Characterization of enzymes involved in formation of ethyl esters of long-chain fatty acids in humans

Elevated fatty acid ethyl ester (FAEE) concentrations have been detected in postmortem organs from alcoholics and patients acutely intoxicated by alcohol, and FAEE have been implicated as mediators of ethanol-induced organ damage. The formation of FAEE is catalyzed by acyl-coenzyme A:ethanol O-acyltransferase (AEAT) and by FAEE synthase, which utilize acyl-CoA and free fatty acids, respectively, as substrates. Because little is known about the capacity of various human tissues to synthesize and hydrolyze FAEE, we investigated formation of FAEE by AEAT and FAEE synthase in tissue homogenates from human gastric ventricular and duodenal mucosa, pancreas, liver, heart, lung, and adipose tissue, gallbladder mucosa, and in serum. Liver, duodenal mucosa, and pancreas were found to have the highest capacities to synthesize FAEE, mainly due to AEAT. FAEE hydrolyzing activity was highest in liver and pancreas, but hardly detectable in adipose tissue or heart. Because fatty acids and alcohol are absorbed by the intestinal mucosa, intestine may be a major site of FAEE synthesis, and FAEE may be delivered via the circulation to other organs and taken up by lipoprotein receptor-mediated uptake. A very low rate of FAEE hydrolysis was detected in heart and adipose tissue, which probably accounts for the previously observed accumulation of FAEE in these organs.

Effects of treatment with deoxycholic acid and chenodeoxycholic acid on the hepatic synthesis of cholesterol and bile acids in healthy subjects

The degradation of cholesterol to bile acids is regulated by a negative-feedback mechanism by the bile acids, especially the hydrophobic bile acids, returning to the liver via the portal vein. Chenodeoxycholic acid (CDCA) is a potent suppressor of the cholesterol 7alpha-hydroxylase, the rate-determining enzyme in bile acid formation. CDCA may also suppress hepatic 3-hydroxy-3-methyl glutaryl coenzyme A (HMG CoA) reductase, the rate-limiting enzyme in cholesterol synthesis. Conflicting reports have appeared regarding the suppression on bile acid synthesis by the most hydrophobic bile acid of human bile, deoxycholic acid (DCA). To study the suppressive effects of CDCA and DCA on hepatic cholesterol and bile acid synthesis in humans, 10 healthy subjects were treated with CDCA or DCA for 3 weeks in a randomized cross-over study with a washout period of 4 weeks in between. Serum levels of 7alpha-hydroxy-4-cholesten-3-one, reflecting cholesterol 7alpha-hydroxylase activity, and 7-dehydrocholesterol, reflecting HMG CoA reductase activity, and bile acids were repeatedly measured during the study periods. After 3 weeks of treatment with CDCA or DCA, CDCA constituted 70% and DCA 74% of the total serum bile acids, respectively. CDCA and DCA decreased the serum levels of 7alpha-hydroxy-4-cholesten-3-one by 80% and 75%, respectively. Negative correlations between the percentages of CDCA and DCA and the serum concentration of 7alpha-hydroxy-4-cholesten-3-one were obtained. CDCA reduced the serum level of 7-dehydrocholesterol by 29%, whereas treatment with DCA tended to increase the level of 7-dehydrocholesterol. Treatment of healthy subjects with CDCA and DCA reduces bile acid synthesis. CDCA also inhibits cholesterol synthesis, whereas DCA does not.

The level of 7-dehydrocholesterol in plasma reflects the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase in the human liver

Plasma levels of the cholesterol precursor 7-dehydrocholesterol (7-DHC) were compared with activities of the rate-limiting enzyme in cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase assayed in liver biopsies from patients undergoing cholecystectomy. Some patients were treated with cholestyramine, deoxycholic acid or chenodeoxycholic acid prior to surgery in order to alter the activity of the enzyme. The median level of 7-DHC and the activity of HMG-CoA reductase in the untreated group were 55 ng/ml and 98 pmol/min/mg protein, respectively. The sterol levels and enzyme activities were increased in patients treated with cholestyramine (85 ng/ml and 439 pmol/min/mg protein) and deoxycholic acid (86 ng/ml and 173 pmol/min/mg protein) and decreased in patients treated with chenodeoxycholic acid (38 ng/ml and 51 pmol/min/mg protein). There was a strong positive correlation (rs=0.75, P<0.0005) between the plasma levels of 7-DHC and the activities of hepatic HMG-CoA reductase in these patients. This correlation was further improved when the plasma levels of 7-DHC were expressed relative to those of cholesterol (rs=0.90, P<0.0001). The results show that the level of 7-DHC in plasma reflects the activity of HMG-CoA reductase in the liver.

