Effects of side chain length on ionization behavior and transbilayer transport of unconjugated dihydroxy bile acids: a comparison of nor-chenodeoxycholic acid and chenodeoxycholic acid

• Published in: Journal of lipid research Vol. 35; no. 5; pp. 883 - 892
• Main Authors: Ko, J, Hamilton, J A, Ton-Nu, H T, Schteingart, C D, Hofmann, A F, Small, D M
• Format: Journal Article
• Language: English
• Published: BETHESDA Elsevier Inc 01.05.1994; Amer Soc Biochemistry Molecular Biology Inc; Elsevier
• Subjects: Biochemistry & Molecular Biology; Biological Transport - physiology; Chenodeoxycholic Acid - analogs & derivatives; Chenodeoxycholic Acid - chemistry; Chenodeoxycholic Acid - metabolism; Ions; Life Sciences & Biomedicine; Lipid Bilayers - metabolism; Micelles; Phosphatidylcholines; Science & Technology; Solubility; RAT-LIVER; PHOSPHOLIPID-VESICLES; TRANSBILAYER DIFFUSION; C-13-NMR SPECTROSCOPY; MEMBRANES; FATTY-ACIDS; MIXED MICELLES; SOLUBILITY; INTESTINAL-ABSORPTION; PHOSPHOLIPID VESICLES; APPARENT PK(A); KINETICS; BILE SALTS; BILE ACID TRANSPORT; PH; ALBUMIN; BILE ACIDS; CHEMICAL-PROPERTIES
• ISSN: 0022-2275; 1539-7262
• DOI: 10.1016/S0022-2275(20)39182-3

13C-NMR spectroscopy was used to examine the effect of side chain length on the ionization properties and transmembrane transport rate of 3 alpha,7 alpha-dihydroxy bile acids. When solubilized in taurocholate micelles, [23-13C]nor-chenodeoxycholic acid (nor-CDCA) had a pKa of 6.1, similar to that of CDCA (pKa 6.2), its C24 homologue. In unilamellar phosphatidylcholine vesicles, the pKa of nor-CDCA was 7.0, whereas that of CDCA was 6.6. Lineshape analysis indicated that the rate of ionization of nor-CDCA as a micellar solute or as a vesicle component was very slow (0.4 x 10(5) sec-1) compared to that of acetic acid in water (8.7 x 10(5) sec-1). Lineshape analysis of spectra of the protonated form of nor-CDCA at acidic bulk pH indicated that the transbilayer transport rate of nor-CDCA (580 sec-1) was six times faster than that of CDCA (100 sec-1). It is proposed that the shorter side chain of the nor-CDCA molecule causes it to reside more deeply inside the vesicle bilayer than CDCA, explaining its weaker ionization and more rapid flip-flop rate. These in vitro experiments imply that, in vivo, a given C23 nor-dihydroxy bile acid will ionize less readily when present in membranes, and it will also flip-flop faster than its C24 homologue.