A novel olefinic rearrangement. The enzymic conversion of cholesta-7,9-dien-3β-ol into cholesta-8,14-dien-3β-ol

1. [3alpha-(3)H]Cholesta-7,9-dien-3beta-ol is converted in high yield into cholesterol by a 10000g(av.) supernatant fraction of rat liver homogenate. 2. Incubation of cholesta-7,9-dien-3beta-ol with [4-(3)H]NADPH and rat liver microsomal fractions under anaerobic conditions resulted in (3)H being incorporated into the 14alpha-position of cholest-7-en-3beta-ol. 3. Under anaerobic conditions in the absence of NADPH cholesta-7,9-dien-3beta-ol was isomerized into cholesta-8,14-dien-3beta-ol by rat liver microsomal fractions.     

Sterol synthesis: studies of the metabolism of 14 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol

[3 alpha-3H]14 alpha-Methyl-5 alpha-cholest-7-en-3 beta-ol has been prepared by chemical synthesis. The metabolism of this compound has been studied in the 10,000 g supernatant fraction of liver homogenates of female rats. Efficient conversion to cholesterol was observed. Other labeled compounds recovered after incubation of [3 alpha-3H]14 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol with the enzyme preparations include the unreacted substrate, 5 alpha-cholesta-7,14-dien-3 beta-ol, 5 alpha-cholesta-8,14-dien-3 beta-ol, cholesta-5,7-dien-3 beta-ol, 5 alpha-cholest-8(14)-en-3 beta-ol, 5 alpha-cholest-8-en-3 beta-ol, and 5 alpha-cholest-7-en-3 beta-ol. In addition, significant amounts of incubated radioactivity were recovered in steryl esters. The steroidal components of these esters were found to contain labeled 14 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol, 5 alpha-cholesta-8,14-dien-3 beta-ol, 5 alpha-cholesta-7,14-dien-3 beta-ol, 5 alpha-cholest-8-en-3 beta-ol, 5 alpha-cholest-7-en-3 beta-ol, and cholesterol.     

Studies on delta8-delta7 isomerization and methyl transfer of sterols in ergosterol biosynthesis of yeast.

The formation of cholesta-7,24-dien-3 beta-ol and its activity as a substrate for the sterol side-chain methyltransferase in yeast have not previously been studied. Experiments with acetone-powder extracts of yeast showed that the sterol is formed from zymosterol by delta8-delta7 isomerization. However, direct conversion of cholesta-7,24-dien-3 beta-ol into zymosterol could not be demonstrated. The reversibility of the reaction was proved by the detection of 3H-incorporation into cholesta-8-en-3 beta-ol (with lathosterol as a carrier) from [3H]H2O in the medium. Incubation of cholesta-7,24-dien-3 beta-ol and S-adenosyl-L-[methyl-14C]methionine with the acetone-powder extract resulted in methylation of the sterol to form episterol. Similar incubation of zymosterol gave fecosterol and episterol, suggesting that fecosterol initially formed by the methylation was isomerized to episterol. In intact cells, however, an alternative pathway (zymosterol yields cholesta-7,24-dien-3 beta-ol yields episterol) may also operate. The relative importance of the two pathways is not known.     

Mechanism of C-5 double bond introduction in the biosynthesis of cholesterol by rat liver microsomes.

The dehydrogenation reaction of cholest-7-en-3beta-ol (I) to cholesta-5,7-dien-3beta-ol (II) in the presence of NADH was studied in rat liver microsomes and in microsomal acetone powder preparations, using [3alpha-3H]cholest-7-en-3beta-ol. It was found that the reaction was inhibited by menadione, adenosine diphosphate, potassium ferricyanide, and cytochrome c while p-cresol had no effect. These results indicated the participation of a microsomal electron transport system in the dehydrogenation of cholest-7-en-3beta-ol. The conversion of cholest-7-en-3beta-ol to cholesta-5,7-dien-3beta-ol was also observed in the absence of NADH when ascorbic acid was included in the incubation mixture. However, the ascorbic acid-catalyzed dehydrogenation was not inhibited by potassium ferricyanide. Immunological evidence that microsomal cytochrome b5 is involved in the dehydrogenation of (I) to (II) was obtained. Antibodies specific for rat liver microsomal cytochrome b5 were elicited in rabbits. The anticytochrome b5 immunoglobulin fraction inhibited rat liver microsomal NADH-cytochrome c reductase but not NADPH-cytochrome c reductase. Also, the extent of reduction of cytochrome b5 was not affected by the antibodies. The conversion of (I) to (II) by rat liver microsomes was inhibited (73%) by anticytochrome b5 immunoglobulin at a ratio of microsomal protein:immunoglobulin of 1:5.6. These results are consistent with the participation of microsomal cytochrome b5 in the introduction of the C-5 double bond in cholesterol biosynthesis. A close analogy of the microsomal dehydrogenation of fatty acids and of cholest-7-en-3beta-ol is apparent and this suggests a possible similarity in the mechanisms of the two reactions.     

