Adenyl cyclase was assayed in vitro by measuring the rate of formation of radioactive cyclic 3',5'-adenosine monophosphate (cyclic 3',5'-AMP) from its14C-labeled precursor, adenosine triphosphate, and phosphodiesterase activity was determined from the rate of hydrolysis of cyclic 3',5'-AMP. Studies of the distribution of these two enzymes in various parts of rat brain revealed that, in general, enzyme activity was higher in those areas containing grey matter (cerebral cortex) than in areas consisting predominantly of white matter (pons, medulla, spinal cord). However, there was no correlation between the relative activities of adenyl cyclase and of phosphodiesterase in the various areas of the brain; the greatest discrepancy was found in the cerebellum, corpora quadrigemina and pineal gland where, relative to other brain areas, there was high adenyl cyclase activity compared with that of phosphodiesterase. Moreover, the distribution pattern of adenyl cyclase was different from that of the catecholamine concentration in the brain. Results of differential centrifugation of bovine pineal gland and of rat cerebellar homogenates indicated that most of the adenyl cyclase is particulate, being in the mitochondrial and microsomal fractions, whereas the majority of the phosphodiesterase is soluble. The sp. act. of the mitochondrial and microsomal fractions of pineal adenyl cyclase was about four times that of the whole homogenate. No such increase in specific activity was obtained with cerebellar homogenates. The possibility that an activator of adenyl cyclase is present in soluble supernatant fractions of cerebellar homogenates is discussed. © 1968.
