Atherosclerosis and sterol 27-hydroxylase: evidence for a role of this enzyme in elimination of cholesterol from human macrophages

27-Hydroxycholesterol was found in surprisingly high amounts in atherosclerotic human femoral arteries. When human macrophages were cultured in a medium containing serum, there was a significant transfer of 27-hydroxy-cholesterol and 3 beta-hydroxy-5-cholestenoic acid from the cells into the medium. Sterol 27-hydroxylase (EC 1.14.13.15) is likely to be responsible for formation of the two products as shown by use of immunoblotting, a specific inhibitor, and the 18O-labeling technique. Sterol 27-hydroxylase has the unusual ability to hydroxylate the same methyl group three times to give a carboxylic acid; thus, 3 beta-hydroxy-5-cholestenoic acid is likely to be a direct product of the enzyme. The production of these steroids increased after addition of cholesterol to the culture medium. By using deuterium-labeled cholesterol, it was ascertained that most of the oxidized products were formed from exogenous cholesterol taken up by the cells. 27-Hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid are present in the circulation and are efficiently converted into bile acids in human liver. It is suggested that conversion of cholesterol into 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid represents a general defence mechanism for macrophages and possibly also other peripheral cells exposed to cholesterol. Absence of this defence mechanism may contribute to the premature atherosclerosis known to occur in patients with sterol 27-hydroxylase deficiency (cerebrotendinous xanthomatosis).

Molecular cloning and identification of a rat serum carboxylesterase expressed in the liver

We have cloned and sequenced a carboxylesterase from rat liver and purified the corresponding protein from rat blood. The cDNA encodes the entire mature serum esterase protein. It is apparently identical to cDNAs cloned from rat liver by several groups (Long, R. M., Satoh, H., Martin, B. M., Kimura, S., Gonzales, F. J., and Pohl, L. R. (1988) Biochem. Biophys. Res. Commun. 156, 866-873; Takagi, Y., Morohashi, K.-i., Kawabata, S.-i., Go, M., and Omura, T. (1988) J. Biochem. (Tokyo) 104, 801-806; and Robbi, M., and Beaufay, H. (1992) Biochem. Biophys. Res. Commun. 183, 836-841). However, the identification of the protein encoded by this cDNA has not been previously reported. The COOH-terminal -TEHT sequence found in the rat serum carboxylesterase does not possess retention properties and is therefore responsible for its secretion and presence in the circulation. The rat serum carboxylesterase was purified to apparent homogeneity by affinity chromatography on immobilized antibody to rat liver microsomal acyl-CoA thioesterase followed by ion exchange chromatography. The purified protein, with a M(r) of approximately 70,000, was cleaved in situ in a polyacrylamide gel with trypsin, and two peptides were isolated and sequenced. Sequence analysis showed that both peptides were identical only to the corresponding deduced amino acid sequence of the cloned cDNA. Antibodies raised to the COOH-terminal amino acid sequence deduced from the cDNA cross-reacted with the purified rat serum carboxylesterase. Changes in serum esterase activity levels followed changes in protein mass in rat serum and changes in liver mRNA levels in response to various nutritional conditions while total liver esterase activity was essentially unchanged. The above experiments confirm the identity of the protein isolated from rat sera with the cDNA cloned from rat liver and suggest a function for the serum esterase in lipid metabolism.

Treatment of IgA nephropathy with omega-3-polyunsaturated fatty acids: a prospective, double-blind, randomized study

In several studies diets supplemented with fish oil containing a high proportion of omega-3-polyunsaturated fatty acids (w-3-PUFA) have been shown to produce beneficial effects, such as a reduction in blood pressure, lipid levels and inflammation, all of which may affect the course of IgA nephropathy. However, the results of hitherto published studies concerning IgA nephropathy have been inconclusive. We therefore carried out a prospective, randomized, placebo-controlled six-month study with a higher daily dose of w-3-PUFA than used in previous studies. Thirty-two adult patients with biopsy-proven IgA nephropathy and proteinuria completed the study: 15 were assigned to a fish-oil product with a high percentage of w-3-PUFA (K 85, with 55% eicosapentenoic and 30% docosahexenoic acid) and 17 to corn oil, 6g daily of either oil. At the start, no significant differences were found between the two groups (K85: 3 females/12 males, mean age 39 years (range 22-64), corn oil: 4 females/13 males, age 42 years (range 26-68). By six months, supplements of K85 resulted in a slight but significant reduction in glomerular filtration rate (GFR) compared to the start: 51Cr-EDTA: 63 +/- 22 to 59 +/- 21 ml/min/1.73 m2 (p < 0.05), creatinine clearance: 91 +/- 31 to 79 +/- 25 ml/min (p < 0.01), s-creatinine: 131 +/- 39 to 139 +/- 39 mumol/l, whereas no change in GFR was observed in the corn oil group. The urinary total protein and red blood cell excretions were not affected in any of the groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Peroxisomal chain-shortening of thromboxane B2: evidence for impaired degradation of thromboxane B2 in Zellweger syndrome

