Intrinsic differences in the perturbing ability of alkanols in bilayer: Action of phospholipase A2 on the alkanol-modified phospholipid bilayer

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Abstract
The kinetic parameters for the steady-state rate of hydrolysis of egg phosphatidylcholine in multilamellar vesicles by bee venom phospholipase A2 are measured in the presence of 27 alkanols and several organic solvents. In general, small nonpolar solutes like enflurane, tetrahydrofuran, benzene, chloroform and diethylether do not promote the hydrolysis of multilamellar vesicles. The rate of hydrolysis shows a biphasic dependence upon the alkanol concentration for all higher (C5–C9) alcohols examined, i.e., an optimal rate of hydrolysis is observed at a characteristic concentration for each alcohol. The alkanol to lipid mole ratio (D/L ratio) in the bilayer at the peak activating concentration of an alkanol was computed from its bilayer/water partition coefficient. The branched chain alcohols induce peak activation of hydrolysis at lowerD/L ratios in the bilayer than the corresponding straight chain analogs. Similarly, the longer chainn-alkanols at peak activating concentration have a lowerD/L ratio than the corresponding lower alcohols. Both theK m andV m for phosphatidylcholine increase as a function of the chain length of the activating alcohol. These kinetic parameters also depend upon the position of the substituents on the activating alcohols. Both theD/L ratio andV m for an alcohol are found to change with the cross-sectional area of the activating alcohol across its long axis: alcohols with a more asymmetric cross-section exhibit higherV m and a lowerD/L ratio. Such correlations ofV m andD/L ratio with the molecular parameters of the alkanols are interpreted to suggest that the accessibility of the substrate molecule in the bilayer to the phospholipase is modulated by the free space introduced by the alkanols in the bilayer. Effect of tetradecane derivatives and A2C (a membrane fluidizing agent) on the phase transition characteristics of DPPC bilayers, and their susceptibility to phospholipase A2 from bee venom and pig pancreas is also reported. These solutes cause a broadening of the transition profile and reduce the size of the cooperative unit and the enthalpy of transition. These effects depend upon the mole fraction of a solute in the bilayer; however, equal concentrations of these solutes do not induce equal response. Susceptibility of the modified bilayers to phospholipase A2 depends not only upon the structure of the solute but also upon the source of the enzyme. The data show that the activity of the membrane-bound enzyme is modulated to different extents by different solutes, and the bilayer perturbing ability of these solutes may be related to the asymmetry of their cross-sectional area and to the free space introduced by the alkanols in a bilayer.