Effects of short-term treatment with pravastatin on the hepatic synthesis of cholesterol and bile acids in gallstone patients

BACKGROUND

HMG-CoA reductase inhibitors are now the therapy of choice in the treatment of hypercholesterolaemia. The effects of long-term treatment with these substances on plasma lipoproteins, cholesterol metabolism and biliary secretion of lipids have been extensively studied in humans. Much less is known about the effects of short-term treatment. The aim of this study was to determine the time course of the effects of HMG-CoA reductase inhibitors on plasma lipoprotein levels as well as cholesterol and bile acid synthesis in gallstone patients.

METHODS

Thirty-six patients undergoing elective cholecystectomy were included in the study. Except for the gallstone disease, these patients were otherwise healthy. Four groups of subjects were treated with the HMG-CoA reductase inhibitor pravastatin (Pravachol), 20 mg twice daily for 12, 24, 48 and 72 h preoperatively. Plasma lipoproteins and plasma levels of lathosterol and 7 alpha-hydroxy-4-cholesten-3-one were determined before initiation of pravastatin treatment and on the morning of the day of the operation, lathosterol reflecting hepatic HMG-CoA reductase activity and 7 alpha-hydroxy-4-cholesten-3-one the activity of cholesterol 7 alpha-hydroxylase, the rate-determining enzyme in bile acid synthesis.

RESULTS

All treatment groups displayed a significant decrease in total cholesterol and low-density lipoprotein (LDL)-cholesterol, by about 12% and 17% respectively. Lathosterol was reduced by about 50% in all treatment groups. Of great interest was the finding that 7 alpha-hydroxy-4-cholesten-3-one was unaffected in all treatment groups.

CONCLUSION

The results show that short-term pravastatin treatment in gallstone patients rapidly inhibits cholesterol synthesis and lowers plasma LDL-cholesterol levels without effects on bile acid synthesis.

The effect of plasma low density lipoprotein apheresis on the hepatic secretion of biliary lipids in humans

BACKGROUND

The liver is a key organ in the metabolism of cholesterol in humans. It is the only organ by which substantial amounts of cholesterol are excreted from the body, either directly as free cholesterol into the bile or after conversion to bile acids. The major part of cholesterol synthesis in the body occurs in the liver. Cholesterol is also taken up by the liver from plasma lipoproteins. The relative contributions of newly synthesised cholesterol and plasma lipoprotein cholesterol to bile acid synthesis and biliary cholesterol secretion, respectively, are not known in detail.

AIMS

To determine how a rapid lowering of plasma low density lipoprotein (LDL) and very low density lipoprotein (VLDL) cholesterol influences the biliary secretion rates of cholesterol and bile acids in patients with cholesterol gallstones and complete biliary drainage. In this model with a completely interrupted enterohepatic circulation, the secretion of bile acids equals the new synthesis of bile acids in the liver.

PATIENTS

Eight patients with common bile duct stones of cholesterol type undergoing conventional cholecystectomy and choledocholithotomy.

METHODS

At operation a balloon occludable Foley catheter attached to a T tube was inserted into the bile duct with the balloon placed just past the distal limb of the T tube. The T tube was allowed to drain the bile externally. One week after the operation the Foley catheter balloon was inflated, creating complete biliary drainage. Twelve hours following the inflation plasma LDL apheresis was carried out for two hours. Bile was collected for 15 minute periods starting one hour before the apheresis and ending two hours after its termination. During the collection of bile, plasma lipids were analysed on several occasions.

