UDP-glucuronosyltransferase-dependent bioactivation of clofibric acid to a DNA-damaging intermediate in mouse hepatocytes

Glucuronidation of a number of carboxyl-containing drugs generates reactive acyl glucuronide metabolites. These electrophilic species alkylate cell proteins and may be implicated in the pathogenesis of a number of toxic syndromes seen in patients receiving the parent aglycones. Whether acyl glucuronides also attack nuclear DNA is unknown, although the acyl glucuronide formed from clofibric acid was recently found to decrease the transfection efficiency of phage DNA and generate strand breaks in plasmid DNA in vitro. To determine if such a DNA damage occurs within a cellular environment, the comet assay (i.e. single-cell gel electrophoresis) was used to detect DNA lesions in the nuclear genome of isolated mouse hepatocytes cultured with clofibric acid. Overnight exposure to 50 microM and higher concentrations of clofibric acid produced concentration-dependent increases in the comet areas of hepatocyte nuclei, with 1 mM clofibrate producing a 3.6-fold elevation over controls. These effects closely coincided with culture medium concentrations of the glucuronide metabolite formed from clofibric acid, 1-O-beta-clofibryl glucuronide. Consistent with a role for glucuronidation in the DNA damage observed, the glucuronidation inhibitor borneol diminished glucuronide formation from 100 microM clofibrate by 98% and returned comet areas to baseline levels. Collectively, these results suggest that the acyl glucuronide formed from clofibric acid is capable of migrating from its site of formation within the endoplasmic reticulum to generate strand nicks in nuclear DNA.     

Simultaneous determination of clofibrate and its active metabolite clofibric acid in human plasma by reversed-phase high-performance liquid chromatography with ultraviolet absorbance detection

A reversed-phase high-performance liquid chromatographic (HPLC) using ultraviolet (UV) absorbance detection method for simultaneous determination of clofibrate (I) and its major metabolite clofibric acid (II) in human plasma has been developed to support a clinical study. I, II and internal standard (I.S., III) are isolated from human plasma by 96-well solid-phase extraction (SPE) C(18)z.ccirf;AR plate and quantified by direct injection of the SPE eluent onto the HPLC with UV detection wavelength at 230 nm. Two chromatographic methods, isocratic and step gradient, have been validated from 1.0 to 100.0 microg/ml and successfully applied to plasma sample analysis for a clinical study. The lower limit of quantitation (LLOQ) is 1.0 microg/ml for both I and II when 500 microl plasma sample is processed. Sample collection and preparation is conducted at 5 degrees C to minimize the hydrolysis of I to II in human plasma.     

Increased activity of stearoyl-CoA desaturation in liver from rat fed clofibric acid

Male rats were fed a diet containing 0.5% (w/w) p-chlorophenoxyisobutyric acid (clofibric acid), a hypolipidemic drug. Activities of stearoyl-CoA desaturation in hepatic microsomes were increased approx. 4 times following the administration of clofibric acid for 7 days. An increase in the activity of desaturation of stearic acid was also observed in the liver of clofibric acid-fed rats in vivo. The increase in the activity of microsomal stearoyl-CoA desaturation by clofibric acid-feeding was due to the increase in the activity of terminal desaturase as measured by the rate constant for cytochrome b5 reoxidation, but not due to the changes in cytochrome b5 content and NADH-cytochrome b5 reductase activity. Increases in the activity of stearoyl-CoA desaturation by clofibric acid-feeding were also observed in rats of hormonally altered state, such as diabetic rats, hyperthyroid rats and hypothyroid rats. Percentages of octadecenoic acid in total fatty acid of hepatic lipid were increased with the increase in the activity of stearoyl-CoA desaturation.     

Induction of hepatic long-chain acyl-CoA hydrolase by clofibric acid administration

The induction of hepatic long-chain acyl-CoA hydrolase in the cytosolic fraction by administration of clofibric acid (p-chlorophenoxyisobutyric acid) was compared in rats, mice and guinea-pigs. In rats, two long-chain acyl-CoA hydrolases were induced by the administration of clofibric acid. In mice, only one long-chain acyl-CoA hydrolase was induced, and this hydrolase had properties similar to those of the lower-molecular-weight hydrolase induced in the hepatic cytosol of rats. In hepatic cytosol of guinea-pig, no hydrolase was induced by the administration of clofibric acid.     

