Z.R. Vlahcevic

Multicompartmental Analysis of Cholesterol Metabolism in Man: CHARACTERIZATION OF THE HEPATIC BILE ACID AND BILIARY CHOLESTEROL PRECURSOR SITES

The present report has presented the first clear evidence in man for the existence of specific hepatic cholesterol precursor sites associated with the formation and secretion of bile acids and biliary cholesterol. These hepatic compartments derive virtually all their cholesterol from newly synthesized and lipoprotein free cholesterol. The model which is presented was formulated on current concepts of cholesterol metabolism in man and is concerned, at this initial stage, with the elucidation of the bile acid and biliary cholesterol compartments. The complexity of cholesterol metabolism in man necessitated an initial approach that would minimize the number of inputs of cholesterol into the system, allow for the sampling of several cholesterol compartments, and permit the simultaneous labeling of newly synthesized cholesterol and preformed cholesterol. To achieve these objectives, we studied the patient with a total bile fistula. Six patients were administered simultaneously pulse injections of labeled mevalonic acid and [(14)C]cholesterol. The qualitative features of the specific activity time course curves after labeled mevalonic acid revealed no precursor-product relationship between bile acid, biliary cholesterol, and plasma free cholesterol. The peak specific activity of the bile acids was reached in approximately 100 min and was higher than the biliary cholesterol, which was higher than the plasma free cholesterol. The plasma free cholesterol specific activity became higher than the other lipids after 12 h and remained higher throughout the period of study. Similar related observations were made with [(14)C]cholesterol. The data were then subjected to simulation analysis and modeling using the SAAM-27 computer program. Computer least-square fits of the data were obtained after the model was evolved. During the model development, the least number of compartments and transport pathways were introduced consistent with a good fit of the data. Of particular importance was the constraint that the model fit the data obtained from both [(14)C]cholesterol and labeled mevalonic acid. The same parameter values were used to fit the data from both tracers. The fluxes arrived at in the model indicate that 31% and 20%, respectively, of the cholesterol input into the bile acid and biliary cholesterol precursor sites were derived directly from the newly synthesized hepatic cholesterol. The remainder had its origin predominantly from lipoprotein free cholesterol. Plasma esterified cholesterol (as free) made a small contribution (11%) to the bile acid compartment. Similarly, 10% of the biliary cholesterol arose from an unknown hepatic site. The present report has provided the basis for a new procedure for studying in vivo cholesterol metabolism in man. Examination of the derived cholesterol flux rates between the compartments suggests the presence of an important mechanism regulating the partitioning of lipoprotein free cholesterol between the bile acid and biliary cholesterol precursor sites. Aberrations in the proportioning of precursor cholesterol between these sites could be a causative factor precipitating the excessive secretion of biliary cholesterol and the production of lithogenic bile.

Bile Acid Metabolism in Patients with Cirrhosis: II. Cholic and chenodeoxycholic acid metabolism

Primary bile acid kinetics and pool sizes were determined in 12 patients with cirrhosis and 10 patients without liver disease. Labeled bile acids (14C-cholic and 14C-chenodeoxycholic acids) were administered to each patient. Patients with cirrhosis were found to have a total bile acid pool about one-half that of the group of patients without cirrhosis. The diminished total bile acid pool was the result of marked reductions in the cholic and deoxycholic acid pools, whereas the chenodeoxycholic acid pool was not significantly altered. The daily synthesis of cholic acid in patients with cirrhosis was about one-fourth that of the group with no liver disease, whereas chenodeoxycholic acid synthesis was reduced by only 30%. The fractional turnover rate of cholic acid was significantly reduced in patients with cirrhosis, but the rate of turnover of chenodeoxycholic acid was similar in both groups of patients. As a consequence of these alterations in the fractional turnover rate of the primary bile acids, chenodeoxycholic acid turned over more rapidly than cholic acid in cirrhotic patients. Changes in the conversion of cholic to deoxycholic acid could account for the decreased cholic acid turnover rate in patients with cirrhosis. The findings of this report suggest that there is a marked impairment in the cholesterol to cholic acid pathway in patients with cirrhosis, possibly due to a deficiency of the enzyme system which converts the dihydroxy intermediate to the trihydroxy precursor of cholic acid.

