Assay of intermediates in bile acid biosynthesis using isotope dilution–mass spectrometry: hepatic levels in the normal state and in cerebrotendinous xanthomatosis

Cholestane-3β,5α,6β-triol: high levels in Niemann-Pick type C, cerebrotendinous xanthomatosis, and lysosomal acid lipase deficiency

Niemann-Pick type C (NPC) is a progressive neurodegenerative disease characterized by lysosomal/endosomal accumulation of unesterified cholesterol and glycolipids. Recent studies have shown that plasma cholestane-3β,5α,6β-triol (CT) and 7-ketocholesterol (7-KC) could be potential biomarkers for the diagnosis of NPC patients. We aimed to know the sensitivity and specificity of these biomarkers for the diagnosis of NPC compared with other diseases that can potentially lead to oxysterol alterations. We studied 107 controls and 122 patients including 16 with NPC, 3 with lysosomal acid lipase (LAL) deficiency, 8 with other lysosomal diseases, 5 with galactosemia, 11 with cerebrotendinous xanthomatosis (CTX), 3 with Smith-Lemli-Opitz, 14 with peroxisomal biogenesis disorders, 19 with unspecific hepatic diseases, 13 with familial hypercholesterolemia, and 30 with neurological involvement and no evidence of an inherited metabolic disease. CT and 7-KC were analyzed by HPLC-ESI-MS/MS as mono-dimethylglycine derivatives. Levels of 7-KC were high in most of the studied diseases, whereas those of CT were only high in NPC, LAL, and CTX patients. Consequently, although CT is a sensitive biomarker of NPC disease, including those cases with doubtful filipin staining, it is not specific. 7-KC is a very unspecific biomarker.

History of hepatic bile formation: old problems, new approaches

Studies of hepatic bile formation reported in 1958 established that it was an osmotically generated water flow. Intravenous infusion of sodium taurocholate established a high correlation between hepatic bile flow and bile acid excretion. Secretin, a hormone that stimulates bicarbonate secretion, was also found to increase hepatic bile flow. The sources of the water entering the biliary system with these two stimuli were differentiated by the use of mannitol. An increase in its excretion parallels the increase in bile flow in response to bile acids but not secretin, which led to a quantitative distinction between canalicular and ductular water flow. The finding of aquaglyceroporin-9 in the basolateral surface of the hepatocyte accounted for the rapid entry of mannitol into hepatocytes and its exclusion from water movement in the ductules where aquaporin-1 is present. Electron microscopy demonstrated that bile acids generate the formation of vesicles that contain lecithin and cholesterol after their receptor-mediated canalicular transport. Biophysical studies established that the osmotic effect of bile acids varies with their concentration and also with the proportion of mono-, di-, and trihydroxy bile acids and provides a basis for understanding their physiological effects. Because of the varying osmotic effect of bile acids, it is difficult to quantify bile acid independent flow generated by other solutes, such as glutathione, which enters the biliary system. Monohydroxy bile acids, by markedly increasing aggregation number, severely reduce water flow. Developing biomarkers for the noninvasive assessment of normal hepatic bile flow remains an elusive goal that merits further study.

A useful multi-analyte blood test for cerebrotendinous xanthomatosis

OBJECTIVES

Cerebrotendinous xanthomatosis (CTX) is a rare genetic disorder of bile acid (BA) synthesis that can cause progressive neurological damage and premature death. Blood (normally serum or plasma) testing for CTX is performed by a small number of specialized laboratories, routinely by gas chromatography-mass spectrometry (GC-MS) measurement of elevated 5α-cholestanol. We report here on a more sensitive biochemical approach to test for CTX particularly useful for confirmation of CTX in the case of a challenging diagnostic sample with 5α-cholestanol that, although elevated, was below the cut-off used for diagnosis of CTX (10 μg/mL or 1.0 mg/dL).

DESIGN AND METHODS

We have previously described liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) methodology utilizing keto derivatization to enable the sensitive quantification of plasma ketosterol BA precursors that accumulate in CTX. We have expanded this methodology to perform isotope dilution LC-ESI-MS/MS quantification of a panel of plasma ketosterol BA precursors, with internal standards readily generated using isotopically-enriched derivatization reagent.

RESULTS

Quantification of plasma ketosterol BA precursors (7α-hydroxy-4-cholesten-3-one, 7α,12α-dihydroxy-4-cholesten-3-one and 7α,12α-dihydroxy-5β-cholestan-3-one) in a single LC-ESI/MS/MS test provided better discrimination between a CTX-positive and negative samples analyzed (n=20) than measurement of 5α-cholestanol alone.

