Abstract.-Both membranes of Mycoplasma laidlawii and water dispersions of protein-free membrane lipids exhibit thermal phase transitions that can be detected by differential scanning calorimetry. The transition temperatures are lowered by increased unsaturation in the fatty acid residues, but in each case they are the same for membranes and lipids. The transitions resemble those observed for synthetic lipids in the lamellar phase in water, which arise from melting of the hydrocarbon chains within the phospholipid bilayers. Such melts are cooperative phenomena and would be greatly perturbed by apolar binding to protein. Thus the identity of membrane and lipid transition temperatures suggests that in the membranes, as in water, the lipids are in the bilayer conformation in which the hydrocarbon chains associate with each other rather than with proteins. Observations of morphological changes indicate that osmotic imbalance occurs when the membrane transition temperature exceeds the growth temperature, and that for transport processes to function properly the hydrocarbon chains must be in a liquid-like state.After many years of research, knowledge of the molecular organization of biological membranes remains meager. Although the concept of a phospholipid bilayer bounded on each side by protein is accepted by some investigators, others question the basic assumptions of the bilayer model and suggest alternative models in which the association between lipid and protein is hydrophobic rather than polar.1 If in fact lipids exist in membranes as bilayers, some unique property of a bilayer array might be detectable in membranes by a direct physical technique. Such a property is the reversible thermotropic gel-liquid crystal phase transition observed in phospholipid myelin forms in water. It has been studied by differential scanning calorimetry, differential thermal analysis, nuclear magnetic resonance spectroscopy, X-ray diffraction, and light microscopy; it arises from the melting of the hydrocarbon interiors of lipid bilayers.2-' Unlike transitions between liquid-crystalline phospholipid mesophases,5 the melt does not result in a molecular rearrangement and the lipids exist in the lamellar conformation both above and below the transition temperature. As in the case of bulk hydrocarbons, the melting point varies with unsaturation and chain lengths of the fatty acids in the phospholipids. Because cholesterol interferes, to detect a phase change above the ice point in a membrane an organism containing rather saturated fatty acids but little or no cholesterol must be chosen.The membranes of Mycoplasma laidlawii satisfy these requirements. Previous studies of this organism have shown that the cell membrane contains no choles-104

No abstract

No abstract

Although uridine and adenosine are converted by membrane-associated nuf cleoside phosphorylases to ribose-1-phosphate (ribose-1-P) and the corresponding bases (uracil and adenine), only ribose-1-P is accumulated within Salmonella typhimurium LT2 membrane vesicles. In accordance with these observations, no uptake is observed when the vesicles are incubated with the bases or nucleosides labeled in their base moieties. The vesicles lack a transport system for ribose-1-P, since excess ribose-1-P does not inhibit the uptake of the ribose moiety of uridine. In addition, there is no exchange with preaccumulated ribose-1-P. Thus, uridine, rather than ribose-1-P, must serve as the initially transported substrate. The uptake of the ribose portion of uridine is coupled to electron transport, and the levels to which ribose-1-P are accumulated may be reduced by adding various bases to the reaction mixtures. The bases appear to inhibit the uridine phosphorylase reaction and/or cause an efflux of ribose-1-P from the vesicles. This loss of ribose-1-P reflects the accumulation of nucleosides in the external medium after being synthesized within the membranes. Synthesis of the nucleosides from intravesicular ribose-1-P and exogenous bases proceeds even though the bases are not accumulated by the vesicles. Furthermore, ribose-1-P cannot significantly inhibit uridine phosphorylase activity unless the membranes are disrupted. These observations indicate that the membrane-associated nucleoside phosphorylases may have a transmembranal orientation with their base and ribose-1-P binding sites on opposite sides of the membranes. Such an asymmetric arrangement of these enzymes may facilitate the uptake of the ribosyl moiety ofnucleosides by a group translocation mechanism. Thus, nucleosides may be cleaved during the membrane transport process, with the resultant bases delivered to the external environment while ribose-1-P is shunted to the intravesicular space.