Two forms of cyclic nucleotide phosphodiesterase (ES 3.1.4.17)--PDE-I and PDE-II--sensitive and resistant to Ca-dependent protein regulator, were isolated from the soluble fraction of rabbit heart by chromatography on DEAE-cellulose. Both forms of enzyme are inhibited by 30--50% by Ca2+ (10(-4) M). Addition of Ca-dependent protein regulator activates PDE-I and eliminates Ca2+-induced inhibition of PDE-II. In heart extract Ca2+ increases the phosphodiesterase activity 1.5-fold. The amount of PDE-I makes up to about 10% of total phosphodiesterase activity of the heart; that of PDE-II is about 90%. In the presence of Ca-dependent protein regulator the rate of 3', 5'-AMP hydrolysis by PDE-I is increased 5--15-fold, while that of 3', 5'-GMP hydrolysis only 2.5-fold. Both PDE-I and PDE-II have close Km values for substrates--(3.5--4.0).10(-6) M for 3', 5'-AMP and 14.10(-6) M for 3', 5'-GMP. Inhibition by Ca2+ and effect of Ca-dependent protein regulator manifest themselves in changes in V for cyclic nucleotide hydrolysis and do not alter the Km value for the enzyme.
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