TRANSITIONAL STEPS IN THE SOLUBILIZATION OF PROTEIN-CONTAINING MEMBRANES AND LIPOSOMES BY NONIONIC DETERGENT

Membrane solubilization by dodecyl maltoside was studied, using Ca2+-ATPase membranes and liposome preparations as prototypes of biological membranes. In equilibrium dialysis experiments, transition from saturable incorporation of monomeric detergent into the membrane to cooperative binding already occurred at a free detergent concentration about 50% of the cmc. This transition was discontinuous for unilamellar liposomes of dioleoylphosphatidylcholine, but gradual for Ca2+-ATPase membranes and multilayered liposomes of sarcoplasmic reticulum lipid. Equilibrium detergent binding by Ca2+-ATPase membranes (expressed on the basis of lipid content) was the same as for detergent binding by multilamellar liposomes of sarcoplasmic reticulum lipid. Equilibration involving cooperative binding was considerably delayed (for many days) if detergent was presented gradually to the membranous preparations in nonmicellar form by diffusion across the dialysis membrane, while equilibration of detergent occurred rapidly if detergent in micellar form was added directly to the membrane preparations. In contrast, equilibration was rapid in both directions if detergent was added at levels below that required to initiate cooperative binding. Detergent interaction resulted in a biphasic decrease in light scattering of Ca2+-ATPase membranes. The first of these decreases coincided with the onset of cooperative binding, while the second one was associated with a decreased sedimentability during ultracentrifugation, i.e., with usual criteria of solubilization. The concentration at which this occurred corresponded to t he level of free detergent at which lipid, after detergent solubilization, segregated from detergent after gel chromatography. By freeze-fracture electron microscopy, the decreases in light scattering were correlated with membrane reorganization, leading to the appearance of large holes in the vesicles before complete fragmentation into membrane sheets of diminishing size and solubilization of Ca2+-ATPase as monomers and oligomers. The data indicate the existence of saturable uptake of nonmicellar detergent by the lipid phase of the membranes, followed by cooperative interaction, ultimately leading to solubilization. Rapid solubilization appears to require direct interaction of detergent micelles with membranes. Except for unilamellar liposomes of dioleoylphosphatidylcholine, our results are not consonant with an abrupt transition from the membra nous to the detergent-solubilized state, but rather indicate a gradual mechanism of solubilization via fragmented vesicles, lamellar sheets, and intermediary solubilized forms.