Alkylating affinity labels are reactive compounds that bind receptors in a covalent manner. This property makes them useful tools with which to study the structure and function of these important biological recognition molecules. To familiarize investigators with the use of these compounds, an overview of their general properties in terms of design, pharmacology, and chemistry is presented. In addition, some experimental approaches to characterizing their irreversible effects are described. Finally, several uses of alkylating affinity labels for the determination of a receptor reserve, receptor turnover, and studies on receptor structure are briefly outlined. important limitations with the use of these compounds are discussed, and examples focusing mainly on catecholamine receptors are given.
Although the existence of four distinct adrenergic receptor subtypes (α1, α2, β1, β2) has been recognized for more than 15 years, it has recently become clear that the adrenergic receptor family is much larger than previously suspected. Development of more selective agonists and antagonists and careful comparison of pharmacological properties have led to the realization that there are nine or more adrenergic receptor subtypes. Molecular cloning of many of these subtypes, discussed in an accompanying article, supports this conclusion. The adrenergic receptors fall into three major families (α1, α2, β) based on pharmacology, structure, and signal transduction, with at least three closely related members within each family. Here, we summarize the known pharmacological differences between these receptors and evaluate the best methods currently available for distinguishing these subtypes using selective drugs.
Affinity purification of the β2-adrenergic receptor from hamster lung has led to the amino acid sequencing of a few of its peptides, followed by the molecular cloning of the corresponding cDNA. Expression of this cDNA confirmed the catecholamine binding properties of the β2-adrenergic receptor. This initial success, that of the cloning of the turkey erythrocyte β1-like and of the platelet α2A-adrenergic receptors, has rapidly led to the identification and cloning by homology of nine different subtypes of α1, α2, and β receptors. All these belong to the very large family of G-protein-coupled membrane receptors, which may include over 1000 proteins that act as receptors for neurotransmitters, hormones, and sensory signals such as light or odors. While a few of the adrenergic receptors had been characterized previously by pharmacologic means, most were actually not known to exist as individual entitles. The "reverse pharmacology" made possible by molecular biology should now lead to the synthesis of new subtype-selective ligands.