Bile acid metabolism in patients with cirrhosis. I. Kinetic aspects of cholic acid metabolism.

Labeled cholic acid was administered orally to 8 patients with cirrhosis and no ascites. Bile-rich duodenal fluid was obtained at daily intervals for periods up to 7 days, and bile acid composition and cholic acid specific activity were determined. The total and component bile acid pools, cholic acid production, and half-life were estimated from the data and compared with a group of our patients without liver disease previously reported. The total and component bile acid pools were markedly reduced in patients with cirrhosis. The total bile acid pool was 46% smaller in cirrhotic patients than the control patients (1.3 g versus 2.4 g). There was a virtual absence of biliary deoxycholic acid in 7 of 8 patients with cirrhosis. Cholic acid production in the cirrhotic patients was approximately one-third that of the normal group. A prolongation of the cholic acid turnover time was also noted in 3 of the 8 cirrhotic patients. It is suggested that in patients with cirrhosis the failure of the liver to produce adequate amounts of bile acids is responsible for the diminished bile acid pool. The nonpancreatogenic steatorrhea observed in some patients with cirrhosis might be attributed in part to a lack of sufficient bile salts in the intestine.

Relationship of bile acid pool size to the formation of lithogenic bile in female Indians of the southwest.

Bile acid kinetics and pool size were determined in 18 female American Indians of the Southwest. Eleven patients had a lithogenic bile and 7 patients had a normal bile. Eight of the patients with a lithogenic bile had cholesterol gallstones which were confirmed both at surgery and by analysis of the gallstones; the remaining 3 patients in this group had no radiological evidence of gallstones. The group of 7 patients with a normal bile had no evidence of gallstones by oral cholecystogram. The patients were administered orally 14C-cholic and 14C-chenodeoxycholic acids. The primary and total bile acid pools were found to be significantly smaller in patients with a lithogenic bile than in patients with a normal bile (1.1 g versus 1.6 g). The 3 patients with a lithogenic bile and no gallstones also had greatly diminished bile acid pools (under 1 g). The primary bile acids of patients with a lithogenic bile turned over at a faster rate than in patients with a normal bile. There was no significant difference in the daily production of the primary bile acids between the two groups. The production of primary bile acids in the female Indian averaged 350 mg per day. The data suggest that the diminished bile acid pool in the female Indians with a lithogenic bile resulted from an inability of the liver to synthesize adequate amounts of bile acids to compensate for bile acid excretion. Biliary lipid excretion was estimated from the total bile acid pool size and biliary lipid composition. Although there was a slight decrease in phospholipid and a slight increase in cholesterol excretion in the patients with a lithogenic bile, these differences were not significant. However, these alterations in biliary lipid excretion were of sufficient magnitude when coupled with the reduction in the bile acid pool to tip the bile out of the micellar zone. Based on the findings of this and earlier reports, it is suggested that a diminished bile acid pool could be responsible for the development of a bile supersaturated with cholesterol and subsequent cholesterol gallstone formation.

Abnormal metabolism of secondary bile acids in patients with cirrhosis.

The composition of bile acids in human bile was determined in bile-rich duodenal fluid on four consecutive days in a group of seven patients with cirrhosis and eight control patients with no liver disease. There was a marked reduction of secondary biliary bile acids in cirrhotic patients. Possible mechanisms for these changes are discussed.

Bile-rich duodenal fluid as an indicator of biliary lipid composition and its applicability to detection of lithogenic bile

A comparison was made of the lipid composition of duodenal and gallbladder bile in 10 patients with cholesterol gallstones and in 11 patients without. The lipid composition of duodenal and gallbladder bile was found to be similar in each patient. A phase diagram plot of the biliary lipid composition data showed that duodenal bile is also a valid indicator of the presence or absence of lithogenic bile. Also, based on the analysis of the duodenal lipids, a distinction could be made between the presence of calcium bilirubinate and that of cholesterol gallstones. Analysis of the lipids of duodenal bile should serve as a useful tool in studying the effect of various agents and factors on the metabolism of the biliary lipids.