We have shown that rat liver peroxisomes can chain-shorten prostaglandins to dinor- and tetranor-metabolites. In a recent in vivo study we could demonstrate that peroxisomes are of major importance for chain-shortening of prostaglandin F2 alpha in humans (1991, Diczfalusy et al. J. Clin. Invest. 88:978-984). This was shown by identifying the major urinary metabolites of radiolabeled prostaglandin F2 alpha given intravenously to a patient lacking functional peroxisomes (Zellweger syndrome). In the present investigation we have studied the peroxisomal chain-shortening of thromboxane B2, a compound structurally related to prostaglandins. Isolated rat liver peroxisomes oxidized thromboxane B2 to a chain-shortened metabolite in an NAD(+)-dependent reaction. The metabolite was identified as 9,11,15-trihydroxy-2,3,4,5-tetranor-thromb-13-enoic acid (tetranor-thromboxane B1). The urinary excretion of the major beta-oxidized metabolites of thromboxane B2 and prostacyclin was determined in three Zellweger patients and six age-matched controls. The controls excreted on an average 1.7 and 1.1 ng/mg creatinine of 2,3-dinorthromboxane B2 and 2,3-dinor-6-keto-prostaglandin F1 alpha, respectively. In none of the three Zellweger patients could these dinor-metabolites be detected, i.e., the urinary excretion was less than 0.2 ng/mg creatinine. This shows that peroxisomes play an important role in the degradation of the carboxyl side chain of thromboxane B2 in vivo.

On the mechanism of oxidation of cholesterol at C-7 in a lipoxygenase system

Incubation of [7-2H2]cholesterol with soybean lipoxygenase and linoleic acid in the presence of oxygen gave a mixture of 5-cholestene-3 beta,7 alpha-diol, 5-cholestene-3 beta,7 beta-diol, 3 beta-hydroxy-5-cholesten-7-one,5 alpha,6 alpha-epoxycholestan-3 beta-ol, and 5 beta,6 beta-epoxycholestan-3 beta-ol. The conversion into the 7-oxygenated products was associated with a very high intermolecular isotope effect (KH/KD = 15-17), suggesting that the rate-limiting step in the overall conversion is likely to be the abstraction of hydrogen at C-7 in a radical reaction. Evidence that linoleic acid is to some extent directly involved was obtained with the use of [7-3H]cholesterol. Incubation of [7-3H]cholesterol resulted in a significant incorporation of 3H in the reisolated linoleic acid fraction. The isotope effect associated with conversion of [7 alpha-2H]cholesterol into 7-oxygenated products in the lipoxygenase system was 2-3, indicating that the extraction of hydrogen is nonstereospecific. Incubation of [7-2H2]cholesterol with 13-hydroperoxy-9,11-octadecadienoic acid gave the above 7-oxygenated products with relatively small isotope effects (KH/KD = 3-4). It is concluded that the most important mechanism for oxidation of cholesterol at C-7 in the lipoxygenase system involves participation of radicals and that a carbon-centered linoleic acid radical can extract hydrogen directly from cholesterol. Fatty acid hydroperoxides and their secondary products seem to be less important as initiators in connection with oxidation of cholesterol.