Direct analysis of salicylic acid,salicyl acyl glucuronide,salicyluric acid and gentisic acid in human plasma and urine by high-performance liquid chromatography

A method for the simultaneous direct determination of salicylate (SA), its labile, reactive metabolite, salicyl acyl glucuronide (SAG), and two other major metabolites, salicyluric acid and gentisic acid in plasma and urine is described. Isocratic reversed-phase high performance liquid chromatography (HPLC) employed a 15-cm C₁₈ column using methanol-acetonitrile-25 mM acetic acid as the mobile phase, resulting in HPLC analysis time of less than 20 min. Ultraviolet detection at 310 nm permitted analysis of SAG in plasma, but did not provide sensitivity for measurement of salicyl phenol glucuronide. Plasma or urine samples are stabilized immediately upon collection by adjustment of pH to 3–4 to prevent degradation of the labile acyl glucuronide metabolite. Plasma is then deproteinated with acetonitrile, dried and reconstituted for injection, whereas urine samples are simply diluted prior to injection on HPLC. m-Hydroxybenzoic acid served as the internal standard. Recoveries from plasma were greater than 85% for all four compounds over a range of 0.2–20 μg/ml and linearity was observed from 0.1–200 μg/ml and 5–2000 μg/ml for SA in plasma and urine, respectively. The method was validated to 0.2 μg/ml, thus allowing accurate measurement of SA, and three major metabolites in plasma and urine of subjects and small animals administered salicylates. The method is unique by allowing quantitation of reactive SAG in plasma at levels well below 1% that of the parent compound, SA, as is observed in patients administered salicylates.     

Pharmacokinetic and metabolic study of itanoxone in the crab Pachygrapsus marmoratus (Decapoda, Brachyura): comparative study with clofibric acid

The LD50 of itanoxone injected i.v. in the crab Pachygrapsus marmoratus is 144 mg/kg. Hypolipidemic drug distribution is distinct according to organ and sex. The fixation power of muscle is comparatively low. Ovary radioactivity is higher than in the testicles. Itanoxone storage is large in the hepatopancreas. Metabolism, increasing over time, is not noticeably different according to sex. Two metabolites, chlorobiphenylcarboxylic acid and chlorobiphenylacetic acid, found in crab, are also identified in rat and man. The pharmacokinetic and metabolic study of itanoxone and clofibric acid in the crab shows a difference between the two drugs. Itanoxone metabolism and fixation is higher than clofibric acid.     

Simultaneous determination of mycophenolic acid and its phenolic glucuronide in human plasma using an isocratic high-performance liquid chromatography procedure

Simultaneous determination of mycophenolic acid (MPA) and mycophenolic acid glucuronide (MPAG) in plasma was accomplished by isocratic HPLC with UV detection. After protein precipitation and phase separation with saturated sodium dihydrogenphosphate, chromatographic separation was achieved on a monolithic column "Chromolith Performance RP-18e", with acetonitrile/0.01 M phosphate buffer, pH 3, (25:75, v/v), as the mobile phase; flow rate 3.3 ml/min and measurement at 214 nm. Linearity was verified up to 40 mg/l for MPA and up to 400 mg/l for MPAG. Detection limits based on the analysis of 50 microl plasma were 0.05 and 0.5 mg/l for MPA and MPAG, respectively. Accuracy was 99.6-104% for MPA and 95.6-105% for MPAG and total imprecision (CV) was <7% for both compounds. Analytical recovery was >95% for MPA and MPAG. The method is simple, rapid, accurate and suitable for routine determination of MPA and MPAG in plasma.     

Induction of peroxisomal beta-oxidation enzymes in primary cultured rat hepatocytes by clofibric acid

Rat hepatocytes were cultured for 72 h with or without the addition of 0.5 mM clofibric acid. The activities of individual enzymes of the peroxisomal beta-oxidation pathway (acyl-CoA oxidase, enoyl-CoA hydratase-3-hydroxyacyl-CoA dehydrogenase bifunctional protein, and 3-ketoacyl-CoA thiolase) decreased in the control culture, but markedly increased synchronously in the clofibric acid-treated culture. The levels of mRNAs coding for these enzymes and the rates of synthesis of the enzymes were also elevated in the clofibric acid-treated culture, although no proportional relationship was observed between the time-dependent changes of these parameters. The increase in mRNAs was much higher than the increase in the rate of synthesis of the enzymes. The activity of catalase, its mRNA level and the rate of its synthesis were slightly affected. The effects of clofibric acid on the peroxisomal beta-oxidation enzymes and catalase in primary cultured hepatocytes were very similar to those observed in vivo. These results, therefore, suggest that primary culture of hepatocytes should provide a useful means for investigating the mechanism of induction of peroxisomal enzymes and the mechanism of action of peroxisome proliferators.     

