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

Oxidation of lipid membrane cholesterol by cholesterol oxidase and its effects on raft model membrane structure

The effects of a peripheral protein - cholesterol oxidase (3β-hydroxysteroid oxidase, ChOx) on the characteristics of model lipid membranes composed of cholesterol, cholesterol:sphingomyelin (1:1), and the raft model composed of DOPC:Chol:SM (1:1:1) were investigated using two membrane model systems: the flat monolayer prepared by the Langmuir technique and the curved model consisting of liposome of the same lipids. The planar monolayers and liposomes were employed to follow membrane cholesterol oxidation to cholestenone catalyzed by ChOx and changes in the lipid membrane structure accompanying this reaction. Changes in the structure of liposomes in the presence of the enzyme were reflected in the changes of hydrodynamic diameter and fluorescence microscopy images, while changes of surface properties of planar membranes were evaluated by grazing incidence X-ray diffraction (GIXD) and Brewster angle microscopy. UV-Vis absorbance measurements confirmed the activity of the enzyme in the tested systems. A better understanding of the interactions between the enzyme and the cell membrane may help in finding alternative ways to decrease excessive cholesterol levels than the common approach of treating hypercholesterolemia with statins, which are not free from undesirable side effects, repeatedly reported in the literature and observed by the patients.

Essentially all excess fibroblast cholesterol moves from plasma membranes to intracellular compartments

It has been shown that modestly increasing plasma membrane cholesterol beyond its physiological set point greatly increases the endoplasmic reticulum and mitochondrial pools, thereby eliciting manifold feedback responses that return cell cholesterol to its resting state. The question arises whether this homeostatic mechanism reflects the targeting of cell surface cholesterol to specific intracellular sites or its general equilibration among the organelles. We now show that human fibroblast cholesterol can be increased as much as two-fold from 2-hydroxypropyl-β-cyclodextrin without changing the size of the cell surface pool. Rather, essentially all of the added cholesterol disperses rapidly among cytoplasmic membranes, increasing their overall cholesterol content by as much as five-fold. We conclude that the level of plasma membrane cholesterol is normally at capacity and that even small increments above this physiological set point redistribute essentially entirely to intracellular membranes, perhaps down their chemical activity gradients.

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.

Study of gramicidin A--phospholipid interactions in Langmuir monolayers: analysis of their mechanical, thermodynamical, and electrical properties

The mechanisms of interactions between gramicidin A (gA) and dimyristoylphosphatidylcholine (DMPC) in monolayers formed at the air-water interface were studied by analyzing their mechanical, thermodynamical, and electrical properties evaluated from measurements of pressure-area isotherms and of Maxwell displacement currents (MDC). A contactless method of recording MDC enabled us to monitor changes in the charge state of the monolayer-constituting molecules and to find the relation between a phase state of the monolayer and structural transitions of gA. The peptide-lipid interactions were quantified in terms of the excess of Gibbs free energy, excess entropy, as well as the molecular dipole moments at various gA/DMPC molar ratios, at various temperatures (in the gel phase and also in the liquid-crystalline phase of DMPC molecule), and at various surface pressures. It was found that the strongest interactions between gA and DMPC took place at the gA/DMPC molar ratio at around 0.25. At this monolayer composition, the phospholipids, via their carbonyl moieties, dominantly interact with the single helical gA, which mostly stands upright on the surface and is anchored by its C-terminus to the water surface, and prevent the formation of the intertwined helical gA dimers. The optimum ratio was confirmed also by anomalous electrical behavior of electrical dipole moments derived from MDC measurements.

Continuous measurement of the lipoxygenase-catalyzed oxidation of unsaturated lipids using the monomolecular film technique

PURPOSE

This paper presents the first detailed kinetic investigation involving the continuous measurement of the soybean lipoxygenase 1 (LOX1)-catalyzed oxidation of unsaturated lipids using the monomolecular film technique at an argon/water interface.

MATERIALS AND METHODS

The presence of oxidation products in the monolayer is qualitatively detected, at a constant area, by an increase in the monolayer surface pressure. Alternatively, the rate of lipid oxidation can be measured, at a constant surface pressure, by a backward movement of the mobile barrier, due to the oxidation-dependent increase in the monolayer area.

