Increase of the fluidity of the lipid bilayer of the inner mitochondrial membrane by succinate and phenylsuccinate: a study by EPR and fluorescence

The kinetic aspects of intracellular fluorescence labeling with TMA-DPH support the maturation model for endocytosis in L929 cells

TMA-DPH (1-(4-trimethylammonium)-6-phenyl-1,3,5-hexatriene), a hydrophobic fluorescent membrane probe, interacts with living cells by instantaneous incorporation into the plasma membrane, where it becomes fluorescent. It then follows the intracellular constitutive membrane traffic and acts as a bulk membrane marker of the endocytic pathway (Illinger, D., P. Poindron, P. Fonteneau, M. Modolell, and J. G. Kuhry. 1990. Biochim. Biophys. Acta. 1030:73-81; Illinger, D., P. Poindron, and J. G. Kuhry. 1991. Biol. Cell. 73:131-138). As such, TMA-DPH displays particular properties mainly due to partition between membranes and aqueous media. From these properties, original arguments can be inferred in favor of the maturation model for the endocytic pathway, against that of pre-existing compartments, in L929 cultured mouse fibroblasts. (a) TMA-DPH labeling is seen to progress from the cell periphery to perinuclear regions during endocytosis without any noticeable loss in fluorescence intensity; with a vesicle shuttle model this evolution would be accompanied by probe dilution with a decrease in the overall intracellular fluorescence intensity, and the labeling of the inner (late) compartments could in no way become more intense than that of the peripheral (early) ones. (b) From TMA-DPH fluorescence anisotropy assays, it is concluded that membrane fluidity is the same in the successive endocytic compartments as in the plasma membrane, which probably denotes a similar phospholipidic membrane composition, as might be expected in the maturation model. (c) TMA-DPH internalization and release kinetics are more easily described with the maturation model.

Intramembraneous hydrogenation of mitochondrial lipids reduces the substrate availability, but not the enzyme activity of endogenous phospholipase A. The role of polyunsaturated phospholipid species

(1) Isolated rat liver mitochondria were subjected to catalytic hydrogenation using a water-soluble Pd complex and molecular H2. This treatment resulted in a reduction of double bonds on phospholipid acyl chains as judged by gas chromatography of fatty acid methyl esters and HPLC of dinitrobenzoyldiacylglycerols. (2) After hydrogenation, mitochondria lost their ability to hydrolyze endogenous phospholipids in alkaline, Ca2+ containing medium, while phospholipase A2 retained full activity against exogenous substrates, regardless of whether those substrates were hydrogenated or not. (3) Inhibition by hydrogenation of endogenous phospholipid hydrolysis correlated with the loss of polyunsaturated fatty acyls, rather than with changes of the bulk membrane fluidity as measured by ESR and fluorescence studies. (4) These data suggest that the unsaturation of mitochondrial membrane lipids might be important for regulation of phospholipid breakdown by endogenous phospholipases. In particular, polyunsaturated molecular species seem to be involved in making phospholipids accessible to phospholipase A-mediated hydrolysis.

Triggering of mannosyltransferase activity in inner mitochondrial membranes by dolichyl-monophosphate incorporation mediated through phospholipids or fatty acids

The activity of GDPmannose:dolichyl monophosphate mannosyltransferase in inner mitochondrial membranes can be triggered by dolichyl-monophosphate incorporation mediated through phospholipids or fatty acids. The efficiency of this incorporation and the efficiency of the enzyme activity are not equivalent. Among a variety of amphiphiles which were tested, the highest mannosyltransferase activity was obtained with the mixture of lipids extracted from the outer mitochondrial membranes. The results presented here appear consistent only with a mechanism involving collisional contacts of the phospholipid vesicles and fusion with the membranes. ESR spectroscopy confirms that (a) the incorporation process is followed by solubilization of dolichyl monophosphate molecules in the lipid phase and (b) the general organization of the inner mitochondrial membranes is not perturbed by the addition of dolichyl monophosphate.

Succinate potentiates the action of platelet agonists

Succinate potentiated the actions of ADP, epinephrine, and the endoperoxide analog U46619 on platelets in plasma. Potentiation was observed at concentrations as low as 400 microM. Potentiation occurred immediately and did not change during incubations as long as five min. It was also observed with maleate and malonate but not with fumarate, oxalate, glutarate or succinate analogs with one or both carboxyls blocked. Succinate was able to reverse the inhibition of platelet activation due to elevated cAMP, and succinate and maleate were comparable to ADP in their ability to inhibit PGI2-induced synthesis of cAMP.