The insertion of D-beta-hydroxybutyrate apodehydrogenase into phospholipid monolayers and phospholipid vesicles

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)

Interactions of the mitochondrial membrane rat liver D-3-hydroxybutyrate dehydrogenase with glass beads during adsorption chromatography. Relationships with the activation of the enzyme by phospholipids

D-3-Hydroxybutyrate dehydrogenase (BDH) is an NAD(+)-dependent dehydrogenase of the mitochondrial inner membrane involved in the energetic balance between the liver and peripheral organs in mammals. It allows the conversion of ketone bodies (acetoacetate and D-3-hydroxybutyrate) and it is one of the best documented lipid-requiring enzymes with a dependence on lecithins. After release of proteins from the membrane by phospholipase A2 treatment of salt-treated mitochondria, the rat liver enzyme is absorbed on controlled-pore glass beads. After batch washing, the enzyme, devoid of lipids (apoBDH), is specifically eluted at pH 8.05-8.15 with a 0.1 M Tris-1 M LiBr buffer under reducing conditions (5 mM dithiothreitol). It appears that during BDH absorption, the glass beads mimic the phospholipid surface of biomembranes.

Noncooperative vs. cooperative reactivation of D-beta-hydroxybutyrate dehydrogenase: multiple equilibria for lecithin binding are determined by the physical state (soluble vs. bilayer) and composition of the phospholipids

D-beta-Hydroxybutyrate dehydrogenase (BDH) is a lecithin-requiring mitochondrial enzyme that catalyzes the interconversion of beta-hydroxybutyrate and acetoacetate. The purified soluble enzyme devoid of lipid (i.e., the apodehydrogenase) can be reactivated with soluble lecithin or by insertion into phospholipid vesicles containing lecithin. Lipid activation curves have a sigmoidal shape, and two models have been proposed to explain them. We have previously reported that the kinetics of reactivation with short-chain lecithins in the soluble state is consistent with a model in which the enzyme enzyme contains two identical, noninteracting lecithin binding sites, both of which must be occupied to activate the enzyme [noncooperative mechanism; Cortese, J.D., Vidal, J.C., Churchill, P., McIntyre, J.O., & Fleischer, S. (1982) Biochemistry 21, 3899-3908]. More recently a kinetic model involving cooperative interactions between lecithin binding sites was proposed for the reactivation of the membrane-bound enzyme [Sandermann, H., Jr., McIntyre, J.O., & Fleischer, S. (1986) J. Biol. Chem. 261, 6201-6208]. This study reinvestigates the basis for the different conclusions in these two studies. The previous study with soluble lecithins was limited to about 34% of maximal activation compared with mitochondrial phospholipid, due to inactivation of the enzyme at the critical micellar concentration. We could now extend this study to 91% activation by increasing the ethanol concentration. This experimental evidence confirms that the soluble system follows a noncooperative equation. We provide a new kinetic approach to test the cooperative model. A velocity equation is derived for a Hill-type cooperative ligand binding system interacting with a mixture of ligands. This equation predicts a proportionality between an overall weighted cooperative dissociation constant [Kcoop(w)] and a dissociation constant for a single lecithin (PC) species from interacting sites (KPC), regulated by the PC molar fraction (XPC): 1/Kcoop(w) = XPC/KPC. The equation was applied to the data of Sandermann et al. [Sandermann, H., Jr., McIntyre, J.O., & Fleischer, S. (1986) J. Biol. Chem. 261, 6201-6208] as well as to newly obtained data. The results obtained over a wide range of PC molar fractions and different mixtures of bilayer phospholipids fit this equation, confirming the cooperative behavior. We conclude that BDH has a different mode of reactivation depending on the nature of the lipid environment. With soluble lecithin, the activation is noncooperative, whereas in the bilayer, mixtures of phospholipids give cooperative behavior that fits a Hill equation.(ABSTRACT TRUNCATED AT 400 WORDS)

Direct evidence for the formation of a monolayer from a bilayer. An ellipsometric study at the nitrogen-water interface

Direct evidence for the formation of a monolayer from a bilayer was measured by ellipsometry after spreading unilamellar vesicles of dioleoyl phosphatidylcholine (DOPC) at the nitrogen-water interface. The ellipsometric isotherms of DOPC vesicles and DOPC spread from an organic solvent were compared and found similar. From the observed ellipsometric angle (delta delta) in the plateau region (-1.04 degrees) and literature data for refractive indices of an anisotropic film similar to DOPC, we have calculated a thickness of 20 +/- 1 A. These results strongly suggest that, similarly to DOPC spread from an organic solvent, DOPC vesicles form a monolayer when spread at the nitrogen-water interface.