RESULTS

For instance, the LOX1-catalyzed oxidation of 1,2-di[cis-9,12-octadecadienoyl]-sn-glycero-3-phosphocholine (diC18:2PC) monolayer was found to be characterized by a time dependent increase in the monolayer area, at constant surface pressure. However, the increase in the monolayer area was thought to be caused first by the penetration of the enzyme into the interface, and secondly, by the formation of hydroperoxides at the interface, due to the LOX1-catalyzed oxidation of the diC18:2PC film. The rate of the LOX1-catalyzed oxidation of diC18:2PC film was measured by subtracting the increase in the area due to the LOX1-penetration into the non-oxidizable 1,2-di[cis-9-octadecenoyl]-sn-glycero-3-phosphocholine (diC18:1PC) film from the increase in the area due to LOX penetration and oxidation of the diC18:2PC film. At a constant optimum surface pressure of 1 mN m(-1), similar initial rates of LOX1-catalyzed oxidation are observed with both linoleic acid methyl ester (C18:2) and diC18:2PC. It is worth noting that the surface density of C18:2 acyl chains is also similar in both films. We observed that a phosphatidylcholine (PC) film with two potentially oxidizable chains (e.g., diC18:2PC) is oxidized at a rate which is twice that obtained with a PC containing a single oxidizable chain (e.g., 1-hexadecanoyl-2-[cis-9,12-octadecadienoyl]-sn-glycero-3-phosphocholine).

CONCLUSIONS

The enzymatic lipid oxidation seems to occur when the monolayer is in the expanded state. This expanded state may possibly result in vivo from the lipolysis of a biomembrane and consequently lipolysis and lipid oxidation are coupled at the membrane level.

Conformation study of the membrane models by the Maxwell displacement current technique and oxidative stress

The role of biological membranes as a target in biological radiation damage is still unclear. Recently much attention has been paid to the dynamic behaviour of the cell membrane. Maxwell displacement current technique (MDC) provides new possibility of conformation study of the membrane models. Oxidative stress can impair macromolecules in the cell on a molecular level. MDC technique enables to study the changes in molecular orientations and/or conformations of cell membranes. The combination of different methods in structural biology can clarify membrane chemical and physical properties.

Formation of stable vesicles from N- or 3-alkylindoles: possible evidence for tryptophan as a membrane anchor in proteins

Twelve indole derivatives have been prepared and studied. Five were 1-substituted: 1, methyl; 2, n-hexyl; 3, n-octyl; 4, n-octadecyl; and 5, cholestanyloxycarbonylmethyl. Four were 3-substituted: 6, methyl; 7, n-hexyl; 8, n-octyl; and 9, n-octadecyl. Three were disubstituted as follows: 10, 1-n-decyl-3- n-decyl; 11, 1-methyl-3-n-decyl; and 12, 1,3-bis(n-octadecyl)indole. Sonication of aqueous suspensions afforded stable aggregates from 3-5 and 8-12. Laser light scattering, dye entrapment, and electron microscopy were used to characterize the aggregates. Aggregates formed from N-substituted indoles proved to be more robust than those formed from 3-alkylindoles. A stable monolayer formed from 3-n-octadecylindole but not from N- or 1,3-disubstituted analogues by using a Langmuir-Blodgett trough. The formation of aggregates was explained in terms of stacking by the relatively polar indole headgroup. In the monolayer experiment, this force was apparently overwhelmed by H-bonding interactions with the aqueous phase.

