Bile salt binding to serum components. Taurocholate incorporation into high-density lipoprotein revealed by photoaffinity labelling.

1. Photoaffinity labelling of human serum albumin with the sodium salts of (3 beta-azido-7 alpha,12 alpha-dihydroxy-5 beta-cholan-24-oyl)-2-amino[2(-3)H (N)]ethanesulfonic acid, (7,7-azo-3 alpha,12 alpha-dihydroxy-5 beta-cholan-24-oyl)-2-amino[2(-3)H (N)]ethanesulfonic acid and (11 zeta-azido-12-oxo-3 alpha,7 alpha-dihydroxy-5 beta-cholan-24-oyl)-2-amino[2(-3)H (N)]ethanesulfonic acid resulted, in each case, in a considerable covalent incorporation of radioactivity into the protein. 2. Photoaffinity labelling of whole serum, obtained from fasting test persons, revealed with all three photolabile derivatives of taurocholate at the physiological concentration of 2.1 microM the incorporation of radioactivity not only into albumin but also into high-density lipoprotein, as demonstrated by density gradient centrifugation and by immunological characterization. 3. The bulk of radioactivity incorporated into high-density lipoprotein by photoaffinity labelling of whole serum was found to have been associated with the lipids. Only 10-20% of the label was covalently bound to apolipoproteins, predominantly to the apolipoproteins A-I and A-II. 4. The interaction of taurocholate with high-density lipoprotein has been confirmed by density gradient centrifugation using 14C-labelled taurcholate. It is assumed that the interaction of taurocholate with high-density lipoprotein is physiologically of significance.