Discovery,synthesis and in combo studies of a tetrazole analogue of clofibric acid as a potent hypoglycemic agent

A tetrazole isosteric analogue of clofibric acid (1) was prepared using a short synthetic route and was characterized by elemental analysis, NMR (¹H, ¹³C) spectroscopy, and single-crystal X-ray diffraction. The in vitro inhibitory activity of 1 against 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) was evaluated, showing a moderate inhibitory enzyme activity (51.17% of inhibition at 10 μM), being more active than clofibrate and clofibric acid. The antidiabetic activity of compound 1 was determined at 50 mg/Kg single dose using a non insulin dependent diabetes mellitus rat model. The results indicated a significant decrease of plasma glucose levels, during the 7 h post-administration. Additionally, we performed a molecular docking of 1 into the ligand binding pocket of one subunit of human 11β-HSD1. In this model, compound 1 binds into the catalytic site of 11β-HSD1 in two different orientations. Both of them, show important short contacts with the catalytic residues Ser 170, Tyr 183, Asp 259 and also with the nicotinamide ring of NADP⁺.     

Inducing effects of clofibric acid on 1-acylglycerophosphorylcholine acyltransferase in kidney and intestinal mucosa of rats

Administration of p-chlorophenoxyisobutyric acid (clofibric acid) markedly increased the activity of microsomal 1-acylglycerophosphorylcholine (1-acyl-GPC) acyltransferase in kidney, intestinal mucosa and liver, but not in brain, heart, lung, spleen, testis or skeletal muscle. In both kidney and liver, a marked dose-dependent increase in the activities of both microsomal 1-acyl-GPC acyltransferase and peroxisomal beta-oxidation was observed. In the rats treated with clofibric acid at a relatively low dose, the increase in the activity of 1-acyl-GPC acyltransferase in kidney was more marked than that in liver. The extent of the relative increase in the activity of 1-acyl-GPC acyltransferase to the activity of peroxisomal beta-oxidation in kidney was more marked than that in liver. The increased activity of 1-acyl-GPC acyltransferase in both kidney and liver lasted throughout the 8-week treatment period of rat with clofibric acid.     

Modulation by dietary oils and clofibric acid of arachidonic acid content in phosphatidylcholine in liver and kidney of rat: Effects on prostaglandin formation in kidney

The manipulation of 20:4(n − 6) contents in phosphatidylcholine of liver and kidney of rats by dietary oils and p-chloropheno-xyisobutyric acid (clofibric acid) as well as the effects on the formation of prostaglandin E₂ in kidney were studied. Three groups of rats were fed diets that contained either safflower oil (SO) or perilla oil (PO) or fish oil (FO) for 1 week. Each dietary group was divided into two groups. One group continued the same diet for another 1 week; the second group continued the same diet and received subcutaneous injections of clofibric acid once a day for 1 week. The content of 20:4(n − 6) in hepatic phosphatidylcholine was markedly lowered by feeding either FO or PO and was further decreased by the administration of clofibric acid. Feeding either FO or PO lowered the content of 20:4(n − 6) in hepatic phosphatidylethanolamine, whereas clofibric acid increased it. The decrease in the level of 20:4(n − 6) in serum phospholipid was produced by feeding either FO or PO and by the administration of clofibric acid as well. There was a high correlation for the levels of 20:4(n − 6) between hepatic phosphatidylcholine and serum phospholipid. The changes brought about by dietary oils and clofibric acid in renal phosphatidylcholine was similar to those observed in liver. The content of 20:4(n − 6) in renal phosphatidylcholine was highly correlated with the level of 20:4(n − 6) in serum phospholipid. Other phospholipids in kidney responded less sensitively to the manipulation by dietary oils and clofibric acid. These results suggest that the level of 20:4(n − 6) in renal phosphatidylcholine is regulated by the level of 20:4(n − 6) in hepatic phosphatidylcholine through the changes in serum level of 20:4(n − 6). Formation of prostaglandin E₂ in kidney slices was dependent on the content of 20:4(n − 6) in renal phosphatidylcholine.     

