Membrane milieu is regarded as the native environment of the cyclic AMP degrading enzyme cluster

The phenomenon of kinetic advantage of nucleoside formation from cyclic AMP, via the intermediate 5'AMP has been observed in the microsomal fraction after subcellular fractionation of beef adrenal cortex tissue. It was explained by the existence of a multienzyme sequence previously evidenced [H. Wombacher, 1982, Arch. Biochem. Biophys. 201, 8-19]. In the present study a similar enzyme cluster was prepared from the soluble fraction of the cell homogenate after two steps of gel-chromatography. An elusive channeling of cyclic AMP degradation could be disclosed. The time course reaction of cyclic AMP degradation to the nucleosides, adenosine and inosine, via 5'AMP as an intermediate compared with the time course reaction of 5'AMP hydrolysis to the nucleosides, adenosine and inosine, under otherwise identical conditions showed that the nucleoside formation from cyclic AMP was faster after the lag phase of the reaction sequence. This kinetic advantage effect, however, was much less pronounced than to be seen in the membrane-bound multienzyme sequence. For an analysis of the influence of the environmental conditions on the activity of both enzyme cluster forms they were treated by chaotropic agents, detergents and ultrasonic power. Common to all results was: the activity of the membrane-bound enzyme cluster is highly stable in comparison with the soluble form. On basis of these and previous findings a hypothesis is suggested explaining the similarities between the membrane-bound enzyme cluster and the soluble form. Thus, the soluble enzyme cluster form is considered a partially preserved form of the membrane-bound form arisen from the cell homogenization process and/or vice versa the soluble form might present a pro-form of the membrane-bound enzyme cluster, and the most stable and active assembly has to be yet first membrane-triggered.