Cholesterol oxidase: sources, physical properties and analytical applications

Since Flegg (H.M. Flegg, An investigation of the determination of serum cholesterol by an enzymatic method, Ann. Clin. Biochem. 10 (1973) 79-84) and Richmond (W. Richmond, The development of an enzymatic technique for the assay of cholesterol in biological fluids, Scand. J. clin. Lab. Invest. 29 (1972) 25; W. Richmond, Preparation and properties of a bacterial cholesterol oxidase from Nocardia sp. and its application to enzyme assay of total cholesterol in serum, Clinical Chemistry 19 (1973) 1350-1356) first illustrated the suitability of cholesterol oxidase (COD) for the analysis of serum cholesterol, COD has risen to become the most widely used enzyme in clinical laboratories with the exception of glucose oxidase (GOD). The use is widespread because assays incorporating the enzyme are extremely simple, specific, and highly sensitive and thus offer distinct advantages over the Liebermann-Burchard analytical methodologies which employ corrosive reagents and can be prone to unreliable results due to interfering substances such as bilirubin. Individuals can now readily determine their own serum cholesterol levels with a simple disposable test kit. This review discusses COD in some detail and includes the topics: (1) The variety of bacterial sources available; (2) The various extraction/purification protocols utilised in order to obtain protein of sufficient clarification (purity) for use in food/clinical analysis; (3) Significant differences in the properties of the individual enzymes; (4) Substrate specificities of the various enzymes; (5) Examples of biological assays which have employed cholesterol oxidase as an integral part of the analysis, and the various assay protocols; (6) New steroidal products of COD. This review is not a comprehensive description of published work, but is intended to provide an account of recent and current research, and should promote further interest in the application of enzymes to analytical selectivity.

Cyclodextrin-catalyzed extraction of fluorescent sterols from monolayer membranes and small unilamellar vesicles

This study examined the kinetics of sterol desorption from monolayer and small unilamellar vesicle membranes to 2-hydroxypropyl-beta-cyclodextrin. The sterols used include cholesterol, dehydroergosterol (ergosta-5,7,9,(11),22-tetraen-3beta-ol) and cholestatrienol (cholesta-5,7,9,(11)-trien-3beta-ol). Desorption rates of dehydroergosterol and cholestatrienol from pure sterol monolayers were faster (3.3-4.6-fold) than the rate measured for cholesterol. In mixed monolayers (sterol: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine 30:70 mol%), both dehydroergosterol and cholestatrienol desorbed faster than cholesterol. clearly indicating a difference in interfacial behavior of these sterols. In vesicle membranes desorption of dehydroergosterol was slower than desorption of cholestatrienol, and both rates were markedly affected by the phospholipid composition. Desorption of sterols was slower from sphingomyelin as compared to phosphatidylcholine vesicles. Desorption of fluorescent sterols was also faster from vesicles prepared by ethanol-injection as compared to extruded vesicles. The results of this study suggest that dehydroergosterol and cholestatrienol differ from cholesterol in their membrane behavior, therefore care should be exercised when experimental data derived with these probes are interpreted.

Interfacial regulation of bacterial sphingomyelinase activity

The objective of this study was to define how the quality of the buffer/membrane interface influences the activity of bacterial sphingomyelinase acting at the interface. The enzyme reaction was carried out in a zero-order trough using a surface barostat. This approach allowed for proper control of the physico-chemical properties of the substrate molecules. Since the molecular area of ceramide is smaller than that of sphingomyelin, the hydrolysis reaction could be followed 'on-line' from the monolayer area decrease at constant surface pressure. The hydrolysis reaction could be divided into two separate phases, the first being the lag-phase (time between enzyme addition and commencement of the monolayer area change), and the second phase being the actual hydrolysis reaction (from which a maximal degradation rate could be determined). The activity of sphingomyelinase (Staphylococcus aureus) toward bovine brain sphingomyelin (bb-SM) was markedly enhanced by Mg2+ (maximal activation at 5 mM). Mg2+ also influenced the lag-phase of the reaction (the lag-time increased markedly when the Mg2+ concentration decreased below 1 mM). Saturated sphingomyelins (bb-SM and N-palmitoyl sphingomyelin [N-P-SM]) were more slowly degraded than the mono-unsaturated N-oleoyl sphingomyelin (N-O-SM). Both bb-SM and N-P-SM monolayers underwent a phase-transition at room temperature, whereas the N-O-SM monolayer did not. The phase-transition (liquid-expanded to liquid-condensed) was observed to greatly increase the lag-time of the hydrolysis reaction. The activity of sphingomyelinase was also sensitive to the lateral surface pressure of the monolayer membrane. Maximal degradation rate was achieved at 20 mN/m (with bb-SM, 30 degrees C); above this pressure the lag-time of the reaction increased sharply. The inclusion of 4 mol% of cholesterol into a [3H]sphingomyelin monolayer markedly increased the extent of [3H]sphingomyelin degradation, and shortened the lag-time of the reaction. The inclusion of 10 mol% of zwitterionic or negatively charged phospholipids to the [3H]sphingomyelin monolayer did not affect the sphingomyelinase reaction significantly. In conclusion, this study has demonstrated that the physico-chemical properties of the substrate molecules have a dominating influence on the activity of a bacterial sphingomyelinase acting at the buffer/membrane interface.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