CONCLUSIONS

Quantification of plasma ketosterol BA precursors provides a more sensitive biochemical approach to discriminate between CTX negative and positive samples. A multiplexed LC-ESI-MS/MS test quantifying a panel of plasma ketosterols, with simple sample preparation, rapid analysis time and readily available internal standards, can be performed by most clinical laboratories. Wider availability of testing will benefit those affected with CTX.

A blood test for cerebrotendinous xanthomatosis with potential for disease detection in newborns

Cerebrotendinous xanthomatosis (CTX) is a rare, difficult-to-diagnose genetic disorder of bile acid (BA) synthesis that can cause progressive neurological damage and premature death. Detection of CTX in the newborn period would be beneficial because an effective oral therapy for CTX is available to prevent disease progression. There is no suitable test to screen newborn dried bloodspots (DBS) for CTX. Blood screening for CTX is currently performed by GC-MS measurement of elevated 5α-cholestanol. We present here LC-ESI/MS/MS methodology utilizing keto derivatization with (O-(3-trimethylammonium-propyl) hydroxylamine) reagent to enable sensitive detection of ketosterol BA precursors that accumulate in CTX. The availability of isotopically enriched derivatization reagent allowed ready tagging of ketosterols to generate internal standards for isotope dilution quantification. Ketosterols were quantified and their utility as markers for CTX was compared with 5α-cholestanol. 7α,12α-Dihydroxy-4-cholesten-3-one provided the best discrimination between CTX and unaffected samples. In two CTX, newborn DBS concentrations of this ketosterol (120-214 ng/ml) were ∼10-fold higher than in unaffected newborn DBS (16.4 ± 6.0 ng/ml), such that quantification of this ketosterol provides a test with potential to screen newborn DBS for CTX. Early detection and intervention through newborn screening would greatly benefit those affected with CTX by preventing morbidity and mortality.

Five decades with oxysterols

I have been involved in research on oxysterols since 1963 and this review is intended to cover some of the most important aspects of this work. The first project dealed with 7α-hydroxy-4-cholesten-3-one. My successful synthesis of this steroid with high specific radioactivity allowed a demonstration that it is a bile acid precursor. The mechanism of conversion of 7α-hydroxycholesterol into 7α-hydroxy-4-cholesten-3-one was investigated and I concluded that only one enzyme is required and that no isomerase is involved. Accumulation of 7α-hydroxy-4-cholesten-3-one in patients with lack of sterol 27-hydroxylase (Cerebrotendinous xanthomatosis was shown to be an important pathogenetic factor. This disease is characterized by cholestanol-containing xanthomas in tendons and brain and we could show that most of this cholestanol is formed from 7α-hydroxy-4-cholesten-3-one. We also showed that 7α-hydroxy-4-cholesten-3-one passes the blood-brain barrier. In contrast to cholesterol itself, side-chain oxidized oxysterols have a high capacity to pass lipophilic membranes. We demonstrated conversion of cholesterol into 27-hydroxycholesterol to be a significant mechanism for elimination of cholesterol from macrophages. We also showed that conversion of cholesterol into 24S-hydroxycholesterol is important for elimination of cholesterol from the brain. Side-chain oxidized oxysterols have a high capacity to affect critical genes in cholesterol turnover in vitro. Most of the published in vitro experiments with oxysteroids are highly unphysiological, however. Mouse models studied in my laboratory with high or low levels of 27-hydroxycholesterol have little or no disturbances in cholesterol homeostasis. 24S-hydroxycholesterol is an efficient ligand to LXR and suggested to be important for cholesterol homeostasis in the brain. We recently developed a mouse model with markedly increased levels of this oxysterol in circulation and brain. This overexpression had however only a very modest effect on cholesterol turnover. We concluded that oxysterols are not the master regulators of cholesterol homeostasis in vivo suggested previously.