Mode of formation of cholesta-5,7-dien-3β-ol from cholest-5-en-3β-ol by larvae of Calliphora erythrocephala

1. The conversion of cholest-5-en-3beta-ol (cholesterol) into cholesta-5,7-dien-3beta-ol by axenic Calliphora erythrocephala larvae was demonstrated. 2. The transformation is probably direct (Delta(5)-->Delta(5,7)) and does not involve a Delta(0) intermediate (Delta(5)-->Delta(0)-->Delta(7)--> Delta(5,7)). 3. Delta(7)-bond formation involves the stereospecific elimination of the 7beta hydrogen atom. 4. The relative amounts of free and esterified sterols were determined in larvae grown on cholesterol as sole sterol source and on 5alpha-cholestan-3beta-ol supplemented with minimal amounts of cholesterol. 5. The significance of the results is assessed in relation to the probable role of cholesta-5,7-dien-3beta-ol as an intermediate in the biosynthesis of ecdysones.     

Cholest-4-en-3-one-delta 1-dehydrogenase, a flavoprotein catalyzing the second step in anoxic cholesterol metabolism

The anoxic metabolism of cholesterol was studied in the denitrifying bacterium Sterolibacterium denitrificans, which was grown with cholesterol and nitrate. Cholest-4-en-3-one was identified before as the product of cholesterol dehydrogenase/isomerase, the first enzyme of the pathway. The postulated second enzyme, cholest-4-en-3-one-Delta(1)-dehydrogenase, was partially purified, and its N-terminal amino acid sequence and tryptic peptide sequences were determined. Based on this information, the corresponding gene was amplified and cloned and the His-tagged recombinant protein was overproduced, purified, and characterized. The recombinant enzyme catalyzes the expected Delta(1)-desaturation (cholest-4-en-3-one to cholesta-1,4-dien-3-one) under anoxic conditions. It contains approximately one molecule of FAD per 62-kDa subunit and forms high molecular aggregates in the absence of detergents. The enzyme accepts various artificial electron acceptors, including dichlorophenol indophenol and methylene blue. It oxidizes not only cholest-4-en-3-one, but also progesterone (with highest catalytic efficiency, androst-4-en-3,17-dione, testosterone, 19-nortestosterone, and cholest-5-en-3-one. Two steroids, corticosterone and estrone, act as competitive inhibitors. The dehydrogenase resembles 3-ketosteroid-Delta(1)-dehydrogenases from other organisms (highest amino acid sequence identity with that from Pseudoalteromonas haloplanktis), with some interesting differences. Due to its catalytic properties, the enzyme may be useful in steroid transformations.     

Cholest-4-En-3-One-Δ1-Dehydrogenase, a Flavoprotein Catalyzing the Second Step in Anoxic Cholesterol Metabolism

The anoxic metabolism of cholesterol was studied in the denitrifying bacterium Sterolibacterium denitrificans, which was grown with cholesterol and nitrate. Cholest-4-en-3-one was identified before as the product of cholesterol dehydrogenase/isomerase, the first enzyme of the pathway. The postulated second enzyme, cholest-4-en-3-one-Δ¹-dehydrogenase, was partially purified, and its N-terminal amino acid sequence and tryptic peptide sequences were determined. Based on this information, the corresponding gene was amplified and cloned and the His-tagged recombinant protein was overproduced, purified, and characterized. The recombinant enzyme catalyzes the expected Δ¹-desaturation (cholest-4-en-3-one to cholesta-1,4-dien-3-one) under anoxic conditions. It contains approximately one molecule of FAD per 62-kDa subunit and forms high molecular aggregates in the absence of detergents. The enzyme accepts various artificial electron acceptors, including dichlorophenol indophenol and methylene blue. It oxidizes not only cholest-4-en-3-one, but also progesterone (with highest catalytic efficiency, androst-4-en-3,17-dione, testosterone, 19-nortestosterone, and cholest-5-en-3-one. Two steroids, corticosterone and estrone, act as competitive inhibitors. The dehydrogenase resembles 3-ketosteroid-Δ¹-dehydrogenases from other organisms (highest amino acid sequence identity with that from Pseudoalteromonas haloplanktis), with some interesting differences. Due to its catalytic properties, the enzyme may be useful in steroid transformations.     

