COMPARATIVE BINDING OF BILE-ACIDS TO SERUM-LIPOPROTEINS AND ALBUMIN

Characteristics of the binding of lithocholic acid (LC), chenodeoxycho lic 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 t he bile acid. Binding of LC, CDC, and cholic acid to the lipoprotein-f ree, albumin-rich plasma fraction was characterized by two classes of binding sites with respective K(D)s of 2, 5, and 51 muM, and of 39, 2, 387, and 5,575 muM, while corresponding B(max) values were similar for the different bile acids, at around 6 and 100 nmol/mg protein. Bile a cid binding to the different lipoprotein fractions was characterized b y a single population of binding sites, with a K(D) ranging from 47 to 66 muM for LC, 695 to 1010 muM for CDC, and 2,511 to 2,562 muM for ch olic acid, B(max) values, at 416-913 nmol/mg protein, were similar amo ng the different bile acids studied. Both glycine- and taurine-conjuga ted, as well as unconjugated LC competitively inhibited [24-C-24]LC bi nding to low density (LDL) and to high density lipoproteins (HDL) to t he same extent, while the more polar LC-3-sulfate, CDC, and cholic aci d were increasingly less potent in displacing LC binding from the resp ective lipoproteins. Furthermore, all bile acids studied shared the sa me lipoprotein binding site. The lipoprotein fluorescence at 330-334 n m, following excitation at 280 nm, was diminished after incubation wit h LC, suggesting that the bile acid masks the tryptophan residues of t he protein moiety. Finally, the initial rate of uptake of 1 mum 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 c oncentration of LC, bound to either LDL or HDL, LC binding resulted in 75-77% of the total [24-C-14]LC nonspecifically bound to the hepatocy te, compared to 65% when bound to albumin, and 45% in the absence of p rotein. The studies show that, under conditions when the serum bile ac id concentration exceeds the capacity of the high affinity class of al bumin binding sites for bile acids, lipoproteins have similar or great er affinity to bind bile acids than does albumin. The ability of lipop roteins to increase the nonspecific association of lithocholic acid wi th liver cells may also facilitate bile acid association with extrahep atic tissues. As lipoproteins, in contrast to albumin, are targeted to most cells, they may play a major role in the transport of potentiall y toxic bile acids to peripheral cells.