Adenosine binding sites in brain; relationship to endogenous levels of adenosine and to its physiological and regulatory roles.

Adenosine is an endogenous component of brain tissue and is present at micromolar concentrations, well above those required to saturate the adenylate cyclase-linked receptors recently demonstrated by binding studies. It is suggested that a major binding site for adenosine may be the enzyme S-adenosyl homocysteinase, and modulation of the equilibrium catalysed by this reaction may regulate the concentration of adenosine available for stimulation of cyclic AMP production and its other physiological roles.