Ca2+/Protein Modulator-Dependent and -Independent Cyclic GMP Phosphodiesterase from Hog Heart1

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Abstract
Ca2+/protein modulator-dependent and -independent guanosine 3 ′:5′-monophosphate (cGMP) phosphodiesterases were separated from hog heart. The protein modulator-free Ca2+/protein modulator-dependent enzyme was partially purified by repeated DEAE-cellulose column chromatography and heat treatment. The final preparation of this enzyme showed no significant basal activity under the standard assay conditions. Lineweaver-Burk plots of the Ca2+/protein modulator-dependent enzyme activity indicated the presence of only a single kinetic form of the enzyme with Km = 2.0×10−6 M for cGMP, whereas the plots for the independent enzyme were anomalous, showing both high and low Km values for cGMP. The Ca2+/protein modulator-dependent enzyme proved relatively stable at 48°C for 1 h, but the independent form lost its activity under the same conditions. Furthermore, 50% inhibition of the dependent enzyme activity, but only 10% inhibition of the independent enzyme activity, was observed with 0.1 mM adenosine 3 ′:5′-monophosphate (cAMP) when 1 μM cGMP was employed as a substrate.