Metabolism of prostaglandin F2 alpha in Zellweger syndrome. Peroxisomal beta-oxidation is a major importance for in vivo degradation of prostaglandins in humans

We have recently shown in vitro that the peroxisomal fraction of a rat liver homogenate has the highest capacity to beta-oxidize prostaglandins. In order to evaluate the relative importance of peroxisomes for this conversion also in vivo, we administered [3H]prostaglandin F2 alpha to an infant suffering from Zellweger syndrome, a congenital disorder characterized by the absence of intact peroxisomes. As a control, labeled compound was administered to two healthy volunteers. Urine was collected, fractionated on a SEP-PAK C18 cartridge, and subjected to reversed-phase high-performance liquid chromatography. The Zellweger patient was found to excrete prostaglandin metabolites considerably less polar than those of the control subjects. The major urinary metabolite in the control subjects was practically absent in the urine from the Zellweger patient. The major urinary prostaglandin F2 alpha metabolite from the Zellweger patient was identified as an omega-oxidized C20-prostaglandin, 9,11-dihydroxy-15-oxoprost-5-ene-1,20-dioic acid. The major urinary prostaglandin F2 alpha metabolite from the control subjects had chromatographic properties of a tetranor (C16) prostaglandin, in accordance with earlier published data. The present results, in combination with our previous in vitro data, indicate that peroxisomal beta-oxidation is of major importance for in vivo chain shortening of prostaglandins.

Impaired intrarenal dopamine production following intravenous sodium chloride infusion in type 1 (insulin-dependent) diabetes mellitus

Type 1 (insulin-dependent) diabetes mellitus is characterized by impaired sodium excretion following NaCl infusion. To investigate the possible role of dopamine in the impaired natriuresis in diabetes, intrarenal sodium handling, sodium excretion and urinary dopamine output, reflecting intrarenal dopamine formation, were studied following a 2 h 0.9% NaCl infusion (25 ml/kg) in eight diabetic patients and nine control subjects. The increase in sodium excretion in response to NaCl infusion was significantly (p less than 0.01) reduced in diabetic patients (19 +/- 7%) as compared with control subjects (46 +/- 8%). Fractional proximal tubular sodium reabsorption (determined by lithium clearance) decreased in the control group (p less than 0.01) following NaCl infusion but not in the diabetic group. Fractional distal tubular reabsorption decreased similarly in both groups. In response to NaCl urinary dopamine excretion increased by approximately 15% (p less than 0.01) in the control group but did not change in the diabetic group. The mean urinary dopamine excretion above basal was significantly greater in the control group (8.4 +/- 2.1 nmol/h) than in the diabetic group (-2.2 +/- 2.1 nmol/h; p less than 0.01). The urinary sodium/dopamine excretion ratio did not differ significantly between the two groups in the basal state or following NaCl. Baseline plasma levels of atrial natriuretic peptide did not differ between control and diabetic patients. In the control group atrial natriuretic peptide levels increased significantly (p less than 0.01) in response to NaCl whereas atrial natriuretic peptide levels did not change in the diabetic group.(ABSTRACT TRUNCATED AT 250 WORDS)

The antioxidant butylated hydroxytoluene protects against atherosclerosis

Rabbits fed a 1% cholesterol diet with or without the antioxidant butylated hydroxytoluene (BHT) developed typical atherosclerotic lesions. The addition of BHT gave higher levels of total cholesterol (+40%), triglycerides (+250%), low density lipoprotein (LDL), and very low density lipoprotein (VLDL) in plasma. Despite the lower plasma lipid levels, the degree of atherosclerosis of the aortic surface was considerably higher in rabbits fed cholesterol than in the group treated with cholesterol and BHT. The mean atherosclerotic involvement was 18.6 +/- 4.4% in the former group and 5.9 +/- 1.7% in the latter group (p = 0.02). In all animals, there was a high correlation between the area of the arterial lesion and cholesterol content (r = 0.96). Serum levels of cholesterol autooxidation products (7-ketocholesterol and cholesterol 5 alpha,6 alpha-epoxide) were lower in the group of rabbits treated with BHT (p less than 0.005). Serum levels of vitamin E were slightly higher in the BHT group. There was no significant difference in the clearance of beta-VLDL between the two treatment groups after using either beta-VLDL from cholesterol-fed animals or beta-VLDL from BHT-fed animals. The results are in accord with the contention that oxidative modification of lipoproteins is important for the development of atherosclerosis and that antioxidants may have a protective effect. At present, however, other explanations cannot be completely excluded, for example, effects of antioxidants on immunologic factors or monocyte adhesion.