Interactions between apo-(D-beta-hydroxybutyrate dehydrogenase) and phospholipids studied by intrinsic and extrinsic fluorescence

Interactions of D-beta-hydroxybutyrate dehydrogenase with phospholipids were investigated by both intrinsic- and extrinsic-fluorescence approaches. The intrinsic fluorescence, mainly caused by tryptophan residues, increased upon re-activation in the presence of phospholipids bearing a positive charge, i.e. phosphatidylcholine, but decreased in the presence of non-re-activating phospholipids with a negative charge. This indicates either that the environment of tryptophan residues is affected by charges rather than by hydrophobic chains of phospholipids, or that the enzyme undergoes different conformational changes depending on the nature of the phospholipids. On the other hand, the graph of the temperature-dependence of the fluorescence intensities of the enzyme embedded in dimyristoylphosphatidylcholine liposomes exhibits a break around 21 degrees C. This indicates either that at least one tryptophan residue is closely in contact with the hydrophobic chains of phospholipids or that there is a change in the environment of tryptophan residues owing to the physical state of the phospholipids. The addition of D-beta-hydroxybutyrate apo-dehydrogenase to phospholipid liposomes containing diphenylhexatriene (a fluorescent probe) increased the diphenylhexatriene fluorescence polarization. Moreover, there was a partial fluorescence energy transfer from tryptophan to diphenylhexatriene. These results strongly favour the possibility that there is a portion of the enzyme polypeptide chain inserted into the phospholipid hydrophobic region. All these results demonstrate that D-beta-hydroxybutyrate apo-dehydrogenase interacts with both polar and hydrophobic parts of phospholipids and leads to small, but essential, conformational changes of the enzyme.

Lipid-protein interactions in biomembranes studied through the phospholipid specificity of D-beta-hydroxybutyrate dehydrogenase

Since the biological membranes are fundamental units in the living cells, the studies of lipid-protein interactions are crucial for the understanding of their structure, functions and properties. Beside hydrophobic interactions between fatty acids chain of phospholipids and intrinsic membrane proteins, the interactions between charged groups of the protein with the polar heads of phospholipids generally confer the specificity which may be absolute or preferential. This paper reports essential results obtained these last few years with D-beta-hydroxybutyrate dehydrogenase (BDH) from inner mitochondrial membrane, one of the most interesting and best documented examples of a lipid-requiring enzyme. This is a review of the molecular basis--knowledge and strategy of study--of the lipid specificity for membrane protein functions.

Kinetic aspects of the role of phospholipids in D-beta-hydroxybutyrate dehydrogenase activity

Interactions of phospholipids with D-beta-hydroxybutyrate dehydrogenase (BDH), a lecithin-requiring enzyme, have been studied by a kinetic approach. The process of reactivation of BDH by phospholipids, which follows a second-order mechanism, reveals that (1) at least 2 mol of lecithins is essential for the reactivation of the enzyme, and (2) the enzyme contains two dependent binding sites for lecithins. The graphic representation of the time course of reactivation shows a latent phase which decreases when there is an increase in the amount of phospholipids. A Scatchard plot treatment of the reactivation kinetic data reveals the presence of two classes of phospholipid binding sites, which exhibit high and low affinities related to the binding of four and two lecithin molecules, respectively. The effect of temperature on BDH activity and on the inactivation of the apoenzyme with N,N'-dicyclohexylcarbodiimide (a specific carboxyl reagent) or with phenylglyoxal (a specific arginine reagent) shows a break at 22-24 degrees C, indicating a slight structural change in the enzyme-active site around this temperature. In addition, the variations in enzyme kinetic parameters, according to the nature of phospholipids, are in agreement with conformational changes related to the nature and to the fluidity state of phospholipids. However, the apparent NAD+ binding constant does not depend on the phospholipid's fluidity.