Direct observation of the action of cholesterol oxidase in monolayers

The oxidation of monolayer cholesterol by cholesterol oxidase has been visualized using monolayer fluorescence microscopy. A direct microscopic visualization was possible because the lateral distribution of a lipid fluorophore, tetramethylrhodamine (TRITC)-labeled phosphatidylethanolamine, was very different in a cholesterol containing monolayer as compared with a cholestenone monolayer. The lipid fluorophore was effectively excluded from the condensed cholesterol phase, but was readily miscible in the cholestenone phase. One could therefore observe the appearance of fluorophore rich cholestenone-domains in the cholesterol monolayer as a result of the cholesterol oxidase catalyzed oxidation reaction. The oxidation experiments were performed at 22 degrees C with a monolayer surface pressure of 5 mN/m (on 50 mM Tris-HCl buffer, containing 140 mM NaCl, pH 7.4). When 40 mU/ml of cholesterol oxidase was injected beneath the monolayer under observation, it appeared that the enzyme penetrated the cholesterol monolayer at random sites and initiated the oxidation reaction. Once the oxidation reaction had commenced, it progressed rapidly and converted the condensed (cholesterol-rich) phase into an expanded (cholestenone-rich) phase. When the oxidation of cholesterol in mixed cholesterol/dimyristoylphosphatidyl-choline monolayers was visualized, it was observed that the enzyme-catalyzed oxidation started from the expanded phases (domains with higher compressibility) and the reaction eventually led to the dissipation of the boundary line between expanded and condensed phases. With time all condensed phases were dissolved and the monolayer became uniformly fluorescent. The association of TRITC-labeled cholesterol oxidase with a non-fluorescent mixed cholesterol/dimyristoylphosphatidylcholine monolayer led to the penetration (or association) of the fluorescent cholesterol oxidase into expanded phases of the mixed monolayers. The monolayer lateral domain morphology was similar whether the fluorescent probe was TRITC-PE or TRITC-labeled enzyme. It is concluded that cholesterol oxidase associated with (or penetrated to some extent into) the expanded phases of a monolayer, and carried out its oxidation reaction in the expanded phase or at the interface between expanded and condensed phases.

Availability for enzyme-catalyzed oxidation of cholesterol in mixed monolayers containing both phosphatidylcholine and sphingomyelin

In this study we have examined the interaction between cholesterol and phospholipids in monolayers using cholesterol oxidase (Streptomyces cinnamomeus) as a probe. Monolayers containing cholesterol and phospholipids in different molar ratios were exposed to cholesterol oxidase at a lateral surface pressure of 20 mN/m (at 30 degrees C). The rate of cholesterol oxidation by cholesterol oxidase was faster in a monolayer consisting of a mono-unsaturated phospholipid (either 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC) or N-oleoyl-sphingomyelin (O-SPM)) and cholesterol than it was in a monolayer of a saturated phospholipid (either 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) or N-stearoyl-sphingomyelin (S-SPM)) and cholesterol. This suggests that the susceptibility of cholesterol to oxidation by cholesterol oxidase was markedly affected by the phospholipid acyl chain composition. In addition, cholesterol was oxidized more readily in a phosphatidylcholine-containing monolayer as compared with a sphingomyelin monolayer (at a similar degree of acyl chain saturation). The average rate of oxidation, as a function of the cholesterol/phospholipid (C/PL) molar ratio in a binary monolayer (with cholesterol and one phospholipid class), was linear except for one discontinuity, at 1:1 for phosphatidylcholine monolayers (either SOPC or DSPC) and at 2:1 for sphingomyelin monolayers (O-SPM or S-SPM). We interpret these discontinuities as indicating the stoichiometry at which cholesterol can exist dispersed in the monolayer without lateral segregation into cholesterol-rich clusters. Next, ternary monolayers were examined (with cholesterol and one phosphatidylcholine and one sphingomyelin species).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of sterol side-chain structure on sterol-phosphatidylcholine interactions in monolayers and small unilamellar vesicles