Profiling sterols in cerebrotendinous xanthomatosis: utility of Girard derivatization and high resolution exact mass LC-ESI-MS(n) analysis

In this study we profile free 3-oxo sterols present in plasma from patients affected with the neurodegenerative disorder of sterol and bile acid metabolism cerebrotendinous xanthomatosis (CTX), utilizing a combination of charge-tagging and LC-ESI-MS(n) performed with an LTQ-Orbitrap Discovery instrument. In addition, we profile sterols in plasma from 24-month-old cyp27A1 gene knockout mice lacking the enzyme defective in CTX. Charge-tagging was accomplished by reaction with cationic Girard's P (GP) reagent 1-(carboxymethyl) pyridinium chloride hydrazide, an approach uniquely suited to studying the 3-oxo sterols that accumulate in CTX, as Girard's reagent reacts with the sterol oxo moiety to form charged hydrazone derivatives. The ability to selectively generate GP-tagged 3-oxo-4-ene and 3-oxo-5(H) saturated plasma sterols enabled ESI-MS(n) analysis of these sterols in the presence of a large excess (3 orders of magnitude) of cholesterol. Often cholesterol detected in biological samples makes it challenging to quantify minor sterols, with cholesterol frequently removed prior to analysis. We derivatized plasma (10 μl) without SPE removal of cholesterol to ensure detection of all sterols present in plasma. We were able to measure 4-cholesten-3-one in plasma from untreated CTX patients (1207±302 ng/ml, mean±SD, n=4), as well as other intermediates in a proposed pathway to 5α-cholestanol. In addition, a number of bile acid precursors were identified in plasma using this technique. GP-tagged sterols were identified utilizing high resolution exact mass spectra (±5 ppm), as well as MS(2) ([M](+)→) spectra that possessed characteristic neutral loss of 79Da (pyridine) fragment ions, and MS(3) ([M](+)→[M-79](+)→) spectra that provided additional structurally informative fragment ions.

Cerebrotendinous xanthomatosis: an inborn error in bile acid synthesis with defined mutations but still a challenge

Cerebrotendinous xanthomatosis [CTX] is a rare disease characterized by the accumulation of cholesterol and cholestanol in brain and tendons caused by a mutation in the sterol 27-hydroxylase gene [CYP27A1] involved in bile acid synthesis. Disruption of this gene in mice does not give rise to xanthomas. The gene defect leads to reduced bile acid synthesis with a compensatory increase in the activity of the rate-limiting enzyme in bile acid synthesis, cholesterol 7alpha-hydroxylase. This leads to a marked accumulation of 7alpha-hydroxylated bile acid precursors, in particular 7alpha-hydroxy-4-cholesten-3-one. The latter oxysterol passes the blood-brain barrier and is an efficient precursor to cholestanol. The activity of cholesterol 7alpha-hydroxylase is normalized by treatment with bile acids. Such treatment reduces the xanthomas in CTX patients in parallel with decreased cholestanol levels. The relationship between the accumulation of cholestanol and the development of cholesterol-rich xanthomas has however not been clarified and a suitable animal model is still lacking.

ESI-MS/MS quantification of 7alpha-hydroxy-4-cholesten-3-one facilitates rapid, convenient diagnostic testing for cerebrotendinous xanthomatosis

BACKGROUND

The genetic disorder cerebrotendinous xanthomatosis (CTX) frequently remains undiagnosed for many years. Left untreated CTX is associated with the development of cataracts, xanthomas and severe neurological dysfunction. The method routinely used to screen for CTX is GC-based measurement of elevated 5alpha-cholestanol from hydrolyzed plasma. A plasma test for CTX utilizing ESI-MS/MS methodology would be beneficial.

METHODS

Development of rapid, simple LC-ESI-MS/MS methodology to test plasma for CTX is described. Two hour Girard derivatization allowed for 7alpha-hydroxy-4-cholesten-3-one quantification by isotope dilution LC-ESI-MS/MS within 12 min from un-hydrolyzed affected patient plasma (5 microl).

RESULTS

Adequate sensitivity and reproducibility were achieved for quantification of 7alpha-hydroxy-4-cholesten-3-one, which demonstrated improved utility as a diagnostic marker of disease and to monitor treatment compared to 5alpha-cholestanol. The mean plasma concentration of 7alpha-hydroxy-4-cholesten-3-one in untreated CTX-affected patients (n=6) was 107-fold that in unaffected subjects (n=9), with the lowest concentration in affected patients >10-fold the highest concentration in unaffected subjects.

CONCLUSION

Quantification of the bile acid precursor 7alpha-hydroxy-4-cholesten-3-one with LC-ESI-MS/MS is a novel approach to improved diagnostic testing of plasma for CTX, amenable to high-throughput analysis and automated sample handling. Development of ESI-MS/MS methodology should make CTX testing more widely available and facilitate easier diagnosis of CTX.