Preliminary media screening for application in the removal of clofibric acid,carbamazepine and ibuprofen by SSF-constructed wetlands

The aim of the present work was to evaluate the sorption capacity of light expanded clay aggregates (LECA) to remove mixtures of ibuprofen, carbamazepine and clofibric acid in water and wastewater. High removal efficiencies were attained for carbamazepine and ibuprofen while a less satisfactory performance was observed for clofibric acid. In a mixture of the three compounds in water a slight decrease in the sorbed amounts is observed in comparison with solutions of the single compounds, indicating some competitive sorption. In wastewater, the pharmaceuticals mixture also undergoes a slight reduction in the sorbed amounts of carbamazepine and ibuprofen, probably due to the presence of dissolved organic matter which increases their solubility. These compounds were removed in the following order of efficiencies in all the tested conditions: carbamazepine > ibuprofen > clofibric acid. Two other clay materials – sepiolite and vermiculite – were tested for the removal of the more recalcitrant clofibric acid, and vermiculite exhibited higher removal efficiency than LECA. The sorption is characterized by an initial fast step, with most pharmaceuticals being removed within the first 24 h. The results of this study are a first step in the process of selecting an appropriate material or combination of materials to be used as media in SSF-CWs designed for the removal of pharmaceuticals from wastewaters.     

Modification by clofibric acid of acyl composition of glycerolipids in rat liver. Possible involvement of fatty acid chain elongation and desaturation

Administration of p-chlorophenoxyisobutyric acid (clofibric acid) to rats induced a marked change in acyl composition of hepatic glycerolipids; a considerable increase in the proportion of octadecenoic acid (18:1) was accompanied by a marked decrease in the proportion of octadecadienoic acid (18:2). Among the glycerolipids, the changes in the proportions of 18:1 and 18:2 were the most marked in phosphatidylcholine. The change in the acyl composition of phosphatidylcholine paralleled the change in free fatty acid composition in microsomes. The treatment of rats with clofibric acid resulted in a 2.3-fold increase in activity of microsomal palmitoyl-CoA chain elongation and a 4.8-fold increase in activity of stearoyl-CoA desaturation. The activities of acyl-CoA synthetase, 1-acylglycerophosphate acyltransferase and 1-acylglycerophosphorylcholine acyltransferase in hepatic microsomes were increased approx. 3-, 1.7- and 3.6-times, respectively, by the treatment of rats with clofibric acid. These findings are discussed with respect to the role of fatty acid modification systems in the regulation of acyl composition of phosphatidylcholine.     

Probenecid-induced changes in the clearance of pranoprofen enantiomers

Probenecid is known to inhibit the elimination of several acidic drugs. Its influence on the pharmacokinetics of pranoprofen was investigated in rabbit after a single intravenous injection of racemic mixture (5 mg/kg). Levels of (-)-(R)- and (+)-(S)-pranoprofen and their glucuronide (after hydrolysis with sodium hydroxide) were determined in plasma, urine, and several tissues. The plasma concentration of the (+)-(S)-isomer was higher than that of the (-)-(R)-form. Oral coadministered probenecid (100 mg/kg) resulted in an increased plasma concentration of both enantiomers. Probenecid reduced the apparent total clearance and excretion of pranoprofen enantiomers in urine. It had a slight effect on the tissue distribution of pranoprofen at the dose used, but significantly reduced the formation of glucuronide for both enantiomers to the same extent in kidney microsomes. The differences caused by probenecid were significant with respect to its ability to inhibit glucuronidation in the kidney and subsequent excretion into urine, but enantioselective effects were negligible.     

Additive effects of clofibric acid and pyruvate dehydrogenase kinase isoenzyme 4 (PDK4) deficiency on hepatic steatosis in mice fed a high saturated fat diet

Although improving glucose metabolism by inhibition of pyruvate dehydrogenase kinase 4 (PDK4) may prove beneficial in the treatment of type 2 diabetes or diet-induced obesity, it may have detrimental effects by inhibiting fatty acid oxidation. Peroxisome proliferator-activated receptor α (PPARα) agonists are often used to treat dyslipidemia in patients, especially in type 2 diabetes. Combinational treatment using a PDK4 inhibitor and PPARα agonists may prove beneficial. However, PPARα agonists may be less effective in the presence of a PDK4 inhibitor because PPARα agonists induce PDK4 expression. In the present study, the effects of clofibric acid, a PPARα agonist, on blood and liver lipids were determined in wild-type and PDK4 knockout mice fed a high-fat diet. As expected, treatment of wild-type mice with clofibric acid resulted in less body weight gain, smaller epididymal fat pads, greater insulin sensitivity, and lower levels of serum and liver triacylglycerol. Surprisingly, rather than decreasing the effectiveness of clofibric acid, PDK4 deficiency enhanced the beneficial effects of clofibric acid on hepatic steatosis, reduced blood glucose levels, and did not prevent the positive effects of clofibric acid on serum triacylglycerols and free fatty acids. The metabolic effects of clofibric acid are therefore independent of the induction of PDK4 expression. The additive beneficial effects on hepatic steatosis may be due to induction of increased capacity for fatty acid oxidation and partial uncoupling of oxidative phosphorylation by clofibric acid, and a reduction in the capacity for fatty acid synthesis as a result of PDK4 deficiency.     

