Cholesterol oxidase as a structural probe of biological membranes: its application to brush-border membrane

Stability and stoichiometry of bilayer phospholipid-cholesterol complexes: relationship to cellular sterol distribution and homeostasis

Is cholesterol distributed among intracellular compartments by passive equilibration down its chemical gradient? If so, its distribution should reflect the relative cholesterol affinity of the constituent membrane phospholipids as well as their capacity for association with the sterol. We examined this issue by analyzing the reactivity to cholesterol oxidase of large unilamellar vesicles (LUVs) containing phospholipids and varied levels of cholesterol. The rates of cholesterol oxidation differed among the various phospholipid environments by roughly 4 orders of magnitude. Furthermore, accessibility to the enzyme increased by orders of magnitude at cholesterol thresholds that suggested cholesterol:phospholipid association ratios of 1:1, 2:3, or 1:2 (moles:moles). The accessibility of cholesterol above these thresholds was still constrained by its particular phospholipid environment. One phospholipid, 1-stearoyl-2-oleoyl-sn-glycero-3-phosphatidylserine, exhibited no threshold. The analysis suggested values for the stoichiometries of the putative cholesterol-phospholipid complexes, their relative stabilities, and the fractions of bilayer cholesterol not in complexes at the threshold equivalence points. Predictably, the saturated phosphorylcholine species had the lowest apparent stoichiometric ratios and the strongest associations with cholesterol. These results are in general agreement with the equilibrium distribution of cholesterol between the various LUVs and methyl-β-cyclodextrin. In addition, the behavior of the cholesterol in intact human red blood cells matched predictions made from LUVs of the corresponding composition. These results support a passive mechanism for the intracellular distribution of cholesterol that can provide a signal for its homeostatic regulation.

Noninvasive neutron scattering measurements reveal slower cholesterol transport in model lipid membranes

Proper cholesterol transport is essential to healthy cellular activity and any abnormality can lead to several fatal diseases. However, complete understandings of cholesterol homeostasis in the cell remains elusive, partly due to the wide variability in reported values for intra- and intermembrane cholesterol transport rates. Here, we used time-resolved small-angle neutron scattering to measure cholesterol intermembrane exchange and intramembrane flipping rates, in situ, without recourse to any external fields or compounds. We found significantly slower transport kinetics than reported by previous studies, particularly for intramembrane flipping where our measured rates are several orders of magnitude slower. We unambiguously demonstrate that the presence of chemical tags and extraneous compounds employed in traditional kinetic measurements dramatically affect the system thermodynamics, accelerating cholesterol transport rates by an order of magnitude. To our knowledge, this work provides new insights into cholesterol transport process disorders, and challenges many of the underlying assumptions used in most cholesterol transport studies to date.

Fluorescence studies of lipid regular distribution in membranes

This article reviews the use of fluorescent lipids and free probes in the studies of lipid regular distribution in model membranes. The first part of this article summarizes the evidence and physical properties for lipid regular distribution in pyrene-labeled phosphatidylcholine (PC)/unlabeled PC binary mixtures as revealed by the fluorescence of pyrene-labeled PC. The original and the extended hexagonal superlattice model are discussed. The second part focuses on the fluorescence studies of sterol regular distributions in membranes. The experimental evidence for sterol superlattice formation obtained from the fluorescent sterol (i.e. dehydroergosterol) and non-sterol fluorescent probes (e.g. DPH and Laurdan) are evaluated. Prospects and concerns are given with regard to the sterol regular distribution. The third part deals briefly with the evidence for polar headgroup superlattices. The emphasis of this article is placed on the new concept that membrane properties and activities, including the activities of surface acting enzymes, drug partitioning, and membrane free volume, are fine-tuned by minute changes in the concentration of bulky lipids (e.g. sterols and pyrene-containing acyl chains) in the vicinities of the critical mole fractions for superlattice formation.

Rapid transbilayer movement of spin-labeled steroids in human erythrocytes and in liposomes

The transbilayer movement and distribution of spin-labeled analogs of the steroids androstane (SLA) and cholestane (SLC) were investigated in the human erythrocyte and in liposomes. Membranes were labeled with SLA or SLC, and the analogs in the outer leaflet were selectively reduced at 4C using 6-O-phenylascorbic acid. As shown previously, 6-O-phenylascorbic acid reduces rapidly nitroxides exposed on the outer leaflet, but its permeation of membranes is comparatively slow and thus does not interfere with the assay. From the reduction kinetics, we infer that transbilayer movement of SLA in erythrocytes is rapid at 4C with a half-time of approximately 4.3 min and that the probe distributes almost symmetrically between both halves of the plasma membrane. We have no indication that a protein-mediated transport is involved in the rapid transbilayer movement of SLA because 1) pretreatment of erythrocytes with N-ethyl maleimide affected neither flip-flop nor transbilayer distribution of SLA and 2) flip-flop of SLA was also rapid in pure lipid membranes. The transbilayer dynamics of SLC in erythrocyte membranes could not be resolved by our assay. Thus, the rate of SLC flip-flop must be on the order of, or even faster than, that of probe reduction rate on the exoplasmic leaflet (half-time approximately 0.5 min). The results are discussed with regard to the transbilayer dynamics of cholesterol.