Stanozolol and experimental atherosclerosis: atherosclerotic development and blood lipids during anabolic steroid therapy of New Zealand white rabbits

Exogenous oestrogens have been shown to reduce low-density lipoprotein (LDL) cholesterol, increase high-density lipoprotein (HDL) cholesterol and reduce the severity of experimental atherosclerosis. In contrast, exogenous progestogens, testosterone and anabolic steroids have been shown to increase the level of LDL cholesterol and decrease the level of HDL cholesterol in man. We induced atheromatosis through cholesterol feeding of New Zealand White rabbits of 1% cholesterol (w/w) food supplementation for three months. Furthermore, control rabbits and cholesterol rabbits respectively, were given stanozolol 5 mg per day orally for the first 6 weeks and thereafter 10 mg per day. We found no significant influence of the anabolic steroid stanozolol either on the extent of atherosclerotic involvement or on HDL or LDL cholesterol. However, we could not exclude an influence of stanozolol on the development of atherosclerosis because of the finding that two out of 10 stanozolol-treated rabbits on normal diet developed macroscopic atherosclerosis compared to none out of 72 rabbits of the same age from previous studies given the same normal diet (p = 0.013).

Identification of intermediates in the peroxisomal beta-oxidation of linoleic acid

Rat liver peroxisomes contain a beta-oxidation system different from that present in the mitochondria. Intermediates in this oxidation have not hitherto been identified by direct methods. Incubation of linoleic acid with isolated peroxisomes (in the absence of detergent) resulted in the accumulation of polar products in addition to the chain-shortened products. Omission of NAD in the incubation mixture considerably increased the accumulation of these products. Two of the products were isolated and characterized by gas chromatography-mass spectrometry. They were identified as 2,3-dehydrolinoleic acid and 3-hydroxylinoleic acid, based on identical chromatographic behaviour and mass spectra compared to synthetic reference compounds. Stereochemical analysis of catalytically hydrogenated 3-hydroxylinoleic acid showed a D/L ratio near to one. The mechanism behind the apparent lack of stereospecificity is discussed in relation to the recently described novel peroxisomal 2-enoyl-CoA hydratase (Smeland, T.E., Li, J., Chu, C.-h., Cuebas, D. and Schultz, H. (1989) Biochem. Biophys. Res. Commun. 160, 988-992 and Hiltunen, J.K., Palosaari, P.M. and Kunau, W.-H. (1989) J. Biol. Chem. 264, 13536-13540). In previous work we have demonstrated that beta-oxidation intermediates accumulate also in the peroxisomal metabolism of C27-bile acid intermediates and prostaglandins. The possibility is discussed that the peroxisomal beta-oxidation system is less tightly coupled than the corresponding system in mitochondria.

Identification of metabolites from peroxisomal beta-oxidation of prostaglandins

We have recently shown that isolated rat liver peroxisomes can chain-shorten prostaglandin F2 alpha and prostaglandin E2 to tetranor-metabolites. In the present report dinor-metabolites of these two prostaglandins were also identified, suggesting that the peroxisomal chain-shortening reaction of prostaglandins is a beta-oxidation reaction. Furthermore, an intermediate containing an extra double bond was isolated from incubates of prostaglandin F2 alpha with peroxisomes. This intermediate was tentatively assigned the structure 2,3-dehydroprostaglandin F2 alpha. Prostaglandin E1 and a major circulating prostaglandin F2 alpha metabolite were also metabolized to chain-shortened products by peroxisomes. The accumulation of the 2,3-dehydro-metabolite and the dinor-metabolites suggest that the peroxisomal beta-oxidation sequence is not tightly coupled, in contrast to mitochondrial fatty acid oxidation.

Experimental atherosclerosis: effects of oestrogen and atherosclerosis on thromboxane and prostacyclin formation

We evaluated the effect of oestrogen and experimental atherosclerosis on the in vivo formation of thromboxane and prostacyclin in rabbits. Twenty-four New Zealand White rabbits were divided into four groups. One group received control diet, one group received control diet and oestrogen, one group received control diet supplemented with 1% cholesterol and one group received cholesterol supplemented diet and oestrogen during 3 months. The in vivo formation of thromboxane and prostacyclin were measured as 2,3-dinor-TxB2 and 2,3-dinor-6-keto-PGF1 alpha in urine by gas chromatography/mass spectrometry. All rabbits on cholesterol diet became hypercholesterolaemic and developed atherosclerosis. As in previous experiments cholesterol and oestrogen-treated rabbits had only minor atherosclerosis compared to purely cholesterol-fed rabbits. The in vivo production of thromboxane in oestrogen-treated rabbits decreased from 1641 +/- 162 pg mg-1 creatinine pretreatment to 808 +/- 92 pg mg-1 creatine at 12 weeks (P = 0.0001). In contrast, the in vivo production of prostacyclin increased during oestrogen treatment (P = 0.0027). The in vivo production of prostacyclin decreased during pure cholesterol feeding without oestrogen 1384 +/- 219 pg mg-1 creatinine to 702 +/- 142 pg mg-1 creatinine (P = 0.0091). The ratio of in vivo prostacyclin to thromboxane formation increased 2-3-fold during oestrogen therapy (P = 0.0007).(ABSTRACT TRUNCATED AT 250 WORDS)

