Membrane lipid phase change

The effects of membrane order on enzymatic activity observed for the Na+-K+-ATPase of hamster ovary cells mirror effects noted for a wide range of membrane-localized proteins involved in enzymatic activity, transport, and cellular signaling. There is a general tendency for proteins to exhibit increased activity as the order of their lipid microenvironment decreases, albeit there is a limit to this relationship at extremes of temperature where membranes undergo phase changes that disrupt function of membrane-localized proteins. 

No drastic change occurred in tail sodium current. When tetramethrin was added to the BTX-treated axon, a large and prolonged tail current characteristic of the tetramethrin modified sodium channel developed. Thus tetramethrin binds to a site different from the binding site of BTX which is located inside of the channel. This result is compatible with the hypothesis that the pyrethroid molecules bind to the channel gating machinery via the membrane lipid phase thereby altering the kinetics of channel gating. 

Correlations between critical temperatures for membrane lipid phases determined with EPR techniques and discontinuities in Arrhenius plots have also been shown for the ATPase from sarcoplasmic reticulum (Eletr and Inesi, 1972), for UDP-glucuronyltransferase, and for G-6-Pase (Eletr et al., 1973) from liver microsomes. In the case of the microsomal membranes, perturbation of the membrane lipids by treatment with detergents or phospholipase A leads to linear Arrhenius plots for both enzyme activities and Tq between 5° and 30 C. For UDP-glucuronyltransferase, the phase change in the lipids also results in a loss of substrate specificity and a loss of sensitivity to an allosteric effector (Vessey and Zakim, 1974). 

Figure 1. A model which relates a change in the fluidity of the membrane lipid phase to a change in the conformation of a tightly-bound protein. On the left is shown a liquid-crystalline lipid phase which will accommodate a relatively large portion of the protein molecule. On the right is represented a gel phase lipid bilayer from which the protein has been partially extruded, thereby exposing hydrophobic residues on the protein to water the protein will thus rearrange its conformation to internalize these hydrophobic residues.

An emergent field dealing with the physical/chemical processes that underlie the changes of lipid phase state during such cellular events as membrane fusion, vesicle trafficking, and cell disjunction. 

Membrane conformational changes are observed on exposure to anesthetics, further supporting the importance of physical interactions that lead to perturbation of membrane macromolecules. For example, exposure of membranes to clinically relevant concentrations of anesthetics causes membranes to expand beyond a critical volume (critical volume hypothesis) associated with normal cellular function. Additionally, membrane structure becomes disorganized, so that the insertion of anesthetic molecules into the lipid membrane causes an increase in the mobility of the fatty acid chains in the phospholipid bilayer (membrane fluidization theory) or prevent the interconversion of membrane lipids from a gel to a liquid form, a process that is assumed necessary for normal neuronal function (lateral phase separation hypothesis). 

Because the area under the peaks in the scanning calorimeter is proportional to the heat of transition, the instrument can be calibrated by running a known amount of membrane lipid suspended in water. It is necessary to assume, of course, that all of the lipid is in the bilayer conformation in water. If the lipid content of the membranes is known, the fraction of the lipids contributing to the peak observed for the membranes can then be calculated by comparing peak areas for the membranes and the lipids in water. Our preliminary results using this approach indicate that at least 60% of the lipids in the membranes participate in the phase change. Work is in progress to obtain more precision. 

Interpretation of the Calorimetric Results. There is little doubt that the transition observed in M. laidlawii membranes arises from the lipids since it occurs at the same temperature in both intact membranes and in water dispersions of membrane lipids. It is reasonable to conclude that in both membranes and membrane lipids the lipid hydrocarbon chains have the same conformation. The lamellar bilayer is well established for phospholipids in water (I, 20, 29) at the concentration of lipids used in these experiments. In the phase change the hydrocarbon core of the bilayer undergoes melting from a crystalline to a liquid-like state. Such a transition, like the melting of bulk paraffins, involves association between hydrocarbon chains and would vanish or be greatly perturbed if the lipids were apolarly bound to protein. We can reasonably conclude that most of the lipids in M. laidlawii membranes are not apolarly bound to protein. 

The influence of substituent size, polarity, and location on the thermotropic properties of synthetic phosphatidylcholines has been studied by Menger et al. [18], The effect of increasing membrane curvature on the phase transition has been investigated by DSC and FTIR [19]. In addition, a data bank, LIPIDAT, on lipid phase transition temperatures and enthalpy changes is available [20, 21],... 

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