Novel sterols synthesized via the CYP27A1 metabolic pathway

A major biologic role of the ubiquitous mitochondrial P450 enzyme CYP27A1 is the generation of ligands such as 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid, which regulate the expression of nuclear receptors that govern many aspects of cholesterol homeostasis. We now report that sterol intermediates in cholesterol synthesis, beginning with the initial post-cyclization sterol, lanosterol, continuing with zymosterol, and ending with desmosterol are also substrates for the enzyme. Using the human enzyme expressed in Escherichia coli, we characterized the retention times and major mass fragments of these novel metabolites. Although sequestration of the enzyme in the inner mitochondrial membrane and normal subcellular organization probably greatly restrict the proportion of these and other intermediates in cholesterol synthesis that undergo side chain oxidation, disruption of compartmentalization can bypass cholesterol as the end product and give rise to potent ligands that further modify gene expression.

Assay method for mitochondrial sterol 27-hydroxylase with 7alpha-hydroxy-4-cholesten-3-one as a substrate in the rat liver

Mitochondrial sterol 27-hydroxylase (EC 1.14.13.15) is an important enzyme, not only in the formation of bile acids from cholesterol intermediates in the liver but also in the removal of cholesterol by side chain hydroxylation in extrahepatic tissues. The enzyme has been assayed by complicated methods using radiolabeled substrates or deuterium-labeled tracers. These methods may be inaccurate for measuring enzyme activity, because the amount of electron-transferring proteins may be insufficient for maximal velocity. To solve this problem, after solubilization of the enzyme from rat liver mitochondria with n-octyl-beta-d-glucopyranoside (OGP), we measured the enzyme activity by incubating the solubilized enzyme with saturated amounts of electron-transferring proteins. In our assay system, using 7alpha-hydroxy-4-cholesten-3-one (HCO) as a substrate, we could easily measure the product, 7alpha,27-dihydroxy-4-cholesten-3-one, with HPLC monitoring absorbance at 240 nm. The product formation was proportionate to the time up to 5 min and the protein concentration up to 0.5 mg of protein/ml. The maximal velocity of the enzyme was 1.1 nmol/min/mg of protein, which was 4- to 16-fold higher than previously reported values. A simple and accurate assay method for sterol 27-hydroxylase in rat liver mitochondria is herein described.

Cholestanol Metabolism, Molecular Pathology, and Nutritional Implications

Cholestanol, not cholesterol, is a minor component in the human body and in foods, but an increase in cholestanol concentration in serum induces a pathological condition named cerebrotendinous xanthomatosis (CTX). In our investigation of this disease for more than 25 years, a procedure for quantification of cholestanol by high-performance liquid chromatography and an assay method for sterol 27-hydroxylase were established, and several mutations of the CYP 27 gene in 10 CTX families were identified. We also established experimental animal models with symptoms of CTX by feeding a high cholestanol diet. Corneal dystrophy and gallstones were produced in mice, and an apoptosis of cerebellar neuronal cells was observed in rats. We propose the following underlying mechanism of CTX pathogenesis: When cholesterol in the plasma membrane is replaced by cholestanol to some extent, the membrane fluidity is reduced, and the calcium channel fails to open, inducing cell death. CTX patients are treated with oral administration of chenodeoxycholic acid, which reduces the cholestanol concentration in serum. Cholestanol has a toxic effect, and an imbalance of the cholesterol/cholestanol ratio in plasma membrane is suspected to cause the disturbance of calcium channel function of the membrane.

Bile acid synthesis in cultured human hepatocytes: support for an alternative biosynthetic pathway to cholic acid

The biosynthesis of bile acids by primary cultures of normal human hepatocytes has been investigated. A general and sensitive method for the isolation and analysis of sterols and bile acids was used, based on anion exchange chromatography and gas chromatography-mass spectrometry (GC/MS). Following incubation for 5 days, 8 oxysterols and 8 C(27)- or C(24)-bile acids were identified in media and cells. Cholic and chenodeoxycholic acids conjugated with glycine or taurine were by far the major steroids found, accounting for 70% and 24% of the total, respectively, being consistent with bile acid synthesis in human liver. Small amounts of sulfated 3beta-hydroxy-5-cholenoic acid and 3beta,7alpha-dihydroxy-5beta-cholanoic acid were also detected. Nine steroids were potential bile acid precursors (2% of total), the major precursors being 7alpha, 12alpha-dihydroxy-3-oxo-4-cholenoic acid and its 5beta-reduced form. These 2 and 5 other intermediates formed a complete metabolic sequence from cholesterol to cholic acid (CA). This starts with 7alpha-hydroxylation of cholesterol, followed by oxidation to 7alpha-hydroxy-4-cholesten-3-one and 12alpha-hydroxylation. Notably, 27-hydroxylation of the product 7alpha, 12alpha-dihydroxy-4-cholesten-3-one and further oxidation and cleavage of the side chain precede A-ring reduction. A-Ring reduction may also occur before side-chain cleavage, but after 27-hydroxylation, yielding 3alpha,7alpha, 12alpha-trihydroxy-5beta-cholestanoic acid as an intermediate. The amounts of the intermediates increased in parallel to those of CA during 4 days of incubation. Suppressing 27-hydroxylation with cyclosporin A (CsA) resulted in a 10-fold accumulation of 7alpha, 12alpha-dihydroxy-4-cholesten-3-one and a decrease of the production of CA and its acidic precursors. These results suggest that the observed intermediates reflect an alternative biosynthetic pathway to CA, which may be quantitatively significant in the cells.

