Role of membranes in bile formation. Comparison of the composition of bile and a liver bile-canalicular plasma-membrane subfraction

Membranes and bile formation. Composition of several mammalian biles and their membrane-damaging properties

The total content and profile of bile salts and phospholipids are reported for several mammalian biles. Rabbit and guinea-pig biles are characterized by high proportions of conjugated dihydroxy bile salts with respect to trihydroxy bile salts, but contain relatively little phospholipid. Both rabbit and guinea-pig biles exhibit little evidence of hepatic cell damage, even though they are able to cause membrane damage (as evidenced by lysis of human erythrocytes) at low (2--3 mM) concentrations of bile salts; this lytic behaviour is also a property of their predominant bile salts. Addition of phosphatidylcholine to the bile or bile salt is able to decrease the lytic behaviour. Perhaps the most significant observation is that these biles, and their predominant bile salts, are dramatically less lytic towards sheep erythrocytes, indicating that some factor(s) in membrane composition and structure may partly explain the resistance of membranes of the biliary tract to the presence of high concentrations of potentially membrane-damaging bile salts.

Sources of the proteins of rat bile

The protein composition of rat bile has been studied systematically using two-dimensional agarose-polyacrylamide gel electrophoresis, with or without prior absorption by immobilised antisera, and by crossed immunoelectrophoresis. Sixteen bile proteins were distinguished. Of these, thirteen are immunologically identical to proteins present in rat serum and only one is identical to a protein present in rat liver plasma membrane but not in rat serum. Of the remaining two proteins, one is bile lipoprotein and the other has many of the properties of immunoglobulin A secretory component. The serum-related proteins in rat bile fall into two distinct groups. In the first group are immunoglobulin A and an alpha2-globulin. These proteins are major constituents of bile but only minor constituents of serum. In the second group are albumin and some other major serum proteins which are found in bile at concentrations less than 1% of their concentrations in serum. The relative proportions of these proteins in bile appear to differ from their proportions in serum. It therefore appears that, although the majority of bile proteins are derived from serum, there cannot be direct leakage of serum into bile. Examination of the proteins contained within liver lysosomes indicates that, although discharge of lysosomal contents at the bile canalicular face of the hepatocyte may contribute to the bile proteins, an additional mechanism, with a considerable degree of selectivity, must also be involved in the transport of proteins from serum to bile.

Effects of bile salts of human erythrocytes. Plasma membrane vesiculation, phospholipid solubilization and their possible relationships to bile secretion

Glycocholate removed approximately 25% of the membrane acetylcholinesterase and 10% of the membrane phospholipid from intact human erythrocytes prior to the onset of cell lysis. At low concentrations (up to 6 mM), glycocholate caused human erythrocytes to become echinocytic and to pinch off microvesicles, whereas at higher concentrations glycocholate also specifically released components from the outer leaflet of the plasma membrane in a 'soluble' form (as defined by their presence in a 150 00 X g/60 min supernatant) and caused the cells to become stomatocytic. Whilst the phospholipdi profile of the 'soluble' material differed from that of the whole membrane, the profile of the microvesicle fraction was similar. The microvesicles were depleted in several membrane proteins with respect to phospholipids. These observations are discussed in relation to the possible role of bile salts in the origins of biliary phospholipid and protein.

Bile 5'-nucleotidase in the serum of jaundiced rats

Rat serum 5'-nucleotidase, L-leucyl-beta-naphthylamidase and beta-glycerophosphatase activities are increased whilst alkaline p-nitrophenylphosphatase and alkaline phosphodiesterase activities are unchanged or decreased three days after bile duct ligation. Affinity chromatography on an immobilised antiserum raised against highly purified liver plasma membranes showed that although 5'-nucleotidase in normal serum is unrelated to the 5'-nucleotidase of liver plasma membrane, the 5'-nucleotidase of bile and much of the 5'-nucleotidase in the jaundiced serum are closely related to the plasma membrane enzyme. Since bile is rich in 5'-nucleotidase, the changes in level of this enzyme after bile duct ligation are most simply explained by leakage of bile into the blood; changes in the patterns of the other enzymes are shown to be consistent with this explanation. The jaundiced serum was examined by gel exclusion chromatography and flotation in sucrose gradients for the presence of small fragments of plasma membrane as reported in human jaundiced sera, but no such fragments could be detected three days after bile-duct ligation.