Cholesterol oxidation reduces Ca(2+)+MG (2+)-ATPase activity, interdigitation, and increases fluidity of brain synaptic plasma membranes

These experiments examined effects of cholesterol oxidation on Ca(2+)+Mg(2+)-ATPase activity, Na(+)+K(+)-ATPase activity, and membrane structure of brain synaptic plasma membranes (SPM). Cholesterol oxidase [E.C.1.1.3.6 from Brevibacterium sp.] was used to oxidize cholesterol. Two cholesterol pools were identified in synaptosomal membranes based on their accessibility to cholesterol oxidase. A rapidly oxidized cholesterol pool was observed with a 1t1/2 of 1.19 +/- 0.09 min and a second pool with a 2t1/2 of 38.30 +/- 4.16 min. Activity of Ca(2+)+Mg(2+)-ATPase was inhibited by low levels of cholesterol oxidation. Ten percent cholesterol oxidation, for example, resulted in approximately 35% percent inhibition of Ca(2+)+Mg(2+)-ATPase activity. After 13% cholesterol oxidation, further inhibition of Ca(2+)+Mg(2+)-ATPase activity was not observed. Activity of Na(+)+K(+)-ATPase was not affected by different levels of cholesterol oxidation (5%-40%). SPM interdigitation was significantly reduced and fluidity was significantly increased by cholesterol oxidation. The relationship observed between SPM interdigitation and Ca(2+)+Mg(2+)-ATPase activity was consistent with studies using model membranes [7]. Brain SPM function and structure were altered by relatively low levels of cholesterol oxidation and is a new approach to understanding cholesterol dynamics and neuronal function. The sensitivity of brain SPM to cholesterol oxidation may be important with respect to the proposed association between oxygen free radicals and certain neurodegenerative diseases.

Sensitive assay for cholesterol in biological membranes reveals membrane-specific differences in kinetics of cholesterol oxidase

Quantification of cholesterol in biological membranes from a variety of sources is an important step toward understanding cholesterol's roles in membrane function. We extend to biological membranes the fluorometric/enzymatic approach (cholesterol oxidase) to measure cholesterol, originally described for whole cells (Heider and Boyett [1978] J. Lipid Res., 19:514-518; Gamble et al. [1978] J. Lipid Res., 19:1068-1070) and serum (Huang et al. [1975] Clin Chem., 21:1605-1608). This method has a detection limit of 0.3 microgram cholesterol. As revealed by comparison with high-performance liquid chromatography, the fluorometric/enzymatic method with biological membranes is accurate (within 4% and 8% for intestinal and hepatic plasma membranes, respectively). The assay may be completed within 3 to 4 hours and requires neither lipid extraction nor chromatographic techniques. Kinetics of the cholesterol oxidase reaction are membrane-specific, and first-order rate constants (k) are positively correlated with membrane order.

Substrate specificity of cholesterol oxidase from Streptomyces cinnamomeus--a monolayer study

The substrate specificity of cholesterol oxidase from Streptomyces cinnamomeus was examined in oriented sterol monolayers at the air/water interface. Of the cholesterol analogues with structural alterations in the A- or B-ring that were examined, it was observed that 5 alpha-cholestan-3 beta-ol was oxidized almost as fast as cholesterol itself. When the delta-5 double bond in cholesterol was instead at the delta-4 position, the oxidation rate became 3.2-fold slower. A similar reduction in the average oxidation rate was observed when the delta-5 double bond in cholesterol was instead at the delta-7 position (5 alpha-cholest-7-en-3 beta- ol). 5,7-Cholestadien-3 beta-ol was oxidized 5.1-fold slower compared to cholesterol, whereas 3 beta-hydroxy-5-cholesten-7-one and 5 beta-cholestan-3 beta-ol were not substrates of the enzyme (also verified from the lack of H2O2-production). With C(17) side chain analogues of cholesterol, it was observed that the complete lack of the C(17) side chain (5-androsten-3 beta-ol), or the insertion of an unsaturation at delta-24 (desmosterol), or even an ethyl group at C(24)(24b-ethyl-5,22- cholestadien-3 beta-ol) had no appreciable effects on sterol oxidation rate, implying that the enzyme did not recognize the side chain in oriented sterol monolayers. This study has shown that the sterol monolayer system is a good technique to examine sterol/cholesterol oxidase interactions, since both the orientation of the substrate molecules, and the quality of the interface can be mastered.