Oestrogen-induced changes in lipoprotein metabolism: role in prevention of atherosclerosis in the cholesterol-fed rabbit

Administration of moderate pharmacological doses of oestrogen to cholesterol-fed rabbits for 12 weeks resulted in a dramatically retarded development of arterial lesions as compared to non-oestrogen-treated, cholesterol-fed rabbits (7% vs. 47% aortal involvement). Oestrogen treatment was associated with a retarded increase in plasma cholesterol, and five times higher high density lipoprotein (HDL) to very low density lipoprotein (VLDL) cholesterol ratio. Expression of hepatic lipoprotein receptor activity, as detected by heparin-releasable binding of 125I-hypercholesterolaemic VLDL, was heavily suppressed by cholesterol feeding. Administration of oestrogen modulated this response and resulted in higher receptor expression. In accordance, oestrogen treatment resulted in a less prominent reduction of 125I-hypercholesterolaemic VLDL clearance in the cholesterol-fed rabbits. VLDL from both groups of cholesterol-fed animals stimulated cholesteryl ester synthesis in cultured macrophages to the same extent. Thus, in rabbits under a dietary cholesterol load, oestrogen counteracted hepatic lipoprotein receptor suppression, reduced plasma VLDL- and increased plasma HDL-cholesterol levels, and to a large extent abolished the development of atherosclerosis.

Peroxisomal chain-shortening of prostaglandin F2 alpha

We have recently reported that prostaglandin F2 alpha can be chain-shortened by isolated rat liver peroxisomes. In the present study it is further established by cell fractionation experiments that the enzymes involved in this reaction are localized to peroxisomes. Under the conditions employed, the highest activity was found in the light mitochondrial fraction. Further fractionation of the light mitochondrial fraction by sucrose density gradient centrifugation showed that the prostaglandin oxidation activity comigrated with peroxisomal marker enzymes. Di(2-ethylhexyl)phthalate treatment resulted in a tenfold increased capacity for the conversion of prostaglandin F2 alpha into tetranorprostaglandin F1 alpha. The reaction was not inhibited by KCN. The reaction was further characterized with respect to cofactor requirements. The prostaglandin oxidation was found to be completely dependent on NAD, CoA, ATP, Mg2+ and was stimulated by FAD. Incubation of prostaglandin E2 with peroxisomes resulted in conversion into several products. After alkaline hydrolysis, one of these was identified as tetranorprostaglandin B1.

Attempts to define the minimal serum level of vitamin A required for normal visual function in a patient with severe fat malabsorption

A case with severe malabsorption of fat soluble vitamins is described. The malabsorption developed after an intestinal bypass operation due to morbid obesity. Night blindness occurred as the first symptom of vitamin A deficiency. The cone visual sensory threshold was elevated about one log unit and the rod threshold abot two and a half log units. No changes of the a- and b-waves of the electroretinogram (ERG) was observed. However, during the initial phase of very low serum reninol level (0.21 mumol/l) the summed amplitudes of the oscillatory potentials (OPs) were lower. After parenteral therapy with vitamin A the night blindness disappeared and the dark-adapted rod and cone threshold sensitivity recovered to normal. However, the time-course of rod adaptation first reached normal levels after 5 months. The amplitudes of the OPs of the ERG response returned to normal when the serum retinol level had increased close to normal. Serum retinol levels of 0.7 mumol/l or higher were always associated with normal or close to normal dark-adapted rod sensitivity. However, a normal serum retinol level (> 0.95 mumol/l) and a normal dark-adapted rod threshold sensitivity were not always associated with a normal time-course of the rod adaptation. It is concluded, that the maintenance dosage of vitamin A must be individualized and that patients who have undergone jejuno-ilea bypass surgery must be carefully monitored for vitamin A deficiency by both serum levels and dark adaptation measurements.