Inhibition of sterol synthesis by delta 5-sterols in a sterol auxotroph of yeast defective in oxidosqualene cyclase and cytochrome P-450

Synthesis of ergosterol is demonstrated in the GL7 mutant of Saccharomyces cerevisiae. This sterol auxotroph has been thought to lack the ability to synthesize sterols due both to the absence of 2,3-oxidosqualene cyclase and to a heme deficiency eliminating cytochrome P-450 which is required in demethylation at C-14. However, when the medium sterol was 5 alpha-cholestan-3 beta-ol, 5 alpha-cholest-8(14)-en-3 beta-ol, or 24 beta-methyl-5 alpha-cholest-8(14)-en-3 beta-ol, sterol synthesis was found to proceed yielding 1-3 fg/cell of ergosterol (24 beta-methylcholesta-5,7,22E-trien-3 beta-ol). Ergosterol was identified by mass spectroscopy, gas and high performance liquid chromatography, ultraviolet spectroscopy, and radioactive labeling from [3H]acetate. Except for some cholest-5-en-3 beta-ol (cholesterol) which was derived from the 5 alpha-cholestan-3 beta-ol, the stanol and the two 8(14)-stenols were not significantly metabolized confirming the absence of an isomerase for migration of the double bond from C-8(14) to C-7. Drastic reduction of ergosterol synthesis to not more than 0.06 fg/cell was observed when the medium sterol either had a double bond at C-5, as in the case of cholesterol, or could be metabolized to a sterol with such a bond. Thus, both 5 alpha-cholest-8(9)-en-3 beta-ol and 5 alpha-cholest-7-en-3 beta-ol (lathosterol) were converted to cholesta-5,7-dien-3 beta-ol (7-dehydrocholesterol), and the presence of the latter dienol depressed the level of ergosterol. The most attractive of the possible explanations for our observations is the assumption of two genetic compartments for synthesis of sterols, one of which has and one of which has not been affected by the two mutations. The ability, despite the mutations, to synthesize small amounts of ergosterol which could act to regulate the cell cycle may also explain why this mutant can grow aerobically with cholesterol (acting in the bulk membrane role) as the sole exogenous sterol.     

Evidence for the presence of 7-hydroperoxycholest-5-en-3 beta-ol in oxidized human LDL.

Low density lipoprotein (LDL) cholesterol is known to be oxidized both in vitro and in vivo giving rise to oxygenated sterols. Conflicting results, however, have been reported concerning both the nature and the relative concentrations of these compounds in oxidized human LDL. We examined the extracts obtained from Cu(2+)-oxidized LDL. Thin layer chromatography analysis showed that the sterol mixture became more complex with reaction time. Analysis of the components by thin layer chromatography and mass spectrometry allowed to establish that 7 alpha- and 7 beta-hydroperoxycholest-5-en-3 beta-ol (7 alpha OOH and beta OOH) are largely prevalent among the oxysterols at early times of oxidation. These hydroperoxy derivatives have not been previously identified in oxidized LDL. The concentration of 7-hydroperoxycholest-5-en-3 beta-ol decreased with oxidation time with a concomitant increase of cholest-5-en-3 beta, 7 alpha-diol (7 alpha OH), cholest-5-en-3 beta, 7 beta-diol (7 beta OH), cholesta-3,5-dien-7-one (CD) and cholest-5-en-3 beta-ol-7-one (7CO). After 24 h of oxidation a minor component of the LDL sterols was cholestan-3 beta-ol-5,6-oxide (EP).     

