Biochemical and functional characterization of 1-benzyl substituted trimetoquinol affinity analogs on rat and human beta-adrenoceptors.
The site of interaction for the 1-(3',4',5'-trimethoxybenzyl) group of trimetoquinol (TMQ) with beta-adrenoceptors (beta-ARs) is important for the rational design of highly potent and beta3-AR-selective analogs. 1-Benzyl ring-substituted TMQ analogs were evaluated for binding affinities and biochemical activities (cyclic AMP accumulations) in Chinese hamster ovary (CHO) cells expressing the rat and human beta3-AR, and for functional activities on isolated rat tissues. Binding affinities (K1 approximately 0.055 to 1.5 microM) for the rat beta3-AR and potencies for adenylyl cyclase activation (K(act) approximately 0.43 to 2;5 nM) of the 3'-monoiodo or 3',5'-diiodo derivatives with 4'-isothiocyanato-, 4'-amino, 4'-acetamido, or 4'-alpha-haloacetamido substitutions were higher than those of (-)-isoproterenol, and comparable to those of BRL 37344 [(+/-)-(R*,R*-[4-[2-[[2-(3-chlorophenyl)-2-hydroxy-ethyl]amino]propyl]ph enoxy]-acetic acid sodium]. A similar rank order of binding affinities (K(i) approximately 0.11 to 2.5 microM) and potencies (K(act) approximately 0.45 to 9.5 nM) was obtained for TMQ analogs on the human beta3-AR. The 4'-acetamido and 4'-alpha-chloroacetamido analogs of 3',5'-diiodoTMQ were more potent than (-)-isoproterenol in rat atria (beta1-AR) and rat trachea (beta2-AR) and exhibited partial agonist activities, whereas full agonist activities were observed in rat esophageal smooth muscle (EC50 approximately 2-8 nM, beta3-AR). 4'-alpha-Chloroacetamido-3',5'-diiodoTMQ-mediated chronotropic responses in atria were sustained and resistant to washout. Further, the 4'-alpha-chloroacetamido and 4'-alpha-bromoacetamido analogs of 3',5'-diiodoTMQ demonstrated significant concentration-dependent irreversible binding to the rat beta3-AR. Reversible beta-AR agonists such as (-)-isoproterenol, BRL 37344, and 4'-acetamido-3',5'-diiodoTMQ or nucleophilic 1-amino acids (lysine, glutathione, cysteine) did not protect against this irreversible binding. Thus, the lipophilic 1-benzyl ring of TMQ analogs interacts with a hydrophobic region of the beta-AR that may represent an exo-site or an allosteric binding site.
Mehta
R C
RC
College of Pharmacy, The Ohio State University, Columbus 43210, USA.
Salazar-Bookaman
M M
MM
Fertel
R H
RH
De Los Angeles
J
J
Nikulin
V I
VI
Fraundorfer
P F
PF
Miller
D D
DD
Feller
D R
DR
D000318:Adrenergic beta-Agonists / Q000737:chemistry / Q000494:pharmacology*; D000818:Animals; D001011:Aorta; D016280:Atrial Function, Right / Q000187:drug effects*; D001667:Binding, Competitive; D016466:CHO Cells; D006224:Cricetinae; D000242:Cyclic AMP / Q000378:metabolism; D006801:Humans; D051381:Rats; D011943:Receptors, Adrenergic, beta / Q000187:drug effects / Q000378:metabolism*; D022702:Receptors, Adrenergic, beta-3; D014213:Tretoquinol / Q000031:analogs & derivatives / Q000737:chemistry / Q000494:pharmacology*
D000318:Adrenergic beta-Agonists; D011943:Receptors, Adrenergic, beta; D022702:Receptors, Adrenergic, beta-3; D000242:Cyclic AMP; D014213:Tretoquinol