Partial purification and properties of the cyclic AMP and the cyclic GMP phosphodiesterases of bovine liver

Abstract

The cyclic AMP phosphodiesterase of bovine liver was purified approximately 100-fold (assayed at a substrate concentration of 10 −6 M cyclic AMP). The enzyme appeared to occur in two interconvertible forms of mol. wts of 120 000 and 240 000, respectively. They differed in their K m values for cyclic AMP. The ionic strength of the buffer, the concentration of Mg 2+, and the presence of either cyclic AMP or cyclic GMP affected the predominance of one or the other form. The cyclic GMP phosphodiesterase also had a mol. wt of approx. 240 000 but appeared to be distinct from the cyclic AMP phosphodiesterase. A fraction, obtained after Sephadex G-200 filtration of crude extracts, of a mol. wt of approx. 360 000 showed both cyclic AMP and cyclic GMP phosphodiesterase activities when unphysiologically high concentrations of the two cyclic nucleotides were employed in the assay. The fact that high concentrations of cyclic GMP inhibited the activity of the cyclic AMP phosphodiesterase in this fraction suggested the existence of a molecular aggregate combining the activities of the two enzymes.