Genetic analysis of a Japanese cerebrotendinous xanthomatosis family: identification of a novel mutation in the adrenodoxin binding region of the CYP 27 gene

Cerebrotendinous xanthomatosis (CTX), an autosomal recessive lipid-storage hereditary disorder, is caused by mutations in the sterol 27-hydroxylase gene (CYP 27). A 24-year-old female Japanese CTX patient and her parents were studied for a CYP 27 mutation. Multiple xanthomas were the main complaint of the patient and plasma cholestanol level was markedly elevated. Sterol analysis of a xanthoma biopsy confirmed cholesterol and cholestanol deposition, and the cholestanol accounted for 8.1% of the total sterols. Sterol 27-hydroxylase activity in fibroblasts derived from the patient was undetectable, while the activities in fibroblasts from her mother and father were 54% and 41% of the normal level, respectively. Direct sequence analysis showed a missense mutation of A for G substitution in the CYP 27 gene at codon 362 (CGT 362Arg to CAT 362His) with a homozygous pattern in the patient, and a heterozygous pattern in the parents. The mutation, which eliminates a normal HgaI endonuclease site at position 1195 of the cDNA and is located at the adrenodoxin binding region of the gene, is most probably responsible for the decreased sterol 27-hydroxylase activity in this Japanese CTX family. The combined data strongly support that the primary enzymatic defect in CTX is the disruption of sterol 27-hydroxylase and that the disease is inherited in an autosomal recessive trait.

Accumulation of 7 alpha-hydroxycholesterol in liver tissue of patients with cholesterol gallstones

Patients with cholesterol gallstones have a reduced pool of bile acids. This study was undertaken to clarify the mechanism by which bile acid biosynthesis does not increase to supranormal levels to cause a reexpansion of the pool. We investigated the first two steps of the bile acid biosynthesis pathway by assaying the activities of cholesterol 7 alpha-hydroxylase, the rate-limiting enzyme in this pathway, and 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase/isomerase, and by measuring the concentrations of 7 alpha-hydroxycholesterol and 7 alpha-hydroxy-4-cholesten-3-one in liver specimens from ten patients with cholesterol gallstones and ten gallstone-free controls. In the patients with gallstones, cholesterol 7 alpha-hydroxylase activity, 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase/isomerase activity, and hepatic 7 alpha-hydroxy-4-cholesten-3-one concentration did not significantly different from levels in controls, but hepatic 7 alpha-hydroxycholesterol concentration was more than twofold that of controls (12.9 +/- 2.6 vs 5.3 +/- 1.2 nmol/g liver, P < 0.01). The concentration of 7 alpha-hydroxycholesterol positively correlated with the ratio of cholesterol 7 alpha-hydroxylase activity to 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase/isomerase activity (r = 0.93; P < 0.005) in the gallstone-free controls. In contrast, this correlation disappeared in the patients with gallstones. These results suggest a derangement of the normal 7 alpha-hydroxycholesterol metabolism in the patients with gallstones. The reason for the accumulation of 7 alpha-hydroxycholesterol remains unclear; however, it is possible that, in patients with cholesterol gallstone, the accumulated 7 alpha-hydroxycholesterol causes inappropriate suppression of cholesterol 7 alpha-hydroxylase activity by product inhibition.

Another cholesterol hypothesis: cholesterol as antioxidant

Current emphasis on cholesterol as agency if not cause of human atherosclerosis and subsequent cardiovascular disease ignores the essentiality of cholesterol in life processes. Additionally ignored is the ubiquitous presence of low levels of oxidized cholesterol derivatives (oxysterols) in human blood and select tissues, oxysterols also implicated in atherosclerosis. Whereas such oxysterols may be regarded putatively as agents injurious to the aorta, an alternative view of some of them is here proposed: that B-ring oxidized oxysterols of human blood represent past interception of blood and tissue oxidants in vivo by cholesterol as an ordinary aspect of oxygen metabolism. Such interception and subsequent efficient hepatic metabolism of oxysterols so formed, with biliary secretion and fecal excretion, constitute as in vivo antioxidant system. Whether cholesterol, oxysterols, oxidized lipoproteins, or oxidants in blood, singly or in concert, cause or exacerbate human atherosclerosis remains to be understood.