RESULTS

The plasma level of LDL cholesterol decreased by 26% from (mean (SEM)) 2.19 (0.29) to 1.63 (0.17) mmol/l during the LDL apheresis while high density lipoprotein (HDL) cholesterol in plasma was unaffected. During LDL apheresis apolipoprotein B containing lipoproteins bind to the column, causing a significant decrease of not only plasma LDL but also of VLDL cholesterol. The secretion rate of bile acids decreased significantly by 31% from 131 (38) to 90 (16) mumol/15 minutes (p = 0.045). The output of phospholipids also decreased by 19%. The biliary secretion rate of cholesterol was not, however, affected by the plasma LDL apheresis.

CONCLUSIONS

The results suggest that, in patients with cholesterol gallstones and complete biliary drainage, lowering of plasma LDL and VLDL cholesterol reduces the biliary secretion rate--synthesis--of bile acids without affecting the biliary secretion rate of cholesterol.

Changes in biliary lipid output after interruption of pravastatin treatment in humans

The use of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (e.g. pravastatin) has gradually increased in the treatment of hypercholesterolaemia. By inhibiting HMG-CoA reductase (the rate-limiting enzyme in cholesterol synthesis) activity, cholesterol synthesis in the liver is reduced and the plasma level of cholesterol, especially low-density lipoprotein (LDL)-cholesterol, is substantially lowered. Simultaneously, inhibition of HMG CoA reductase activity is associated with increased synthesis and accumulation of larger amounts of HMG CoA reductase enzyme protein. The main purpose of this study was to determine if the cessation of pravastatin treatment causes a rapid increase in the synthesis and biliary secretion of cholesterol, a condition which might lead to a temporarily increased cholesterol saturation of bile. Nine patients undergoing surgery for stones in the common bile duct were fitted with T-tubes in the common bile duct peroperatively; the side arm of the T-tube was left open postoperatively, creating a biliary fistula. All patients were given 6 days' treatment with pravastatin (20 mg b.i.d.) following the operation. Bile was collected from the T-tube, in 12-h fractions during this period and for another 3 days after termination of the treatment. Plasma levels of lipoproteins and lathosterol--reflecting cholesterol synthesis--were determined on several occasions. After cessation of pravastatin treatment, the plasma levels of total cholesterol and LDL-cholesterol increased by 21% and 33% respectively. High-density lipoprotein (HDL)-cholesterol did not change. The plasma level of lathosterol was increased two- to fourfold. Outputs of bile acids and phospholipids were significantly increased (23% and 10% respectively) after termination of treatment, whereas the output of cholesterol was not significantly changed. Cholesterol saturation was reduced by 20%, from 175 +/- 37% to 140 +/- 19%, but this change was not significant. The results indicate that, with the present experimental model (biliary diversion), the synthesis and biliary secretion of bile acids seem to be largely dependent on the de novo synthesis of cholesterol in the liver, whereas the biliary output of cholesterol is not.

Concentration of unsulfated lithocholic acid in portal and systemic venous plasma: evidence that lithocholic acid does not down regulate the hepatic cholesterol 7 alpha-hydroxylase activity in gallstone patients

It has been proposed that lithocholic acid may have a physiological role for the regulation of bile acid synthesis in humans. In this study, the portal concentration and hepatic uptake of unsulfated lithocholic acid was determined in 21 gallstone patients-untreated, cholestyramine-treated and chenodeoxycholic acid-treated-at cholecystectomy. Lithocholic acid was analyzed by a combined gas-liquid mass-fragmentographic technique. In most of the patients a liver biopsy was obtained for assay of the cholesterol 7 alpha-hydroxylase activity. The portal venous concentration of unsulfated lithocholic acid averaged 0.32 mumol/l in untreated patients, constituting about 4% of the total bile acids. The apparent hepatic uptake of lithocholic acid averaged 78%, being as high as that of cholic acid. No significant correlation was obtained between the portal venous concentration of unsulfated lithocholic acid and the hepatic cholesterol 7 alpha-hydroxylase activity. This study thus confirms an enterohepatic circulation of lithocholic acid in humans. No evidence was obtained that the portal venous inflow of small amounts of lithocholic acid to the liver is of regulatory importance for the cholesterol 7 alpha-hydroxylase activity.