In this study we have characterized the monolayer behavior of analogues of cholesterol having different side-chain structures and their interaction with phosphatidylcholines in mixed monolayers and small unilamellar vesicles (SUVs). Two series of side-chain analogues of cholesterol were synthesized, one with an unbranched side chain (the n-series, from 3 to 7 carbons in length), and the other with a single methyl-branched side chain (the iso-series, from 5 to 10 carbons in length). The length and conformation of the sterol side chain markedly influenced both the mean molecular area of the pure sterols and their monolayer stability (i.e., collapse pressure). Shorter side chains gave smaller mean molecular areas and decreased monolayer stability. The sterols from the n-series also had smaller mean molecular areas than the corresponding sterols in the iso-series. In mixed 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/sterol monolayers (equimolar ratio; at 22 degrees C), all of the sterols tested decreased the monolayer stability as judged by the lower collapse pressure with sterol than without sterol. A similar trend was observed in mixed monolayers containing 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), except that sterols from the iso-series with a chain length of 8 or 10 carbon atoms actually stabilized the monolayer compared with the sterol-free SOPC monolayer. The ability of the sterols to condense the molecular packing of DPPC was similar with all sterols (3-5% condensation at 10 mN/m), irrespective of the length or structure of the side chain. 5-Androsten-3 beta-ol, however, which lacks the side chain, did not at all condense the monolayer packing of DPPC. With SOPC mixed monolayers, all side chain containing sterols caused a 18-20% condensation (at 10 mN/m) of monolayer packing. The condensing effect of 5-androsten-3 beta-ol on SOPC packing was again much smaller (about 10%) compared with that of the side-chain sterols. The rate of sterol oxidation by cholesterol oxidase (at 37 degrees C) in DPPC-containing SUVs increased as a function of increasing the side-chain length (iso-series). With sterols from the n-series, the same trend was seen, except that the n-C7 analogue was oxidized much slower than the n-C4, n-C5, and n-C6 analogues. With SOPC SUVs, a similar side-chain dependent oxidation pattern was observed. Our results support and extend previous knowledge about the importance of the sterol side chain in determining sterol-sterol and sterol-phospholipid interactions, both in mono- and bilayers.

Effect of cholesterol oxidase treatment on physical state of renal brush border membranes: evidence for a cholesterol pool interacting weakly with membrane lipids

Treatment with cholesterol oxidases has shown that cholesterol is heterogeneously distributed in brush borders isolated from the apical membrane domain of the renal and intestinal epithelial cells [Bloj, B., & Zilversmit, D. B. (1982) J. Biol. Chem. 257, 7608-7614; El Yandouzi, E. H., & Le Grimellec, C. (1992) Biochemistry 31, 547-551]. To better understand the origin of cholesterol heterogeneity, the effects of cholesterol oxidation by Brevibacterium sp. cholesterol oxidase on the physical state of renal brush border membrane vesicles were determined using steady-state fluorescence polarization and differential phase fluorescence of 1,6-diphenyl-1,3,5-hexatriene (DPH). Selective quenching by trinitrobenzenesulfonate indicated that DPH distributes equally between outer and inner membrane leaflets. Oxidation of 90% of the cholesterol decreased the steady-state anisotropy of DPH, determined at 37 degrees C, by 14%. This modification corresponded to a change in the lipid order, the rotational rate of the probe being unaffected. Oxidation of the cholesterol corresponding to the readily accessible pool (30% of the total cholesterol), on the other hand, had a very limited effect on the membrane physical state. This contrasted with the linear decrease in both anisotropy and fluorescence lifetime of DPH obtained when cholesterol was replaced by cholestenone in liposomes made of phosphatidylcholine/sterol (1/1 molar ratio). These results indicate that, in brush border membranes, the cholesterol readily accessible to cholesterol oxidase interacts only weakly with the other membrane lipids.