Determination of total mycophenolic acid and its glucuronide metabolite using liquid chromatography with ultraviolet detection and unbound mycophenolic acid using tandem mass spectrometry

Two simple, sensitive and reproducible methods for determination of total mycophenolic acid (MPA) and its glucuronide metabolite (MPAG) as well as unbound MPA (fMPA) was developed by the use of HPLC-UV and LC-MS/MS methods, respectively. For the total MPA/MPAG method, the analytes were extracted using Isolute C(2) solid-phase extraction (SPE) cartridges and analyzed at 254 nm over a Zorbax Rx C(8) column (150 mm x 4.6 mm, 5 microm). The mobile phase was a gradient mixture of methanol and water (containing 0.1% (v/v) phosphoric acid). The total run time was 18 min and the extraction recovery was 77% for MPA and 84% for MPAG. The method was precise and accurate with a lower limit of quantification (LLOQ) of 0.5 mg/l for MPA and 5.0 mg/l for MPAG. For the fMPA method, plasma was subjected to ultrafiltration followed by SPE using C(18) cartridges. Analytical column was the same as the HPLC-UV method and the mobile phase was a gradient composition of methanol:0.05% formic acid with a flow rate of 0.6 ml/min for the first 3 min and 0.7 ml for the last 4 min. The chromatographic method separated MPA from its metabolites MPAG and Acyl-MPAG. Mass transitions in negative ionization mode for MPA and the internal standard, indomethacin were m/z: 319-->190.9 and m/z: 356-->312.2, respectively. The assay was linear in the concentration range of 1-1000 microg/l for fMPA with a LLOQ of 1 microg/l and an accuracy of >95%. The two methods reported have an adequate degree of robustness and dynamic concentration range for the measurement of MPA, MPAG and fMPA for therapeutic drug monitoring purposes or pharmacokinetics investigations.     

Effects of clofibric acid and tiadenol on cytosolic long-chain acyl-CoA hydrolase and peroxisomal beta-oxidation in liver and extrahepatic tissues of rats

The effects of two peroxisome proliferators, p-chlorophenoxyisobutyric acid (clofibric acid) and 2,2'-(decamethylenedithio)diethanol (tiadenol), on cytosolic long-chain acyl-CoA hydrolase and peroxisomal beta-oxidation were studied in several organs of rat. Among organs of control rats, the brain had the highest activity of long-chain acyl-CoA hydrolase, followed by testis, and a low activity was found in other tissues. Administration of the peroxisome proliferators caused a marked increase in activity of long-chain acyl-CoA hydrolase in both liver and intestinal mucosa and a slight increase in the activity in kidney, but little affected acyl-CoA hydrolase activity in either brain, testis, heart, spleen and skeletal muscle. In accordance with the change in the activity of acyl-CoA hydrolase, the activity of peroxisomal beta-oxidation was markedly increased in liver, intestinal mucosa and kidney, and a slight increase was found in brain and testis, whereas peroxisome proliferators little affected the activity in other organs tested. Gel filtration of cytosol from intestinal mucosa showed that clofibric acid caused an appearance of a new peak in intestinal mucosa. Although cytosol of liver, intestinal mucosa, brain and testis contained two 4-nitrophenyl acetate esterases with different molecular weights (about 105,000 and about 55,000), these esterases are different from cytosolic long-chain acyl-CoA hydrolases of these four organs in respect of molecular weight. The administration of clofibric acid little affected cytosolic 4-nitrophenyl acetate esterases. Comparative studies on cytosolic long-chain acyl-CoA hydrolases from these four organs showed that liver hydrolase I (molecular weight of about 80,000) had properties similar to those of brain and testis enzymes. On the other hand, intestinal mucosa enzyme was different from either hepatic hydrolase I or II (molecular weight of about 40,000). The results from the present study suggest that inductions of peroxisomal beta-oxidation and cytosolic long-chain acyl-CoA hydrolases are essential responses of rats to peroxisome proliferators not only in liver but also in intestinal mucosa and that induced hydrolases are not attributable to non-specific esterases.     

