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.
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.
Ko
J
J
Department of Biophysics, Housman Medical Research Center, Boston University School of Medicine, MA 02118.
Hamilton
J A
JA
Ton-Nu
H T
HT
Schteingart
C D
CD
Hofmann
A F
AF
Small
D M
DM
D001692:Biological Transport / Q000502:physiology; D002635:Chenodeoxycholic Acid / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism*; D007477:Ions; D008051:Lipid Bilayers / Q000378:metabolism*; D008823:Micelles; D010713:Phosphatidylcholines; D012995:Solubility
D007477:Ions; D008051:Lipid Bilayers; D008823:Micelles; D010713:Phosphatidylcholines; D002635:Chenodeoxycholic Acid; C081330:24-norchenodeoxycholic acid