Effect of fatty acid supplementation on thermotropic behavior of membrane lipids and leucine transport in Saccharomyces cerevisiae

Geometrical isomerism of monounsaturated fatty acids: thiyl radical catalysis and influence of antioxidant vitamins

Thiyl radicals generated either from thiols or disulfides act as the catalyst for the cis-trans isomerization of a variety of monounsaturated fatty acid methyl esters in homogeneous solution. Similar results have also been obtained using alpha-lipoic acid and its reduced form. The effectiveness of the isomerization processes in the presence of the most common antioxidants has been addressed. The ability of thiyl radical scavenging was found to increase along the series alpha-tocopherol < ascorbic acid < all-trans retinol. The cis-trans isomerization of fatty acid residues in multilamellar vesicles of dioleoyl phosphatidyl choline by thiyl radical, in the absence and presence of the various antioxidants, has also been studied in detail. The influence of the isomerization process on the phospholipid bilayer has been tested by permeability measurements of vesicles and it is clearly shown that trans fatty acid-containing membranes have intermediate properties between those formed by all-cis and saturated components. This study contributes to the understanding of radical processes that can alter or protect the naturally occurring cis geometry of unsaturated lipids in cell membranes and demonstrates a new role of essential antioxidants.

Carrier-mediated L-lactate transport in brush-border membrane vesicles from rat placenta during late gestation

The mechanism for L-lactate transport across microvillous membrane vesicles prepared from rat placenta was examined. Uptake of L-lactate into these vesicles was mainly the result of transport into the intravesicular (osmotically active) space. The initial rate of L-lactate uptake was not affected by the presence of an inward gradient of either Na+ or K+. In the presence of an inward-directed proton gradient, L-lactate uptake was markedly stimulated, accumulating at concentrations 6-7-fold higher than the equilibrium. Lower transmembrane pH gradients were associated with slower initial uptakes and smaller overshoots. L-Lactate uptake determined under an inside-directed pH gradient was strongly inhibited by p-chloromercuriphenylsulphonic acid, a protein-thiol oxidizing agent. L-Lactate uptake was: (1) saturable as a function of the concentration of L-lactate, (2) inhibited by monocarboxylic acids such as pyruvate, D-lactate, beta-hydroxybutyrate and alpha-cyano-4-hydroxycinnamic acid, and (3) temperature-dependent. When present inside the vesicles, L-lactate, pyruvate and beta-hydroxybutyrate caused trans-stimulation of L-lactate uptake both in the presence and in the absence of an inside-directed pH gradient, indicating that L-lactate transport is a reversible process that can be shared by other monocarboxylic acids. There were no significant changes in maximal initial rate or in the kinetic parameters of L-lactate transport during the last 3 days of gestation.

Lipid composition and amino acid transport in a nystatin-resistant mutant of Aspergillus niger

A nystatin-resistant mutant of Aspergillus niger has been isolated and used as a model system to study the effect of altered sterol levels on lipid composition, transport behavior and physical properties of membrane lipids. There is a decrease in the sterol to phospholipid ratio in the mutant compared to the wild type. Although there is no qualitative change in phospholipid composition, the mutant contains a higher amount of phosphatidylcholine and a lower amount of phosphatidylethanolamine compared to the wild type. The most significant change is the elevated level of linoleic acid in the mutant, concomitant with a decreased level of oleic acid. These adaptive changes to nystatin resistance are manifested in the altered thermotropic behavior of membrane lipids as studied by the steady-state fluorescence polarization technique. These changes are also associated with altered membrane permeability as evidenced by the change in Vmax values for uptake of some amino acids in the mutant compared to the wild type.