Pigeon-liver malic enzyme has a tendency to aggregate at a large concentration of protein. The larger aggregates (hexamer and octamer) were demonstrated to be enzymically active with specific activity similar to that of the tetramer. When the enzyme was embedded in a reverse micellar system prepared by dissolving the surfactant sodium bis(2-ethylhexyl)-sulfosuccinate (AOT) in isooctane, the tetrameric enzyme dissociated into monomers. The dissociated monomers were also enzymically active but with diminished specific activity relative to the activity in aqueous media. The decreased enzyme activity in reverse micelles was due to interactions of surfactant with the enzyme molecules, suggesting that the cytosolic malic enzyme is located near the plasma membrane. When the dissociation was monitored by altering the degree of hydration of the system (represented by the ratio [H2O]/[AOT]), the detergent and organic solvent slightly affected KTD, the dissociation constant of tetramer to dimers (T <--> 2 D), but increased KDM, the dissociation constant of dimer to monomers (D <--> 2 M), by 1-2 orders of magnitude; this change caused a 2-3 orders of magnitude increase in the overall dissociation constant KTM (T <--> 4 M). The dissociation of the tetrameric malic enzyme to monomers was favored by approximately 16 kJ/mol in AOT/isooctane reverse micelles 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 quaternary structure of the enzyme [Lee, H. J. & Chang, G. G. (1990) FEBS Lett. 277, 175-179] was re-evaluated in terms of the subunit interactions and various interconvertible enzyme forms.