Levels of 7 alpha-hydroxy-4-cholesten-3-one in plasma reflect rates of bile acid synthesis in man

A method for analysis of 7 alpha-hydroxy-4-cholesten-3-one in plasma is described. Following solid-phase extraction/purification the compound is determined by high-performance liquid chromatography using a UV detector. The median concentration in healthy subjects was 12 ng/ml (range 3-40). The levels were lower in diseases associated with a low bile acid production: extrahepatic cholestasis, less than 1.5 ng/ml (range less than 0.9-3); liver cirrhosis less than 1.5 ng/ml (range less than 0.9-38), and higher in diseases associated with a high bile acid production: cholestyramine treatment, 188 ng/ml (range 54-477); ileal resection 397 ng/ml (range 128-750). The levels were essentially normal in patients with colon resection. The results are consistent with a strong positive correlation between the levels of 7 alpha-hydroxy-4-cholesten-3-one in plasma and the rate of bile acid synthesis.

Clinical biochemistry of peroxisomal disorders

Peroxisomes have been shown to participate in a variety of pathological processes. Peroxisomal anomalities are central features of Zellweger's cerebro-hepato-renal syndrome, neonatal adrenoleukodystrophy, infantile Refsum's disease and several other genetic metabolic disorders (pseudo-Zellweger syndrome, Leber congenital amaurosis, cerebrotendinous xanthomatosis, rhizomelic chondrodysplasia punctata). In disorders with general loss of peroxisomal functions (Zellweger syndrome, neonatal adrenoleukodystrophy, infantile Refsum's disease) an accumulation of very long-chain fatty acids and pathological bile acids are found. Patients have a defective synthesis of plasmalogens and show increased excretion of dicarboxylic acids of medium chain length and of pipecolic acid in the urine. These anomalities which are due to the lack of peroxisomal enzymes, supply the basis for clinical laboratory tests. The study of these peroxisomal disorders has presented valuable information on the normal function of peroxisomes.

Metabolism of the cholestanol precursor cholesta-4,6-dien-3-one in different tissues

Cerebrotendinous xanthomatosis (CTX) is a lipid storage disease where the basic defect is a lack of the mitochondrial C27-steroid 26-hydroxylase involved in bile acid synthesis (EC 1.14.13.15). Cholestanol and cholesterol accumulate in all tissues. At least part of the accumulation of cholestanol is due to a 7 alpha-dehydroxylation of early bile acid intermediates. Cholesta-4,6-dien-3-one, a proposed intermediate in this pathway, is found in increased concentrations in serum of the patients. This study shows that cholesta-4,6-dien-3-one may be metabolized to 4-cholesten-3-one and cholestanol by liver, adrenals and brain. No conversion was found in intestinal mucosa or in kidneys. The capacity to convert cholesta-4,6-dien-3-one into 4-cholesten-3-one and cholestanol varied in different tissues as well as in different species. The results are discussed in relation to the cholestanol accumulation in CTX.

Cholesterol autoxidation 1981-1986

Literature published between 1980 and 1986 dealing broadly with the topic of cholesterol autoxidation is reviewed. The review builds on the detailed 1981 monographic treatment of the topic by the author and covers new items of chemistry, analysis, and metabolism.

Determination of some hydroxycholesterols in human serum samples

The simultaneous determination of some hydroxycholesterols in human serum samples is described. The procedure is based on hydrolysis and extraction of these compounds in serum samples, followed by removal of especially cholesterol (making use of reversed-phase high-performance liquid chromatography) and derivatization of the purified compounds to their trimethylsilyl ethers and subsequent gas chromatography using flame ionization detection. Serum levels of 7 alpha-hydroxycholesterol, 7 beta-hydroxycholesterol and 26-hydroxycholesterol were determined in several groups of patients: normals, untreated patients suffering from cerebrotendinous xanthomatosis, patients suffering from cerebrotendinous xanthomatosis and treated with either chenodeoxycholic acid or cholic acid in an effective dose, patients suffering from cerebro-hepato-renal syndrome, patients suffering from hypercholesterolemia and treated with cholestyramine for prolonged periods and one patient presumed to be suffering from an inborn error of metabolism in bile acid synthesis. It can be concluded that the 7 alpha-hydroxycholesterol concentration in serum is a good parameter for establishing disorders involving the metabolic conversion of 7 alpha-hydroxycholesterol towards bile acids. In addition, 26-hydroxycholesterol levels in patients suffering from cerebrotendinous xanthomatosis are beyond detectable limits, even during treatment with bile acids in an effective dose, whereas in all other conditions this compound is substantially present.