Oxidation/isomerization of 5-cholesten-3 beta-ol and 5-cholesten-3-one to 4-cholesten-3-one in pure sterol and mixed phospholipid-containing monolayers by cholesterol oxidase

In this study we have examined the cholesterol oxidase (Streptomyces cinnamomeus) catalyzed conversion of either 5-cholesten-3 beta-ol or 5-cholesten-3-one into 4-cholesten-3-one in pure sterol or mixed phospholipid-containing monolayers at the air/buffer interface. The mean molecular area requirement of 5-cholesten-3-one in a pure monolayer was slightly smaller than the comparable area required by 5-cholesten-3 beta-ol (although the collapse pressure was markedly lower for 5-cholesten-3-one), and both sterols were about equally capable of condensing the lateral packing density of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine at a lateral surface pressure of 20 mN/m. Both sterols were converted by cholesterol oxidase to 4-cholesten-3-one, the reaction being faster with 5-cholesten-3-one as compared to 5-cholesten-3-beta-ol. When the temperature-dependency of the cholesterol oxidase catalyzed conversion of the sterols to 4-cholesten-3-one was examined, the Arrhenius activation energy was calculated to +30 kJ/mol and +27 kJ/mol for 5-cholesten-3 beta-ol and 5-cholesten-3-one, respectively, when the sterols were presented to the enzyme as pure sterol monolayers at a lateral surface pressure of 20 mN/m. With a mixed monolayer containing 40 mol% sterol and 60 mol% EPC, the corresponding activation energies were +107 kJ/mol and +96 kJ/mol for 5-cholesten-3 beta-ol and 5-cholesten-3-one, respectively. With the monolayer system used, it appeared that the over all rate-limiting step in the enzyme-catalyzed conversion of 5-en-sterols to 4-en-3-one was the desorption of the sterol molecules from the monolayer into the active site of the enzyme at the interface. This appeared to be true both with pure sterol monolayers as well as with mixed monolayers containing phosphatidylcholine.

Enzyme-catalyzed oxidation of cholesterol in mixed phospholipid monolayers reveals the stoichiometry at which free cholesterol clusters disappear

In this study, we have used cholesterol oxidase as a probe to study cholesterol/phospholipid interactions in mixed monolayers at the air/water interface. Mixed monolayers, containing a single phospholipid class and cholesterol at differing cholesterol/phospholipid molar ratios, were exposed to cholesterol oxidase at a lateral surface pressure of 20 mN/m (at 22 degrees C). At equimolar ratios of cholesterol to phospholipid, the average rate of cholesterol oxidation was fastest in unsaturated phosphatidylcholine mixed monolayers (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and egg yolk phosphatidylcholine), intermediate in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, and slowest in sphingomyelin monolayers (egg yolk or bovine brain sphingomyelin). The average oxidation rate in mixed monolayers was not exclusively a function of monolayer packing density, since egg yolk and bovine brain sphingomyelin mixed monolayers occupied similar mean molecular areas even though the measured average oxidation rate was different with these two phospholipids. This suggests that the phospholipid acyl chain composition influenced the oxidation rate. The importance of the phospholipid acyl chain length on influencing the average oxidation rate was further examined in defined phosphatidylcholine mixed monolayers. The average oxidation rate decreased linearly with increasing acyl chain lengths (from di-8:0 to di-18:0). When the average oxidation rate was examined as a function of the cholesterol to phospholipid (C/PL) molar ratio in the monolayer, the otherwise linear function displayed a clear break at a 1:1 stoichiometry with phosphatidylcholine mixed monolayers, and at a 2:1 C/PL stoichiometry with sphingomyelin mixed monolayers.(ABSTRACT TRUNCATED AT 250 WORDS)

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.