J. Mallol

Receptor–receptor interactions involving adenosine A1 or dopamine D1 receptors and accessory proteins

Summary.The molecular basis for the known intramembrane receptor–receptor interactions among heptahelical receptors (G protein coupled receptors, GPCR) was postulated to be heteromerization based on receptor subtype specific interactions between different types of homomers of GPCR. Adenosine and dopamine receptors in the basal ganglia have been fundamental to demonstrate the existence of receptor heteromers and the functional consequences of such molecular interactions. The heterodimer is only one type of heteromeric complex and the evidence is equally compatible with the existence of higher order heteromeric complexes, where also adapter proteins such as homer proteins and scaffolding proteins can exist, assisting in the process of linking the GPCR and ion channel receptors together in a receptor mosaic that may have special integrative value and may constitute the molecular basis for learning and memory. Heteromerization of D2 dopamine and A2A adenosine receptors is reviewed by Fuxe in another article in this special issue. Here, heteromerization between D1 dopamine and A1 adenosine receptors is reviewed. Heteromers formed by dopamine D1 and D2 receptors and by adenosine A1 and A2A receptors also occur in striatal cells and open new perspectives to understand why two receptors with apparently opposite effects are expressed in the same neuron and in the nerve terminals. The role of accessory proteins also capable of interacting with receptor–receptor heteromers in regulating the traffic and the molecular physiology of these receptors is also discussed. Overall, the knowledge of the reason why such complex networks of receptor–receptor and receptor–protein interactions occur in striatal cells is crucial to develop new strategies to combat neurological and neuropsychiatric diseases.

G‐protein‐coupled receptor heteromers: function and ligand pharmacology

Almost all existing models for G‐protein‐coupled receptors (GPCRs) are based on the occurrence of monomers. Recent studies show that many GPCRs are dimers. Therefore for some receptors dimers and not monomers are the main species interacting with hormones/neurotransmitters/drugs. There are reasons for equivocal interpretations of the data fitting to receptor dimers assuming they are monomers. Fitting data using a dimer‐based model gives not only the equilibrium dissociation constants for high and low affinity binding to receptor dimers but also a ‘cooperativity index’ that reflects the molecular communication between monomers within the dimer. The dimer cooperativity index (DC) is a valuable tool that enables to interpret and quantify, for instance, the effect of allosteric regulators. For different receptors heteromerization confers a specific functional property for the receptor heteromer that can be considered as a ‘dimer fingerprint’. The occurrence of heteromers with different pharmacological and signalling properties opens a complete new field to search for novel drug targets useful to combat a variety of diseases and potentially with fewer side effects. Antagonists, which are quite common marketed drugs targeting GPCRs, display variable affinities when a given receptor is expressed with different heteromeric partners. This fact should be taken into account in the development of new drugs.

A method for binding parameters estimation of A1 adenosine receptor subtype: A practical approach

Working with pig brain striatum in which A1 and A2 adenosine receptor subtypes coexist, we describe an uncomplicated method for unequivocally obtaining the equilibrium parameters (KD and binding capacity) of A1 receptor without interference from ligand binding to A2 receptor. Also, the equilibrium parameter estimation method we propose avoids the experimental determination of nonspecific binding by the inclusion of the corresponding unknown parameter in the function. This not only saves time but also avoids the use of expensive radioligands in saturation experiments. The method is suitable for any system with two different receptor subtypes for the same physiological ligand, and good estimates of the equilibrium parameters corresponding to the subtype displaying the higher affinity for the ligand can be obtained.

The binding of [3H]R-PIA to A1 adenosine receptors produces a conversion of the high- to the low-affinity state

Kinetic evidence for negative cooperativity on the binding of [3H]R‐PIA to A1 adenosine receptors was obtained from dissociation experiments at different ligand concentrations and from the equilibrium isotherm. The dissociation curves indicate that there is an apparent ligand‐induced transformation of high‐ to low‐affinity states of the receptor. At concentrations of 18.2 nM R‐PIA or higher there was only found the low‐affinity state of the receptor. In view of these results equilibrium binding data were analyzed by the usual two‐state model (assuming that there is an interconversion between them) and by the negative cooperativity model employing the Hill equation.

Relationships Between Metabolic Enzymes and the Nucleoside Transport

The understanding of nucleoside incorporation into cells requires the integration of all factors related directly or indirectly with the transport phenomenon. This is implicitely assumed when using the term Uptake whose definition is: the total intracellular accumulation of radioactivity from exogenous labelled substrate regardless of metabolic conversions.

Theoretical study of the acidic strength of amino acid side chains

Abstract The acidic strengths in gas phase of three groups (NH 4 + , H 2 S, and HCOOH) that mimic the most common amino acid side chains of enzymes are studied by means of quantum mechanical methods. The results demonstrate that in gas phase the acidities of such groups change drastically with respect to those reported in aqueous phase. Moreover, the dependence between the energetics of the proton-transfer process and the distance separating the acid and base groups is stated. The biological implications of these results are discussed.