Optical rotatory dispersion of cytochrome c phospholipid complexes

Hydrophobic interaction between the monomer of mitochondrial malate dehydrogenase and phospholipid membranes

Porcine mitochondrial malate dehydrogenase (EC 1.1.1.37) dissociates into subunits on dilution. The enzyme monomer caused large increases in the surface pressure of monolayers of 1:1 phosphatidylserine/phosphatidylcholine at air/water and oil/water interfaces. The monomer increased the permeability of phospholipid vesicles to 22Na+. Both effects were significantly greater than the corresponding effects of ribonuclease A, cytochrome c and the dimeric form of malate dehydrogenase. Changes in the circular-dichroism spectra of the enzyme indicated that conformational changes may be associated with dimer formation or when monomer interacts with lysophosphatidyl-choline. Similar interactions to those described may occur in situ when mitochondrial malate dehydrogenase is transported to the mitochondrial matrix from its site of synthesis on cytosolic ribosomes.

Lipid-polypeptide interactions in bilayer lipid membranes

The modifications of the electrical properties of bilayer lipid membranes (BLM) composed of cholesterol and an ionic surfactant upon interaction with charged polypeptides were studied. The addition of 10(-8) M polylysine (Ps(+)) to one side of anionic cholesterol dodecylphosphate BLM increases the specific membrane conductance over 1000-fold (from 10(-8) to 10(-5) mho/cm(2)) and develops a cationic transmembrane potential larger than 50 mV. This potential is reverted by addition of polyanions such as RNA, polyglutamic or polyadenilic acid to the same side on which Ps(+) is present, by addition of Ps(+) to the opposite side, or by addition of trypsin to either side. Both conductance and potential changes are hindered by increasing the ionic strength or by raising the pH of the bathing medium, disappearing above pH 11.5 where it is known that Ps(+) folds into an α-helix. The interaction of polyglutamic acid (PGA) with a cationic cholesterol-hexadecyltrimethylammonium bromide BLM results in increased membrane conductance and development of an anionic transmembrane potential which is reverted by addition of polycations to the same aqueous phase where PGA is present. Addition of either Ps(+) or PGA to one or both sides of a neutral BLM composed of 7-dehydrocholesterol induces no significant change. The observations suggest the formation of a lipid polymer membrane resultant from the interaction, predominantly electrostatic, of the isolated components. The implications of these results are discussed in terms of the current models of membrane structure.