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Studies on the hydrogen belts of membranes: I. Diester, diether, and dialkyl phosphatidycholines and polyoxyethylene glycerides in monolayers with cholesterol.

The hydrogen belts of membranes are defined as the regions consisting of hydrogen bond acceptors, i.e., the C=O groups of glycero- and sphingolipids, and hydrogen bond donors, i.e., cholesterol-OH, sphingolipid-OH, proteins, and water. Lipid-lipid hydrogen bonding in these belts has been suggested. The connection of such hypothetical bonding with the condensation effect, i.e., the apparent reduction of surface area occupied by phospholipids in mixed monolayers with cholesterol, has been tested with lipids possissing and lacking C=O groups: diester, diether, and dialkyl phosphatidylcholine, and analogous polyoxyethylene diglycerides. Condensation by cholesterol was observed for all lipids. Consequently, the hypothetical lipid-C=O-cholesterol hydrogen bonding is not a prerequisite for the condensation effect.

Cholesterol↗

Decreased utilization of [2-3H]glycerol in phospholipid and neutral glyceride biosynthesis in the retina of streptozotocin-diabetic rats.

The effect of streptozotocin (STZ)-induced diabetes on lipid uptake was studied in the rat retina. The intravitreal incorporation of [2-3H]glycerol was followed in retinal lipids from the first day of diabetes (acute state) up to 20 wk (chronic state). Total lipid incorporation decreased 50% 2 d after injection of STZ; the labeling remained lower than the control values throughout the 20 wk period studied. In the chronic state of diabetes, the time-course of the incorporation of [2-3H]glycerol during the first 2 h after its injection displayed a pattern of incorporation similar to that of the controls. The possibility that the decreased utilization of [2-3H]glycerol in the lipid pathway results from a competition between the injected [3H]glycerol and the glycolytic pathway during diabetes in rat retina in vivo is discussed.

Animals↗

Abiotic condensation synthesis of glyceride lipids and wax esters under simulated hydrothermal conditions.

Precursor compounds for abiotic proto cellular membranes are necessary for the origin of life. Amphipathic compounds such as fatty acids and acyl glycerols are important candidates for micelle/bilayer/vesicle formation. Two sets of experiments were conducted to study dehydration reactions of model lipid precursors in aqueous media to form acyl polyols and wax esters, and to evaluate the stability and reactions of the products at elevated temperatures. In the first set, mixtures of n-nonadecanoic acid and ethylene glycol in water, with and without oxalic acid, were heated at discrete temperatures from 150 ( composite function)C to 300 ( composite function)C for 72 h. The products were typically alkyl alkanoates, ethylene glycolyl alkanoates, ethylene glycolyl bis-alkanoates and alkanols. The condensation products had maximum yields between 150 ( composite function)C and 250 ( composite function)C, and were detectable and thus stable under hydrothermal conditions to temperatures < 300 ( composite function)C. In the second set of experiments, mixtures of n-heptanoic acid and glycerol were heated using the same experimental conditions, with and without oxalic acid, between 100 ( composite function)C and 250 ( composite function)C. The main condensation products were two isomers each of monoacylglycerols and diacylglycerols at all temperatures, as well as minor amounts of the fatty acid anhydride and methyl ester. The yield of glyceryl monoheptanoates generally increased with increasing temperature and glyceryl diheptanoates decreased noticeably with increasing temperature. The results indicate that condensation reactions and abiotic synthesis of organic lipid compounds under hydrothermal conditions occur easily, provided precursor concentrations are sufficiently high.

Esters↗