Comparative binding of bile acids to serum lipoproteins and albumin

Characteristics of the binding of lithocholic acid (LC), chenodeoxycholic acid (CDC), and cholic acid to human plasma proteins were studied. Affinity of the different plasma protein fractions for the bile acids studied decreased with increased polarity of the steroid nucleus of the bile acid. Binding of LC, CDC, and cholic acid to the lipoprotein-free, albumin-rich plasma fraction was characterized by two classes of binding sites with respective KDs of 2, 5, and 51 micromolar, and of 39, 2,387, and 5,575 micromolar, while corresponding Bmax values were similar for the different bile acids, at around 6 and 100 nmol/mg protein. Bile acid binding to the different lipoprotein fractions was characterized by a single population of binding sites, with a KD ranging from 47 to 66 micromolar for LC, 695 to 1010 micromolar for CDC, and 2,511 to 2,562 micromolar for cholic acid. Bmax values, at 416-913 nmol/mg protein, were similar among the different bile acids studied. Both glycine- and taurine-conjugated, as well as unconjugated LC competitively inhibited [24-14C]LC binding to low density (LDL) and to high density lipoproteins (HDL) to the same extent, while the more polar LC-3-sulfate, CDC, and cholic acid were increasingly less potent in displacing LC binding from the respective lipoproteins. Furthermore, all bile acids studied shared the same lipoprotein binding site. The lipoprotein fluorescence at 330-334 nm, following excitation at 280 nm, was diminished after incubation with LC, suggesting that the bile acid masks the tryptophan residues of the protein moiety. Finally, the initial rate of uptake of 1 micromolar LC, in isolated hamster hepatocytes, at around 0.045 nmol.sec-1 mg cell wt-1, was not affected by the protein carrier. However, for the same concentration of LC, bound to either LDL or HDL, LC binding resulted in 75-77% of the total [24-14C]LC nonspecifically bound to the hepatocyte, compared to 65% when bound to albumin, and 45% in the absence of protein. The studies show that, under conditions when the serum bile acid concentration exceeds the capacity of the high affinity class of albumin binding sites for bile acids, lipoproteins have similar or greater affinity to bind bile acids than does albumin. The ability of lipoproteins to increase the nonspecific association of lithocholic acid with liver cells may also facilitate bile acid association with extrahepatic tissues. As lipoproteins, in contrast to albumin, are targeted to most cells, they may play a major role in the transport of potentially toxic bile acids to peripheral cells.