Vasopressin-sensitive Adenylate Cyclase From the Mammalian Kidney: Mechanisms of Activation

Abstract

Adenylate cyclase has been found in membranes from all animal cells studied thus far. The catalytic activity of the enzyme can be modified in vitro by a large variety of chemical and physical agents. Some of these agents are active on adenylate cyclases from almost all cell types; examples of such agents are sodium fluoride (1) and to a lesser extent guanylnucleotides (2). Other agents, e. g., many peptide hormones, several neurotransmitters, and prostaglandins are active on the adenylate cyclase from one or a limited number of cell types. It is generally accepted that the specificity of the response is due to the presence on the membrane of specific receptor molecules. These receptors are able (1) to bind the regulatory agent and (2) to initiate the sequence of molecular events leading to a change (in most cases an increase) in the catalytic activity of adenylate cyclase. In the past few years considerable effort has been devoted to the characterization of membrane receptors functionally coupled to adenylate cyclase (for review, see Ref. 3). Some of these receptors have been extracted from the membrane in an active form. However, in none of the receptor adenylate cyclase systems investigated so far were the receptors present in a sufficiently great amount to allow complete purification and physicochemical characterization. On the other hand, very little is known about adenylate cyclase itself and about the molecular mechanisms involved in its activation by specific and nonspecific regulatory agents Almost all available data were derived from kinetic studies using intact membranes. Adenylate cyclases from various sources were found to be extractable from the membrane under the influence of nonionic detergents (4–6). However, it was not possible to obtain a purified and stable form of the soluble enzyme. Furthermore, the receptor-mediated enzyme activation was lost during membrane solubilization.