Zone electrophoresis study of the bile lipoprotein complex

Sucrose gradient column electrophoresis was performed with human hepatic and gallbladder bile. It is shown that bile phosphatidylcholines exhibit a more rapid anodic mobility than do bile salts and serum albumin. This high mobility of bile phosphatidylcholines is not due to the negatively charged lipids which are present in bile, i.e. bile salts or free fatty acids. It is demonstrated that phosphatidylcholines are associated with anionic polypeptides. Electrophoresis of reassociations between these purified polypeptides and dilaurylphosphatidylcholine showed that these anionic polypeptides are primarily responsible for the high anodic mobility of the bile lipoprotein complex. This work describes a procedure for the purification of the bile lipoprotein complex which can be useful for the study of other kinds of lipid-polypeptide associations.

Bile proteins in the serum of jaundiced rats

The major bile proteins can be detected by two-dimensional electrophoresis in the serum of cholestatic rats, three days after ligation of the common bile duct. The novel electrophoresis system used agarose gel electrophoresis in the first dimension followed by equilibrium electrophoresis in a polyacrylamide gel gradient.

Membranous localization and properties of ATPase of rat liver lysosomes

Lysosomes isolated from rat liver were found to have ATPase activity (EC No 3.6.1.3). Subfractionation of the lysosomes revealed a membranous localization of ATPase activity. The enzyme has half maximal activity at 0.2 mM ATP and is inhibited by high concentrations of ATP. The apparent Km for divalent metal is 0.2 mM, and either Ca2+ or Mg2+ give maximal activity. The ATPase activity has latency when lysosomes are isolated from rats treated with Triton WR-1339. This latency may be due to the presence of internalized sucrose because the activity of L fraction lysosomes is much less latent and Triton WR-1339 itself is not inhibitory. The latency of glucosaminidase, a marker enzyme for lysosomes, contrasts with the low latency of the ATPase and points to an ATPase with an exposed active site in intact lysosomes.

The lipid composition of plasma membrane subfractions originating from the three major functional domains of the rat hepatocyte cell surface

1. The neutral and phospholipid compositions of three rat liver plasma membrane subfractions originating predominantly from the three major functional domains of the hepatocyte viz the blood sinusoidal, contiguous and bile canalicular fractions, were determined. 2. The sinusoidal and canalicular plasma membrane subfractions, both of which were vesicular, contained a higher lipid to protein weight ratio than the contiguous plasma membrane subfraction that consisted of membrane strips, junctional complexes and some larger vesicles. The three plasma membrane subfractions contained a similar neutral lipid to phospholipid ratio. The highest unesterified cholesterol content was associated with the canalicular plasma membrane subfraction. 3. The phospholipid profiles of the three subfractions were generally similar. However, the canalicular plasma membrane subfraction contained a higher proportion of sphingomyelin than the other subfractions. 4. Correlations between the neutral and phospholipid composition of the subfractions and membrane integrity and function are discussed, especially with respect to a possible role of lipids in governing the resilience of the canalicular plasma membrane to the action of bile salts.

Plasma-membrane components can be removed from isolated lymphocytes by the bile salts glycocholate and taurocholate without cell lysis

Glycocholate and taurocholate removed from isolated pig lymphocytes a proportion of the cells' complement of 5'-nucleotidase, alkaline phosphatase and alkaline phosphodiesterase I before cell lysis. This may indicate a loss of externally orientated plasma-membrane components.