Quantification of free mycophenolic acid and its glucuronide metabolite in human plasma by liquid-chromatography using mass spectrometric and ultraviolet absorbance detection

The immunosuppressant drug mycophenolic acid (MPA) and its major metabolite, mycophenolic acid glucuronide (MPAG), are highly bound to albumin. An HPLC-tandem-MS (HPLC/MS/MS) and an HPLC-UV assay were developed to measure free (unbound) concentrations of MPA and MPAG, respectively. Ultrafiltrate was prepared from plasma (500 microl) by ultrafiltration at 3000 x g for 20 min (20 degrees C). Both MPA and MPAG were isolated from ultrafiltrate (100 microl) by acidification and C18 solid-phase extraction. Free MPA was measured by electrospray tandem mass spectrometry using selected reactant monitoring (MPA: m/z 338.2--> 206.9) in positive ionisation mode. Chromatography was performed on a PFPP column (50 mm x 2 mm, 5 microm). Total analysis time was 7 min. The assay was linear over the range 1-200 microg/l with a limit of quantification of 1 microg/l. The inter-day accuracy and imprecision of quality controls (7.5, 40, 150 microg/l) were 94-99% and < 7%, respectively. Free MPAG was chromatographed on a C18 Nova-Pak column (150 mm x 3.9 mm, 5 microm) using a binary gradient over 20 min. The eluent was monitored at 254 nm. The assay was linear over the range 1-50 mg/l with the limit of quantification at 2.5 mg/l. The inter-day accuracy and imprecision of quality controls (5, 20, 45 mg/l) was 101-107% and < 8% (n = 4), respectively. For both methods no interfering substances were found in ultrafiltrate from patients not receiving MPA. The methods described have a suitable dynamic linear range to facilitate the investigation of free MPA and MPAG pharmacokinetics in transplant patients. Further, this is the first reported HPLC-UV method to determine free MPAG concentrations.     

Differential effects of altered hormonal state on the induction of acyl-CoA hydrolases and peroxisomal beta-oxidation by clofibric acid

Induction of hydrolase I, hydrolase II, peroxisomal beta-oxidation and hepatomegaly caused by clofibric acid (rho-chlorophenoxyisobutyric acid) administration was investigated in relation to alterations in hormonal state of glucocorticoid, thyroid hormone and insulin. (1) In adrenalectomized rats, the ability to induce hydrolase I was depressed effectively and little hepatomegaly was produced. Hydrolase II and peroxisomal beta-oxidation were induced to a similar extent, compared to those of intact rats. (2) In hypothyroid rats, induction of hydrolase I, hydrolase II, peroxisomal beta-oxidation and hepatomegaly was reduced. In hyperthyroid rats, the ability to induce hydrolase I, hydrolase II and peroxisomal beta-oxidation was depressed, although hepatomegaly was produced by the same or a greater extent as in intact rats. (3) In diabetic rats, marked reduction of ability to induce both hydrolase I and II was observed and induction of hepatomegaly was depressed slightly, although peroxisomal beta-oxidation was induced normally. Differences in the response of four parameters (hydrolase I, hydrolase II, peroxisomal beta-oxidation and liver size) to alterations in hormonal state suggest that the four biological responses to clofibric acid may each be mediated through distinct mechanism(s) other and regulated by distinct hormone(s).     

Effects of oral L-tyrosine administration on CSF tyrosine and homovanillic acid levels in patients with Parkinson's disease

To determine whether 1-tyrosine administration can enhance dopamine synthesis in humans as it does in rats, we measured levels of tyrosine and the major dopamine metabolite, homovanillic acid, in lumbar spinal fluids of 23 patients with Parkinson's disease before and during ingestion of 100 mg/kg/day of tyrosine. Nine patients took 100 mg/kg/day of probenecid in six divided doses for 24 hours prior to each spinal tap; 14 patients did not receive probenecid. L-tyrosine administration significantly increased CSF tyrosine levels in both groups of patients (p less than .01) and significantly increased homovanillic acid levels in the group of patients pretreated with probenecid (p less than .02). These data indicate that l-tyrosine administration can increase dopamine turnover in patients with disorders in which physicians wish to enhance dopaminergic neurotransmission.