Increased aortic thromboxane production in experimental atherosclerosis

It is of great importance to clarify the role of thromboxane and prostacyclin in the process of atherosclerosis. In order to study this we induced atherosclerosis in rabbits through cholesterol feeding (1% w/w) for three to 12 weeks. After sacrifice the aorta was quickly removed and incubated. The in vitro thromboxane production measured as immunoreactive thromboxane B2 increased by 86 per cent in atherosclerotic vessels compared to control vessels (p less than 0.0005). The in vitro production of prostacyclin measured as immunoreactive 6-keto-prostaglandin F1 alpha showed a tendency to higher values in the atherosclerotic vessels compared to the control vessels (31%, p less than 0.025). This suggests that an increased thromboxane production rather than a decreased prostacyclin production might be important for the progression of atherosclerosis.

Chain-shortening of prostaglandin F2 alpha by rat liver peroxisomes

Liver peroxisomes were isolated from di(2-ethylhexyl)phthalate treated rats by isopycnic sucrose gradient centrifugation of a light mitochondrial fraction. Incubation of prostaglandin F2 alpha with purified peroxisomes resulted in conversion into a more polar product(s). In contrast, incubation with mitochondrial fractions and microsomal fractions under the same conditions did not result in any detectable conversion. The polar material obtained from a preparative incubation was purified by high performance liquid chromatography and characterized by radio-gas chromatography and gas chromatography-mass spectrometry. The structure of the polar compound was shown to be 5,7,11-trihydroxy-tetranorprost-9-enoic acid (tetranor-prostaglandin F1 alpha). Prostaglandin F2 alpha was thus chain-shortened by four carbon atoms.

Products, kinetics, and substrate specificity of homogeneous thromboxane synthase from human platelets: development of a novel enzyme assay

Homogeneous thromboxane synthase from human platelets converted prostaglandin H2 (PGH2) to thromboxane A2 (measured as thromboxane B2, TxB2), 12(L)-hydroxy-5,8,10-heptadecatrienoic acid (HHT), and malondialdehyde (MDA) in equimolar amounts under a variety of experimental conditions. PGG2 was transformed to MDA and corresponding 15- and 12-hydroperoxy products. PGH1 was enzymatically transformed into 12(L)-hydroxy-8,10-heptadecadienoic acid (HHD) and PGH3 into TxB3 and 12(L)-hydroxy-5,8,10,14-heptadecatetraenoic acid (delta 14-HHT) as earlier reported for solubilized and partially purified thromboxane synthase preparations. The ratio of thromboxane to C17 hydroxy fatty acid formation was 1:1 with PGG2, PGH2, and PGH3 as substrates. These results confirm and extend earlier observations with partially purified enzyme that the three products are formed in a common enzymatic pathway (Diczfalusy, U., Falardeau, P., and Hammarström, S. (1977) FEBS Lett. 84, 271-274). A convenient spectrophotometric assay for thromboxane synthase activity measuring the ultraviolet light absorption of the C17 hydroxy acid formed (e.g., HHT) was developed. The validity of the assay was determined employing specific inhibitors for thromboxane synthase. The substrate specificity of thromboxane synthase was determined using this assay. PGG2 and PGH3 showed Vmax and KM values similar to those of PGH2. The KM value of PGH1 was also identical to that of PGH2 but the Vmax value PGH1 was more than twice as high as that of PGH2.

Determination of 4-methylpyrazole in plasma using solid phase extraction and HPLC

A rapid method for the determination of 4-methylpyrazole in plasma is described. Internal standard (3-methylpyrazole) is added to the plasma which is subsequently applied to a BondElut SCX column. After washing the column the 3- and 4-methylpyrazoles are eluted with a phosphate buffer and analyzed by isocratic reversed-phase high performance liquid chromatography with UV detection. The concentration of 4-methylpyrazole is calculated from the 4-methylpyrazole/3-methylpyrazole peak height ratio. The method is linear between 2.5 and 100 mumol/L with a within-day precision (CV) of 2.2% (n = 10) and a day-to-day precision of 2.8% (n = 30). The sensitivity is sufficient for analysis of plasma levels in the low micromolar range.