Studies on the Structure and Function of Muscle Aldolase

In 0.05 m potassium phosphate buffer (pH 7.0, 20°), native rabbit muscle aldolase has been found to exhibit a molecular weight (Mn ∼ Mw ∼ Mz) close to 160,000, in agreement with previous findings. Upon exposure to cold alkaline borate buffer (pH 12.5, µ = 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 NH2-terminal 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, but no evidence is obtained for other NH2-terminal residues except for a small amount of proline.