[Topochemistry of hormone action].
The site of hormone action is the cells of target tissues. Now that ultrastructural research has revealed the complexity of eukaryotic cells, we must ask which cellular structures are involved. Some hormones act at the cell membrane, mainly via adenylate cyclase and the "second messenger" cAMP. The cytosol and the mitochondria, the main sites of intermediate metabolism and enzyme activity, are apparently not involved, although in the past interaction of hormones with enzymes has been postulated. Most of the steroid hormones act at the level of the cell nucleus, regulating transcription of specific genes and inducing enzymes or other proteins. The mechanisms by means of which this regulation is achieved are not fully understood, but it is likely that derepression is involved. The active agent seems to be a complex of the hormone with a receptor protein.