Use of determinations of 7-lathosterol (5 alpha-cholest-7-en-3 beta-ol) and other cholesterol precursors in serum in the study and treatment of disturbances of sterol metabolism, particularly cerebrotendinous xanthomatosis

The sterol composition of sera from patients with cerebrotendinous xanthomatosis (CTX) was investigated by gas chromatographic analysis of saponified extracts, using a polar (CP Wax 52CB) and an apolar (CP Sil 5CB) capillary column. Apart from already known sterols, the presence of increased amounts of 8-lathosterol (5 alpha-cholest-8(9)-en-3 beta-ol) and significant amounts of 8-dehydrocholesterol (cholesta-5,8-dien-3 beta-ol) were noticed. The latter compound has not been detected previously in human serum and possibly represents a hitherto unknown cholesterol precursor. The apparently elevated levels of delta 8-sterols in CTX serum suggests partial inhibition of migration of the 8,9 double bond to the 7,8 position in this condition. The concentration of 7-lathosterol, an indicator of cholesterol production rate, is also highly elevated in CTX serum and quickly returns to normal values after oral bile acid therapy. Determinations of serum lathosterol are not only useful in the follow-up of therapy of CTX patients, but also in the follow-up of hypercholesterolemic patients treated with either HMG-CoA reductase inhibitors or bile acid sequestrants.     

Correlation between serum levels of some cholesterol precursors and activity of HMG-CoA reductase in human liver

The possibility that the serum concentrations of various cholesterol precursors may reflect the activity of the hepatic HMG-CoA reductase was investigated in humans under different conditions. The serum levels of squalene, free and esterified lanosterol, (4 alpha, 4 beta, 14 alpha-trimethyl-5 alpha-cholest-8, 24-dien-3 beta-ol), two dimethylsterols (4 alpha, 4 beta-dimethyl-5 beta-cholest-8-en-3 beta-ol and 4 alpha, 4 beta-dimethyl-5 alpha-cholest-8, 24-dien-3 beta-ol), two methostenols (4 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol and 4 alpha-methyl-5 alpha-cholest-8-en-3 beta-ol), two lathosterols (5 alpha-cholest-7-en-3 beta-ol and 5 alpha-cholest-8-en-3 beta-ol) and desmosterol (cholest-5, 24-dien-3 beta-ol) were measured in untreated patients (n = 7) and patients treated with cholestyramine (QuestranR, 8 g twice daily for 2-3 weeks, n = 5) or chenodeoxycholic acid (15 mg/kg body weight daily for 3-4 weeks, n = 8) prior to elective cholecystectomy. The activity of the hepatic microsomal HMG-CoA reductase was measured in liver biopsies taken in connection with the operation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Conversion of sterols and triterpenes by mycobacteria. II. Transformation of 7-oxygenated sterols into androstane derivatives via a 7-deoxygenation

The bioconversion of 7-oxygenated sterols by Mycobacterium aurum was studied in a preliminary investigation of the microbial conversion of wool wax. 7-Oxocholesterol was found to be transformed mainly into 3,17-dioxygenated androstane derivatives. 7 xi-Hydroxylated sterols were formed in an initial reduction step, and the C-7 hydroxyl group was then eliminated in a dehydration reaction. This was thought to take place during the isomerisation of cholest-4-en-3-one to cholest-5-en-3-one. Deuterium labelling experiments showed that this elimination proceeded faster for the C-7 alpha isomer, although it was not stereospecific. The C-7 alpha and C-7 beta-hydroxy isomers were weakly interconverted via the 7-oxo derivatives. Cholest-4-en-3-one, cholest-1,4-dien-3-one and cholest-4,6-dien-3-one all lost their side chains following a hydrogenation/dehydrogenation reaction. The resulting 3,17-dioxoandrostene or 3,17-androstadiene derivatives were mainly hydrogenated into 5 alpha-androstane-3,17-dione and 5 alpha-androstane-3 beta-ol-17-one. Elimination of the 3 beta-hydroxyl groups giving cholesta-3,5-dien-7-one, and subsequent microbial degradation of the side chain was not observed to any significant extent. The convergence of the bioconversion pathways of cholesterol and the 7-oxygenated cholesterols enabled crude, partially auto-oxidised cholesterol to be used as a substrate for the production of 3,17-dioxygenated androstane derivatives by M. aurum.     

