Lipid-protein interactions in membranes: effect of lipid composition on mobility of spin-labeled cysteine residues in yeast plasma membrane

Response of the electrochromic dye, merocyanine 540, to membrane potential in rat liver mitochondria

Merocyanine binds extensively to rat liver mitochondria in spite of the presence of a sulfonic acid group which would suggest only limited penetration through the membrane. Passive binding shows both tight and weak binding components and is dependent on salt concentration and ionic strength in accord with the Gouy-Chapman theory. The binding of merocyanine to mitochondria is accompanied by both a fluorescence enhancement and a spectral shift. Induction of an electrical field by either respiration or K+ diffusion potential results in a partial reversal of the spectral shift seen on dye binding. At low temperature, the merocyanine spectral response to an electrical field is biphasic, consisting of a fast phase with a t1/2 of less than 1 sec at 15 degrees C and a slower phase which may vary considerably in rate and extent with conditions. The spectral shift during the two phases appears similar, but differ in sensitivity to ionic strength and temperature. The spectral shift during the fast phase at 15 degrees C indicates that the major component is a decrease in bound monomer and an increase in the aqueous dimer, indicating an "on-off" mechanism. It is suggested that the fast and slow phases of the merocyanine response may be due to two different populations of dye, possibly located at the outer and inner surfaces, respectively, of the mitochondrial membrane. The electrophoretic movement of the dye located in the membrane interior would result in the temperature-sensitive slow phase response. Demonstration of the proportionality of the fast phase response to the magnitude of the membrane potential suggests the usefulness of merocyanine in studies with mitochondrial systems.

A requirement for cholesterol and its structural features for a human macrophage-like cell line

The lipid requirements of a human macrophagelike cell line were studied. The cells grew only about one generation in a medium supplemented with delipidated serum; during the growth the cholesterol content of the cells was depleted. Growth was restored by including in the medium serum lipids subjected to alkaline hydrolysis or cholesterol. The extent of growth was dependent on cholesterol concentration. No growth was obtained with 5-cholestene, 5-cholesten-3-one, cholesteryl chloride, coprostanol, beta-sitosterol, or stigmasterol. Very limited growth occurred with cholesterol methylether, epicholesterol, or beta-cholestanol. Therefore, for optimal growth of these cells there is a stringent requirement for the structural features of cholesterol, which include a 3-beta OH group, a delta 5-double bond, a trans ring A/B configuration, and freedom of the side chain from bulky groups. This stringency far exceeds what was previously reported for other cells. Of the six sterols that failed to support growth at all, five were incorporated into cells moderately to extensively. This suggests that assembly of a functional membrane is impaired when these sterols are used as substrates for growth.

Effect of exogenous fatty acids on growth, membrane fluidity, and phospholipid fatty acid composition in yeast

The growth response of a double-mutant fatty acid auxotroph of yeast Saccharomyces cerevisiae to exogenous saturated fatty acids of a homologous series from 12:0 to 16:0, each supplied with oleate, linoleate, linolenate, or cis-delta 11-eicosenoate, cannot be explained in terms of the efficiency of incorporation of the fatty acids into phospholipids or alteration of membrane fluidity. There is, however, a negative correlation between growth and levels of 12:0 plus 13:0 in phospholipids, as well as a positive correlation between growth and levels of 14:0, 15:0, and 16:0. We, therefore, conclude that the predominant factor in these phospholipid fatty acyl chain modifications is maintenance of an optimal concentration of C14:0 through C16:0 in phospholipids of this organism.