Evidence for the existence of definitive hepatic cholesterol precursor compartments for bile acids and biliary cholesterol in man.

A bile fistula patient was administered intravenously a constant infusion of [3H]mevalonic acid for 4 hrs, and then after 2 weeks he was given a pulse of [3H]mevalonic acid. Bile and blood were collected at frequent intervals. The specific activity-time course curves did not show a precursor-product relationship between biliary cholesterol, plasma free cholesterol and bile acids. Both the constant infusion and pulse labeling data indicated that the bile acid precursor had a more rapid rate of turnover than plasma or biliary cholesterol; the biliary cholesterol precursor turned over more rapidly than plasma cholesterol. The data suggest the presence of multiple hepatic cholesterol precursor compartments. Bile acids may be derived predominantly from newly synthesized cholesterol in man.     

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.Supported in part by USPHS Research Grant-in-Aid 1RO1-AM-14668-01 from the National Institute of Arthritis and Metabolic Diseases, National Institutes of Health.     

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.     

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.     

Mechanism of secretion of biliary lipids. I. Role of bile canalicular and microsomal membranes in the synthesis and transport of biliary lecithin and cholesterol.

The role of bile canalicular and microsomal membranes in the synthesis and transport of biliary lipids was investigated by using the isolated perfused rat liver model. Labeled lecithin precursors ((3H)-palmitic acid, (14C)linoleic acid, (3H)choline, and 32PO4) and a cholesterol precursor ((3H)mevalonic acid) were administered with and without sodium taurocholate. The incorporation pattern of these labeled precursors into linoleyl and arachidonyl lecithins and cholesterol fractions of microsomes, bile canaliculi, and bile were examined at 30-min intervals up to 90 min. Marker enzymes and electron microscopy indicated that isolated subfractions of plasma membranes were enriched with bile canaliculi (less than 10 percent microsomal contamination). Taurocholate significantly stimulated the incorporation of 32PO4, (3H)choline, (3H)palmitic acid, and (14C)linoleic acid into linoleyl and arachidonyl lecithin with parallel incorporation curves for microsomal and bile canalicular membranes throughout the 90-min study period. During the 30-60-min period, however, these same lecithin fractions in bile significantly exceeded the specific activity of the membrane lecithins. The enzyme CDP-choline diglyceride transferase was virtually absent from canaliculi relative to microsomes, indicating that canaliculi lack the capacity for de novo lecithin synthesis. Incorporation of (3H)mevalonic acid into membranous and biliary cholesterol followed a pattern similar to that for lecithin. These data provide evidence that (a) biliary lecithin and cholesterol are derived from a microsomal subpool regulated by the flux of enterohepatic bile acids, (b) the role of the bile canalicular membranes with respect to biliary lipids is primarily transport rather than synthesis, and (c) lecithin and cholesterol are transported together from microsomes to bile. The findings are consistent with the existence of a cytoplasmic lipid complex within the hepatocyte which is actively involved in the intermembrane transport of biliary lipid.     

Multicompartmental Analysis of Cholesterol Metabolism in Man: QUANTITATIVE KINETIC EVALUATION OF PRECURSOR SOURCES AND TURNOVER OF HIGH DENSITY LIPOPROTEIN CHOLESTEROL ESTERS

The purpose of this study is to delineate the immediate sources and fractional turnover of high density lipoprotein (HDL) esterified cholesterol in man. Various labeled preparations were administered in 11 experiments to six subjects who had either a complete bile fistula (maximally stimulated cholesterol metabolism) or an intact enterohepatic circulation. The administered tracers included [³H]mevalonic acid; [¹⁴C]cholesterol bound to albumin; low density lipoprotein (LDL) free [³H] or [¹⁴C]cholesterol; HDL free [³H] or [¹⁴C]cholesterol; HDL esterified [³H]cholesterol; and LDL esterified [³H]cholesterol. Blood samples were obtained at frequent intervals for up to 5 d after the administration of tracers. The mass and radioactivity in individual plasma lipoprotein (very low density lipoprotein [VLDL], HDL, and LDL) free and esterified cholesterol were determined.     

Influence of bile acids on biliary lipid excretion in man

Bile salts orally administered to a human subject, who had a short-arm Ttube in his common bile duct, markedly increased the concentrations of bile acids, phospholipid, and cholesterol in the hepatic bile. The increase in phospholipid was proportionately greater than that of cholesterol. When administration of oral bile acids was stopped, there was a decrease in the bile acids and phospholipid levels, and in the phospholipid to cholesterol ratio. These data provide evidence that biliary lipid excretion is regulated by the availability of bile acids.