Submitted on May 30, 2008
Revised on July 14, 2008
Accepted on July 23, 2008
Structural determinants of monohydroxylated bile acids to activate 
1 subunit-containing BK channels
Anna N. Bukiya, Jacob McMillan, Abby L. Parrill, and Alejandro M. Dopico
Pharmacology, The University of Tennessee Hlth. Sci. Ctr., Memphis, TN 38163
Corresponding Author: adopico{at}utmem.edu
Lithocholate (LC) (10-300 
M)in physiological solution is sensed by vascular myocyte BK channel 
1 accessory subunits leading to channel activation and arterial dilation. However, the structural features in steroid and target that determine LC action are unknown. We tested LC and close analogs on BK channel (pore-forming cbv1+1 subunits) activity (NPo). LC (5-cholanic acid-3
-ol), 5-cholanic acid-3-ol, and 5-cholanic acid-3-ol increased NPo (EC50~45 M). At Emax, LC increased NPo by 180% while 5-cholanic acid-3-ol and 5-cholanic acid-3-ol raised NPo by 40%. Thus, the -hydroxyl and the cis A-B ring junction are both required for robust channel potentiation. Lacking both features, 5-cholanic acid-3-ol and 5-cholenic acid-3-ol were inactive. Three-dimensional structures show that only LC displays a bean shape with clear-cut convex and concave hemispheres; 5-cholanic acid-3-ol, and 5-cholanic acid-3-ol partially matched LC shape, and 5-cholanic acid-3-ol and 5-cholenic acid-3-ol did not. Increasing polarity in steroid rings (5-cholanic acid-3-sulfate) or reducing polarity in lateral chain (5-cholanic acid 3-ol methyl ester) rendered poorly active compounds, consistent with steroid insertion between 1 and bilayer lipids, the steroid charged tail being nearby the aqueous phase. Molecular dynamics identified two regions in 1 TM2 that meet unique requirements for bonding with LC concave hemisphere, where the steroid functional groups are located.
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