DISSOCIATION OF PIGEON-LIVER MALIC ENZYME IN REVERSE MICELLES

Pigeon-liver malic enzyme has a tendency to aggregate at a large conce ntration of protein. The larger aggregates (hexamer and octamer) were demonstrated to be enzymically active with specific activity similar t o that of the tetramer. When the enzyme was embedded in a reverse mice llar system prepared by dissolving the surfactant sodium bis(2-ethylhe xyl)-sulfosuccinate (AOT) in isooctane, the tetrameric enzyme dissocia ted into monomers. The dissociated monomers were also enzymically acti ve but with diminished specific activity relative to the activity in a queous media. The decreased enzyme activity in reverse micelles was du e to interactions of surfactant with the enzyme molecules, suggesting that the cytosolic malic enzyme is located near the plasma membrane. W hen the dissociation was monitored by altering the degree of hydration of the system (represented by the ratio [H2O]/[AOT]), the detergent a nd organic solvent slightly affected K-TD, the dissociation constant o f tetramer to dimers (T reversible arrow 2 D), but increased K-DM, the dissociation constant of dimer to monomers (D reversible arrow 2 M), by 1-2 orders of magnitude; this change caused a 2-3 orders of magnitu de increase in the overall dissociation constant K-TM (T reversible ar row 4 M). The dissociation of the tetrameric malic enzyme to monomers was favored by approximately 16 kJ/mol in AOT/isooctane reverse micell es versus aqueous media. We propose water-shell and induced-fit models for the enzyme in AOT/isooctane reverse micelles at large and small [ H2O]/[AOT] ratios to explain this data, respectively. The asymmetric q uaternary structure of the enzyme [Lee, H. J. & Chang, G. G. (1990) FE BS Lett. 277, 175-179] was re-evaluated in terms of the subunit intera ctions and various interconvertible enzyme forms.