Accumulation of 7 alpha-hydroxy-4-cholesten-3-one and cholesta-4,6-dien-3-one in patients with cerebrotendinous xanthomatosis: effect of treatment with chenodeoxycholic acid

Evidence was recently presented that an essential part of the accumulation of cholestanol in patients with cerebrotendinous xanthomatosis is due to acceleration of a novel pathway, involving 7 alpha-hydroxylated intermediates in bile acid biosynthesis as precursors (J. Clin. Invest. 1985; 75:448-456). Such intermediates accumulate in patients with cerebrotendinous xanthomatosis due to lack of the mitochondrial 26-hydroxylase involved in the major pathway for bile acid biosynthesis. The new pathway may involve the following steps: 7 alpha-hydroxycholesterol----7 alpha-hydroxy-4-cholesten-3-one----cholesta-4,6- dien-3-one----4-cholesten-3-one----cholestanol. Accurate methods have been developed for assay of 7 alpha-hydroxy-4-cholesten-3-one and cholesta-4,6-dien-3-one in serum, based on isotope dilution-mass spectrometry. The serum levels of 7 alpha-hydroxy-4-cholesten-3-one as well as those of cholesta-4,6-dien-3-one were found to be markedly elevated in the three patients with cerebrotendinous xanthomatosis. Treatment of two of the patients with chenodeoxycholic acid reduced the serum levels of the two steroids by more than 80%. The concentration of cholestanol was reduced by 72% in one patient and by 48% in the other. The possibility is discussed that accumulation of cholestanol in patients with cerebrotendinous xanthomatosis is secondary to accumulation of 7 alpha-hydroxy-4-cholesten-3-one and cholesta-4,6-dien-3-one.

Effects of cholestanol feeding and cholestyramine treatment on the tissue sterols in the rabbit

Rabbits were fed diets enriched with cholestanol or cholesterol (3.5 g/wk) for 4-12 weeks. During cholestanol feeding, the concentration of cholestanol in blood serum, liver, heart and aorta increased 15-30 times. In serum and liver, the concentration of cholesterol also increased. Cholestanol-fed rabbits developed inflammatory changes in the liver, with proliferation of small bile ducts. Liver tests were only slightly abnormal. Morphological atherosclerosis of the aorta was only occasionally seen in rabbits receiving cholestanol for eight weeks or less. During cholesterol feeding, the amounts of cholesterol in different tissues increased dramatically, most in the aorta. Morphological atherosclerosis in the aorta was found in all rabbits fed cholesterol-enriched diets for more than four weeks. Brain cholestanol was doubled in rabbits fed cholestanol for eight weeks, whereas brain sterols did not change significantly during cholesterol feeding. After an additional regression period with cholestyramine for eight weeks, the increased content of cholestanol in the brain was unchanged in cholestanol-fed rabbits. These observations are discussed in relation to the cholestanolosis of the brain that develops in the rare inherited human disease cerebrotendinous xanthomatosis.

Increased (23R)-hydroxylase activity in patients suffering from cerebrotendinous xanthomatosis, resulting in (23R)-hydroxylation of bile acids

Patients suffering from cerebrotendinous xanthomatosis, an inborn error of metabolism in bile acid synthesis, excrete excessive amounts of 23-hydroxylated bile alcohols, 23-norcholic acid and 23-hydroxycholic acid into urine. In this study the configuration of this excreted 23-hydroxycholic acid was established as (23R)-hydroxycholic acid. Urine samples of two treated patients, receiving chenodeoxycholic acid, were investigated to see whether this administered bile acid was partly converted into 23-hydroxychenodeoxycholic acid. One patient was treated with ursodeoxycholic acid for 1 month and subsequently with chenodeoxycholic acid, and the urinary excretion of both (23R)-hydroxychenodeoxycholic acid and (23R)-hydroxyursodeoxycholic acid were followed. Indeed, all three patients excreted (23R)-hydroxylated chenodeoxycholic acid during oral treatment with chenodeoxycholic acid, and the patient treated with ursodeoxycholic acid excreted (23R)-hydroxylated ursodeoxycholic acid. During treatment with chenodeoxycholic acid the excretion of (23R)-hydroxychenodeoxycholic acid increases at first and later on decreases markedly. These findings suggest increased (23R)-hydroxylase activity in patients suffering from cerebrotendinous xanthomatosis, acting both on endogenously synthesized bile alcohols and on exogenously administered bile acids; during continuation of chenodeoxycholic acid treatment in an effective dose (750 mg/day) this enzyme activity gradually disappears.