Novel nuclear methylation of sterols by the nematode Caenorhabditis elegans

Caenorhabditis elegans possesses a unique sterol methylation pathway not reported to occur in any other organism and also removes the C-24 ethyl group of sitosterol (a plant sterol). This nematode produced substantial quantities of 4 alpha-methyl-5 alpha-cholest-8(14)-en-3 beta-ol and smaller amounts of lophenol from dietary cholesterol, desmosterol or sitosterol. When C. elegans was propagated in media containing sitosterol plus 25-azacoprostane hydrochloride (25-aza-5 beta-cholestane hydrochloride), an inhibitor of delta 24-sterol reductase in insects, its 4 alpha-methylsterol fraction largely consisted of equal amounts of 4 alpha-methyl-5 alpha-cholesta-7,24-dien-3 beta-ol and 4 alpha-methyl-5 alpha-cholesta-8(14),24-dien-3 beta-ol. Thus 25-azacoprostane hydrochloride inhibited both a delta 24-sterol reductase and a delta 7-sterol isomerase in C. elegans.     

Microsomal enzymes of cholesterol biosynthesis from lanosterol. Characterization, solubilization, and partial purification of NADPH-dependent delta 8,14-steroid 14-reductase

The membrane-bound enzyme of microsomes that catalyzes NADPH-dependent reduction of the 14-double bond of conjugated delta 8,14- and delta 7,14-sterols has been studied both as collected in microsomes from broken cell preparations of rat liver and after solubilization. Optimal incubation conditions for assay of the membrane-bound enzyme have been determined, and properties of the microsomal enzyme have been established with respect to cofactor requirements, kinetics, pH, addition of inhibitors, addition of glycerol phosphatides, and sterol substrate specificity. The 14-reductase is readily solubilized with a mixture of octylglucoside and taurodeoxycholic acid. The solubilized enzyme has been enriched by precipitation with polyethylene glycol and chromatography on DEAE-Sephacel and hydroxylapatite columns. The resulting partially purified enzyme has been obtained free of other microsomal enzymes of cholesterol biosynthesis: 4-methyl sterol oxidase, delta 5,7-sterol 7-reductase, delta 8,24-sterol 24-reductase, 3-ketosteroid reductase, and steroid 8----7-ene isomerase, plus microsomal cytochrome P-450, cytochrome P-450 reductase, cytochrome b5 reductase, and cytochrome b5. The partially purified enzyme is stimulated by addition of phospholipids. All of the properties exhibited by partially purified 14-reductase are consistent with the suggestion that the solubilized and enriched enzyme catalyzes the microsomal reduction of the 14-double bond of the sterol-conjugated dienes. However, presence of the enzyme does not prove that the sterol-conjugated dienes are obligatory precursors of cholesterol.     

Delta8(14)-steroids in the bacterium Methylococcus capsulatus.

The 4,4-dimethyl and 4alpha-methyl sterols of the bacterium Methylococcus capsulatus were identified as 4,4-dimethyl- and 4alpha-methyl-5alpha-cholest-8(14)-en-3beta-ol and 4,4-dimethyl- and 4alpha-methyl-5alpha-cholesta-8(14),24-dien-3beta-ol. Sterol biosynthesis is blocked at the level of 4alpha-methyl delta8(14)-sterols.     

The role of a 5alpha-hydroxylated intermediate in the formation of the 5, 6-double bond in cholesterol biosynthesis.

If the biological conversion of cholest-7-en-3beta-ol (I) into cholesterol (IV) occurred thorugh the intermediacy of cholest-7-ene-3beta,5alpha-diol (II) then the factor(s) adversely affecting the convwesion of the 5alpha-hydroxy sterol (II) into cholesterol must at least equally adversely affect the formation of cholesterol from cholest-7-en-3beta-ol. By using partial denaturation techniquws and dual-labelled precursors it was shown that the enzyme system responsible for the conversion of the 5alpha-hydroxy sterol (II) into cholesterol denatured faster than that for the corresponding conversion from cholest-7-en-3beta-ol (I).     

Marine sterols. VI(1). Sterols of the scallop, Patinopecten yessoensis.