Enzyme-catalyzed oxidation of cholesterol in pure monolayers at the air/water interface

Mean molecular area vs. lateral surface pressure isotherms were determined for monolayers containing cholesterol, 4-cholesten-3-one (cholestenone), or binary mixtures of the two. At all lateral surface pressures examined, cholestenone had a larger mean molecular area requirement than cholesterol. Results with the binary mixtures of cholesterol and cholestenone suggested that the sterols did not mix ideally (non additive mean molecular area) with each other in the monolayer; the observed mean molecular area for mixtures was less than would be expected based on ideal mixing. The mixed sterol monolayers also displayed a reduction in the lateral collapse pressure which appeared to be a linear function of the mole fraction of cholestenone in the monolayer, suggesting that cholesterol and cholestenone were completely miscible in the mixed monolayer. The pure cholesterol monolayer was next used to examine the cholesterol oxidase-catalyzed (Brevibacterium sp.) oxidation of cholesterol to cholestenone at different lateral surface pressures at 22 degrees C. The difference in mean molecular area requirements of cholesterol and cholestenone was directly used to convert monolayer area changes (at constant lateral surface pressure) into average reaction rates. It was observed that the average catalytic activity of cholesterol oxidase increased linearly with increased lateral surface pressure in the range of 1 to 20 mN/m. In addition, the enzyme was capable to oxidize cholesterol in monolayers with a lateral surface pressure close to the collapse pressure of cholesterol monolayers (collapse pressure 45 mN/m; oxidation was observed at 40 mN/m). The adsorption of cholesterol oxidase to an inert sterol monolayer film at low surface pressures (around 9 mN/m) was marginal, although clearly detectable at very low (0.5-4 mN/m) lateral surface pressures, suggesting that the enzyme did not penetrate deeply into the monolayer in order to reach the 3 beta-hydroxy group of cholesterol. This interpretation is further supported by the finding that a maximally compressed cholesterol monolayer (40 mN/m) was readily susceptible to enzyme-catalyzed oxidation. It is concluded that cholesterol oxidase is capable of oxidizing cholesterol in laterally expanded monolayers as well as in tightly packed monolayers, where the lateral surface pressure is close to the collapse pressure. The kinetic results suggested that the rate-limiting step in the overall process was the substrate availability per surface area (or surface concentration) at the water/lipid interface.

Cholesterol heterogeneity in the plasma membrane of epithelial cells

The distribution of cholesterol in the plasma membrane of epithelial cells has been determined using renal brush border vesicles as a model. In brush borders treated with Brevibacterium sp. or Nocardia erythropolis cholesterol oxidases, a significant fraction of the free cholesterol was oxidized rapidly, without glutaraldehyde fixation, and the remaining cholesterol was oxidized at a slower rate. The size of the readily accessible cholesterol pool, however, depended on the enzyme used, varying from 16% of the total in membranes treated with N. erythropolis oxidase, to 27% using the Brevibacterium sp. enzyme. The slowly accessible pool detected by the Brevibacterium oxidase was suppressed upon sphingomyelinase addition. On the other hand, the restricted activity of the Nocardia oxidase might depend on phosphatidylcholine/cholesterol interactions. These results indicate that cholesterol distribution is heterogeneous in intact renal brush border vesicles. They suggest that, as proposed for model system [Demel, R.A. Jansen, J.W.C.M., van Dijck, P.W.M., & van Deenen, L.L.M. (1977) Biochim. Biophys. Acta 465, 1-10], preferential interactions between some classes of phospholipids and cholesterol define cholesterol pools in the plasma membrane of epithelial cells.