Gas chromatography-mass spectrometry of isobutyl ester trimethylsilyl ether derivatives of bile acids and application to the study of bile sterol and bile acid biosynthesis in rat liver epithelial cell lines

The derivatization of bile acids into trimethylsilyl ether isobutyl ester (IBTMS) and of neutral sterols into trimethylsilyl ether (TMS) allowed the separation on an OV-1 capillary gas chromatography column of 15 bile steroids as follows: cholesterol, 7 alpha-hydroxycholesterol, 6 beta-hydroxycholesterol, 6 alpha-hydroxycholesterol, 7 beta-hydroxycholesterol, lithocholate, deoxycholate, 25-hydroxycholesterol, chenodeoxycholate, cholate, murocholate, hyodeoxycholate, ursodeoxycholate, hyocholate, and beta-muricholate. Fragmentation data of the coupled gas chromatographic-mass spectrometric (GC-MS) analysis of these nine bile acids as IBTMS derivatives under electron impact and chemical ionizations (methane, isobutane, and ammonia) are given. The ammonia chemical ionization appears to be the best mode for compound identification and quantitation due to fragmentations into high mass ions. The comparison of methylene units of the five sterols as TMS derivatives and of each type of methyl, TMS, or isobutyl ester of the nine bile acids as TMS ethers showed that isobutyl esterification increased dramatically the retention time of the bile acids, allowing their separation after the neutral sterols. Different methods of GC-MS analysis were applied to the study of bile steroid secretion in long-term rat liver epithelial cell lines, either serum-supplemented cell lines or serum-free cell lines, growing in serum-free medium since the primary explanation or after adaptation of serum-supplemented lines to this medium. It is demonstrated for the first time that liver epithelial cell lines maintain the metabolic pathway leading from synthesized cholesterol to dioxygenated sterols and the two normal main primary bile acids of the liver, chenodeoxycholic acid and cholic acid, up to 32-47% of the in vivo daily rate, and in addition the production of alpha-muricholic acid, the bile acid marker of murine liver.

Studies of the mechanism of the increased biosynthesis of cholestanol in cerebrotendinous xanthomatosis. The activity of delta 5-3 beta-hydroxysteroid dehydrogenase

It was recently proposed that the increased biosynthesis of cholestanol in cerebrotendinous xanthomatosis (CTX) is due to increased activity of the delta 5-3 beta-hydroxysteroid dehydrogenase involved in bile acid biosynthesis, causing increased conversion of cholesterol into cholestanol through 4-cholesten-3-one. Our attempts to confirm this hypothesis have failed. Liver biopsy specimens from two patients with CTX did not have any increased capacity to catalyze conversion of 7 alpha-hydroxycholesterol into 7 alpha-hydroxy-4-cholesten-3-one. Further, we did not find any changes in the activity of liver microsomal delta 5-3 beta-hydroxysteroid dehydrogenase after feeding rabbits with cholestanol or cholesterol. The findings are discussed in relation to our hypothesis that the accelerated biosynthesis of cholestanol in CTX is due to an increased conversion of early bile acid intermediates into cholestanol.

Diagnosis of cerebrotendinous xanthomatosis (CTX) and effect of chenodeoxycholic acid therapy by analysis of urine using capillary gas chromatography

By means of capillary gas chromatography urine samples of patients with cerebrotendinous xanthomatosis (CTX) were investigated before and during treatment by oral administration of chenodeoxycholic acid. The occurrence of various conjugated bile alcohols, presumably glucuronides, was demonstrated, the major compound being 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha, 23 xi, 25-pentol. In the bile acid fraction norcholic acid and hydroxycholic acid were shown to be present in considerable amounts. In this way the presence of CTX can be demonstrated conclusively. After chenodeoxycholic acid therapy the excretion of both abnormal bile acids as well as of bile alcohols rapidly decreased within a few weeks, showing the effectiveness of the treatment. By early discovery and subsequent therapy it may be possible to prevent the onset of the detrimental symptoms such as mental deficiency, caused by the accumulation of cholestanol and cholesterol in CTX patients.