The sterols of the scallop, Patinopecten yessoensis Jay, was found to contain over 20 components. The major components were delta5-sterols, and lesser amount of ring-saturated sterols were also present. Biogenetically unusual C26 sterols (24-norcholesta-5,22-dien-3beta-ol and 24-norcholest-22-en-3beta-ol) and 24(28)-cis-24-propylidenecholest-5-en-3beta-ol (29-methylisofucosterol), 22-trans-27-nor-(24S)-24-methylcholesta-5,22-dien-3beta-ol (occelasterol), and a new sterol, 22-trans-27-nor-(24S)-24-methylcholest-22-en-3beta-ol (patinosterol), were isolated and their structures were confirmed. Occurrence of 22-trans-(24S)-24-methylcholesta-5,22-dien-3beta-ol (24-epibrassicasterol) was confirmed. 22-cis-Cholesta-5,22-dien-3beta-ol was not found.     

Sterol metabolism. XXVI. Pyrolysis of some sterol allylic alcohols and hydroperoxides

The thermal decomposition of the allylic alcohols 5α-cholest-6-ene-3β,5-diol, cholest-5-ene-3β,7α-diol, and cholest-5-ene-3β,7β-diol and of the allylic hydroperoxides 3β-hydroxy-5α-cholest-6-ene-5-hydroperoxide, 3β-hydroxycho lest-5-ene-7α-hydroperoxide, and 3β-hydroxycholest-5ene-7β-hydroperoxide to six common major pyrolysis products cholest-5-ene-3β,7α-diol, cholest-5-ene-3β,7β-diol, 3β-hydroxycholest-5-en-7-one, cholesta-3,5-dien-7-one, cholesta-4,6-dien-3-one, and cholesta2,4,6-triene was established.     

Defective 3-ketosteroid reductase activity in a human monocyte-like cell line

The human monocyte-like cell line U937, which is a cholesterol auxotroph, does not grow on mevalonate, squalene, or 4,4-dimethyl cholest-7-en-3 beta-ol. It grows on cholest-7-en-3 beta-ol and converts it to cholesterol. When deprived of an exogenous source of cholesterol, the cells accumulate 4 alpha-methyl-cholest-8-en-3-one. The cell-free extracts of U937 are also devoid of 3-ketoreductase activity. The present studies indicate that the lesion in cholesterol synthesis by these cells is located at 3-ketosteroid reductase, making this the first report of a deficiency of this enzyme. In contrast, another U937 strain (U937-N) synthesizes cholesterol, does not accumulate 4 alpha-methyl-cholest-8-en-3-one, and has 3-ketosteroid reductase activity. The two strains should be valuable in studies of the regulation of cholesterol metabolism and of the role of cholesterol in membrane structure and function.     

Construction of a catalytically inactive cholesterol oxidase mutant: investigation of the interplay between active site-residues glutamate 361 and histidine 447

Cholesterol oxidase catalyzes the oxidation of cholesterol to cholest-5-en-3-one and its subsequent isomerization into cholest-4-en-3-one. Two active-site residues, His447 and Glu361, are important for catalyzing the oxidation and isomerization reactions, respectively. Double-mutants were constructed to test the interplay between these residues in catalysis. We observed that the k(cat) of oxidation for the H447Q/E361Q mutant was 3-fold less than that for H447Q and that the k(cat) of oxidation for the H447E/E361Q mutant was 10-fold slower than that for H447E. Because both doubles-mutants do not have a carboxylate at position 361, they do not catalyze isomerization of the reaction intermediate cholest-5-en-3-one to cholest-4-en-3-one. These results suggest that Glu361 can compensate for the loss of histidine at position 447 by acting as a general base catalyst for oxidation of cholesterol. Importantly, the construction of the double-mutant H447E/E361Q yields an enzyme that is 31,000-fold slower than wild type in k(cat) for oxidation. The H447E/E361Q mutant is folded like native enzyme and still associates with model membranes. Thus, this mutant may be used to study the effects of membrane binding in the absence of catalytic activity. It is demonstrated that in assays with caveolae membrane fractions, the wild-type enzyme uncouples platelet-derived growth factor receptor beta (PDGFRbeta) autophosphorylation from tyrosine phosphorylation of neighboring proteins, and the H447E/E361Q mutant does not. Thus maintenance of membrane structure by cholesterol is important for PDGFRbeta-mediated signaling. The cholesterol oxidase mutant probe described will be generally useful for investigating the role of membrane structure in signal transduction pathways in addition to the PDGFRbeta-dependent pathway tested.