in agreement with previous findings. Upon exposure to cold alkaline borate buffer (pH 12.5, p = 0.17, 0”), the enzyme spontaneously dissociates into its monomers which subsequently undergo slow hydrolytic degradation. Compensating for electrostatic charge effects and extrapolating the data to zero time in alkali, a subunit molecular weight of 41,400 to 42,000 is obtained. If the native enzyme is exposed to alkaline conditions at 20°, the subunits undergo an initial rapid degradation which eventually yields alkali-resistant material. Similar results are obtained with succinyl aldolase subunits under equivalent conditions. Acrylamide gel electrophoresis of alkali-treated succinyl aldolase (pH 12.5,30’) results in the appearance of a limited number of electrophoretic bands. This finding suggests that specific alkali-labile peptide bonds are involved in the degradation process. As a further consequence of the action of alkali, native aldolase shows increased solubility properties. Both native and succinyl aldolase manifest new NH&erminal serine, threonine, and glycine residues after treatment with borate buffer at pH 12.5 and 30’. Prolonged exposure to alkali results in increased liberation of the above end groups,