Bile Acid Metabolism in Cirrhosis: III. Biliary lipid secretion in patients with cirrhosis and its relevance to gallstone formation

Abstract Biliary lipid secretion and bile acid pools were determined in 10 patients without and 13 patients with cirrhosis. Patients with cirrhosis were found to have a greatly diminished total bile acid pool. This alteration in the bile acid pool was associated with a significantly decreased phospholipid secretion, and a very marked decrease in biliary cholesterol secretion. The concentration of the biliary lipids in duodenal bile paralleled the changes in biliary lipid secretion. A triangular phase diagram plot of the biliary lipid composition data indicated that the bile of patients with cirrhosis was entirely normal (no insoluble cholesterol) and had a greater cholesterol holding capacity than the bile of control subjects. These findings indicate that cirrhotic patients are not predisposed to develop cholesterol gallstones. The reported high incidence of gallstones in cirrhotic patients may be related to an abnormality in bilirubin metabolism since a high proportion of the stones in these patients have been reported to be of the pigmented type.

Bile Acid Metabolism In Cirrhosis: IV. Characterization of the abnormality in deoxycholic acid metabolism

Several recent studies have demonstrated that patients with cirrhosis frequently lack deoxycholic acid in bile and plasma. In order to explain this observation, comparative experiments on the colonic absorption of deoxycholic acid and on the colonic conversion of cholic to deoxycholic acid were carried out in the cirrhotic patients with normal and very low percentages of deoxycholic acid. Deoxycholic or cholic acid (100 mg) plus 5 muc of each [14C] bile acid were administered by enema to 8 patients with and 5 without liver disease. Deoxycholic acid produced a significant increase in the percentage of biliary deoxycholic acid in patients with cirrhosis. However, the rate of appearance of 14C-deoxycholic acid in patients with cirrhosis was slower than in normal control subjects. Distribution of the 14C activity among the bile acids indicated that rehydroxylation of deoxycholic to cholic acid did not occur. The distribution of 14C activity in biliary bile acids after the rectal administration of [14C]cholic acid showed that patients with severe cirrhosis converted [14C]cholic to [14C]deoxycholic acid at a much slower rate than did cirrhotic patients with normal percentages of biliary deoxycholic acid. Feeding of cholic acid (450 mg per day) for 3 days to 4 cirrhotic patients resulted in a 2-fold increase in the percentage of biliary cholic acid, but only a small increase in the percentage of deoxycholic acid. In a separate group of 9 cirrhotic patients, fecal bile acid analysis indicated that cirrhotic patients had a significantly lower percentage of deoxycholic acid than 12 patients without liver disease; there was no significant difference in fecal lithocholic acid. The data suggest that alteration of bacterial flora and/or altered conditions for bacterial 7alpha-dehydroxylase enzyme activity in the colon could account for the virtual absence of biliary deoxycholic acid in severely cirrhotic patients.

A quantitative evaluation of the conversion of 25-hydroxycholesterol to bile acids in man

The present study was directed toward providing additional information in man on the nature of a potential alternative pathway to cholic acid not involving an initial 7 alpha-hydroxylation of cholesterol. Two bile fistula patients and one normal subject each received 25-hydroxy[G-3H]cholesterol; [14C]cholic and [14C]chenodeoxycholic acids were also simultaneously administered to one bile fistula patient and normal subject. The labeled 25-hydroxycholesterol was found to be poorly converted to primary bile acids by all three patients; the range of conversion was 9.7 to 18.9%. Cholic acid was favored over chenodeoxycholic acid by a margin of about 1.4/1. It is concluded that a pathway to primary bile acid via the 25-hydroxylation of cholesterol is of minor importance under conditions of normal or accelerated synthesis in man.