Cholesterol transport between cells and high-density lipoproteins

Various types of studies in humans and animals suggest strongly that HDL is anti-atherogenic. The anti-atherogenic potential of HDL is thought to be due to its participation in reverse cholesterol transport, the process by which cholesterol is removed from non-hepatic cells and transported to the liver for elimination from the body. Extensive studies in cell culture systems have demonstrated that HDL is an important mediator of sterol transport between cells and the plasma compartment. The topic of this review is the mechanisms that account for sterol movement between HDL and cells. The most prominent and easily measured aspect of sterol movement between HDL and cells is the rapid bidirectional transfer of cholesterol between the lipoprotein and the plasma membrane. This movement occurs by unmediated diffusion, and in most situations its rate in each direction is limited by the rate of desorption of sterol molecules from the donor surface into the adjacent water phase. The net transfer of sterol mass out of cells occurs when there is either a relative enrichment of sterol within the plasma membrane or a depletion of sterol in HDL. Recent studies suggest that certain minor subfractions of HDL (with pre-beta mobility on agarose gel electrophoresis and containing apoprotein A-I but no apo A-II) are unusually efficient at promoting efflux of cell sterol. To what extent efflux to these HDL fractions is balanced by influx from the lipoprotein has not yet been established clearly. The prevention and reversal of atherosclerosis require the mobilization of cholesterol from internal (non-plasma membrane) cellular locations. To some extent, this may involve the retroendocytosis of HDL. However, most mobilization probably involves the transport of internal sterol to the plasma membrane, followed by desorption to extracellular HDL. Several laboratories are investigating the transport of sterol from intracellular locations to the plasma membrane. Studies on biosynthetic sterol (probably originating mostly in the smooth endoplasmic reticulum) suggest that there is rapid transport to the plasma membrane in lipid-rich vesicles. Important features of this transport are that it bypasses the Golgi apparatus and may be positively regulated by the specific binding of HDL to the plasma membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Transmembrane distribution of sterol in the human erythrocyte

The transbilayer cholesterol distribution of human erythrocytes was examined by two independent techniques, quenching of dehydroergosterol fluorescence and fluorescence photobleaching of NBD-cholesterol. Dehydroergosterol in conjunction with leaflet selective quenching showed that, at equilibrium, 75% of the sterol was localized to the inner leaflet of resealed erythrocyte ghosts. NBD-cholesterol and fluorescence photobleaching displayed two diffusion values in both resealed ghosts and intact erythrocytes. The fractional contribution of the fast and slow diffusion constants of NBD-labelled cholesterol represent its inner and outer leaflet distribution. At room temperature the plasma membrane inner leaflet of erythrocyte ghosts as well as intact erythrocytes cells contained 78% of the plasma membrane sterol. The erythrocyte membrane transbilayer distribution of sterol was independent of temperature. In conclusion, dehydroergosterol and NBD-cholesterol data are consistent with an enrichment of cholesterol in the inner leaflet of the human erythrocyte.

Intracellular trafficking of sterols

Cavalier-Smith (1981) has identified 22 characters that are universally present in eukaryotes but absent in prokaryotes. Of these, he argues that one, exocytosis, might have been the driving force behind the evolution of modern eukaryotic cells. Bloom and Mouritsen (1988) further argue that sterols may have removed an evolutionary bottleneck to cytosis. Therefore, the advent of sterols in membranes might have been the single feature that led to eukaryote evolution. The evolutionary advantage conferred by cholesterol is associated primarily with plasma membrane function, since the majority of cellular free cholesterol resides in that membrane. However, sterol synthesis occurs in the ER; therefore, the cell must have a mechanism for transporting sterol to the plasma membrane and its regulation. As has been pointed out in this review, much remains to be elucidated in the study of intracellular sterol trafficking. To date, neither diffusion nor vesicle-mediated transport can be fully confirmed or ruled out. Microtubule and microfilament involvement appears important in some routes (e.g., mitochondria) but not in others. In addition, trafficking roles of cytoplasmic lipoproteinlike particles have not been addressed. Finally, although some "sterol carrier proteins" demonstrate the ability to mediate intervesicular transfer of cholesterol in vitro, the true physiological role of these proteins remains obscure. Future research in this field awaits the refinement of available techniques. Particularly valuable would be cytochemical methods for detection of sterol at the ultrastructural level. Possibly, direct microscopic visualization of radiolabeled components in cells represents the necessary approach. Purification of elements carrying newly synthesized sterols would allow the proteins mediating transport to be identified. Continued analysis of mutants defective in transport, such as in type C Niemann-Pick disease, will shed light on this complex problem. The importance of extracellular trafficking of cholesterol owing to its involvement in the progression of atherosclerosis, has been emphasized in recent years. Little emphasis has been placed on intracellular trafficking of sterol; however, it can be argued that such transport also plays a major role in atherosclerosis, possibly by fueling retrotransport of cholesterol to the liver and secretion in the bile. Therefore, we hope this review will serve to stimulate research interest in this area.

Cytotoxicity of cholesterol oxidase to cells of hypercholesterolemic guinea pigs

1. A high cholesterol diet caused guinea pig erythrocytes to become sensitive to lysis by cholesterol oxidase (CO), a protein not hemolytic to normal cells. 2. Lysis was associated with conversion of membrane cholesterol to its oxidation product (delta-4-cholesten-3-one). 3. Intravenous injection of CO to hypercholesterolemic guinea pigs produced a reduction in serum cholesterol, but was not lethal as it was in rabbits. 4. Homogenized spleen, liver and kidney from the hyperlipidemic animals were sensitive to in vitro cholesterol oxidation while tissues from non-lipemic animals were resistant to modification.