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Beef heart cAMP phosphodiesterase (EC 3.1.4.17) was isolated and partially purified using fractionation by ammonium sulfate and gel filtration on the columns with Sephadex G-200 and Sepharose 6B. This method allowed to preserve the enzyme binding to the low-molecular weight thermostable protein regulator of the phosphodiesterase activity. The enzyme preparation was purified 130--180-fold as compared to the original homogenate. The pH-dependence of the enzyme activity in the imidazole and tris -- buffers for the fraction with maximal activity was carried out. The kinetic analysis of this fraction revealed an abnormal kinetic behaviour with two Km values. The enzyme is represented by four forms differing in their molecular weights and possessing different capacity for activation by Ca2+ and protein regulator. No activation was observed in the forms with higher molecular weights, whereas the activity of the forms with lower molecular weights depended on the presence of Ca2+ and protein regulator. It is assumed that some of the above-described forms are capable of interconversions.
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