The Mode of Action of Drugs on Cells

Abstract

Recent modifications of Clark's classical theory of drug action are analyzed. Some basic deficiencies of the Ariens-Stephenson modification and the rate theory of Raton are pointed out. It is shown that these current concepts suffer from the absence of a biophysical basis and that they fail in providing a qualitative interpretation of drug properties at the receptor level. A molecular theory, defined as the macromolecular perturbation theory (MPT), is elaborated for the muscarinic cholinergic receptor. On the basis of the postulate that the receptor is a protein with acetylcholinesterase (AChE) like properties (with regard to Michaelis complex formation only), evidence is presented for the hydrophobic properties of the protein-binding surface which suggests that two modes of interaction of drugs is operative. The first, involving only the acetylcholine-binding portion of the surface would lead to a specific conformational perturbation (SCP) of the protein, the second requiring alterations with the hydrophobic periphery would be accompanied by a nonspecific conformational perturbation (NSCCP) of the enzyme. Induction of a SCP by a molecule Jl, (agonist) would produce a complex symbolized by P*M"; for the case of the involvement of a NSCP, the complex produced by a molecule (antagonist) is denoted by P *M¡ (eq. 1 and 2). The molecular basis for these rationalizations is discussed in detail. The structural requirements for the induction of a P*M, complex by an Ms molecule and a , , complex by an M ¡ molecule are analyzed and it is shown that hydrophobic interactions (Fig. L) largely condition the transition between the two types of complexes. A third class of molecules symbolized by M5¡ is shown to induce in the protein the formation of an equilibrium mixture of P*M,¡ and P-M*i complexes, thus accounting for the occurrence of partial agonists (eq. 3). Also, the possibility for M"¡ molecules to combine with a P*Msi complex is discussed. Ternary complex formation (eq. 4) accounts for bell-shaped dose-response curves. The MPT serves in the prediction of the general form of dose-response curves usually encountered in the study of drug effects on tissues. The expressions "intrinsic activity" and "efficacy" are shown to reflect the molecular mechanism described by eq. 3. The validity of the MPT was tested by evaluating its capacity to provide a basis for the interpretation of structure-activity relationships. With regard to muscarinic drugs, the theory was strikingly successful in accounting for the qualitative properties of various series of quaternary ammonium salts. A variety of parallel-isms between the muscarinic receptor and AChE arc summarized. Near identity of the two proteins is suggested. Discrepancies in their chemical reactivity such as towards the organophospliorns inhibitors are accounted for. On the basis of known kinetic evidence, it is suggested that the muscarinic receptor is acetylated AChE. In this form, the normal chemical reactivity of the esterarte sites of AChE would be masked but the enzyme would retain (as is now known) the ability to form complexes. The acetylated form of the enzyme would be produced in specific regions of sensitive membranes by the continuous quanta! discharge of acetylcholine (ACh). The muscarinic receptor would therefore have its biochemical origin in the pool of AChE. This hypothesis may account for a variety of previously inexplicable observations. Current theories of the mechanism of action of drugs at the receptor level rest primarily on the classical work of Clark2 and Gaddum3 who showed that drug receptor interactions closely approximate the relationships encompassed by the Langmuir adsorption iso-t herm. This subject has been reviewed in detail several times in the recent past4 5"7 and it will suffice here to briefly summarize and evaluate some recent modifications of the basic concept as they pertain to the quantitiative interpretation of the anomalies that the classical theory fails to explain. Presently, among a number of modifications of the theory, two appear to have acquired momentum; the first may be referred to as the Ariens-St ephciisoi modification''·3 9and the oilier as the Baton theory.11 (1) Published as Part, IV uf the series "The Chemical Basis fur Cholino-mimetic and Chulinolytic Activity." For part III see, B. Beileau and (. In a paper exposing the rate theory. Baton11 presented some valid and pertinent criticism of the Ariens Stephenson kinetic model and pointed out for instance that the concept: of intrinsic activity or efficacy still fails to explain why drugs vary in their type of action (or their efficacy), " so that when one drug occupies a receptor it: stimulates, whereas another occupying the same receptor blocks?"1' The question is a valid one indeed and clearly focuses attention on the fundamental deficiency of the Ariens-Stephenson theory, mainly that it does not afford an approach to mechanism at the molecular level in structural terms. Ariens attempted to answer this (¡ues I ion6·10 (after drawing heavily from the field of enzyme kinetics) by stating that "Tlie intrinsic activity is analogous to the reaction velocity constant which determines the formation of the final product B in the case of an enzymologi-cal react ion as for inst anee" k