CYCLIC NUCLEOTIDE PHOSPHODIESTERASE IN RAT NEOSTRIATUM: PROPERTIES OF THE ENZYME IN CRUDE HOMOGENATES

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Abstract
—: Homogenates of rat neostriatum hydrolysed cGMP faster than cAMP at both high (100μM) and low (1 μM) substrate concentrations, although the hydrolysis of both nucleotides exhibited similar kinetic properties. Kinetic analysis of the effect of substrate concentration on the rate of cAMP and cGMP hydrolysis gave results characteristic of a negatively cooperative enzyme species, with two apparent Km's for each nucleotide. The ratio between the Vmax, of the high Km, form and the Vmax of the low Km form was similar in various subcellular fractions of neostriatal tissue, in a preparation of synaptic membranes from whole brain, and in homogenates of other brain regions, including both neural‐rich and glial‐rich tissues. In homogenates of neostriatum cAMP could almost completely block cGMP hydrolysis and vice versa. The kinetics of this inhibition were competitive at low (1 μM) substrate concentrations, and non‐competitive at high (100 μM) substrate concentrations. Various phosphodiesterase inhibitors failed to preferentially inhibit the hydrolysis of either nucleotide at high or low nucleotide concentrations.Preliminary studies of the effect of a Ca2+ dependent endogenous activator preparation on the hydrolysis of cyclic nucleotides in homogenates of rat neostriatum showed a specific activation of cGMP hydrolysis at low nucleotide concentrations. The rate of cGMP hydrolysis at 1 μM substrate concentration was doubled in the presence of the activator preparation and 100μM‐CaCl2, while cGMP hydrolysis at 100 μM or cAMP hydrolysis at both 1 μM and 100μ.M remained unaffected.These observations raise the possibility that cAMP and cGMP may be hydrolysed by the same enzyme in rat neostriatum and that an endogenous activating factor may determine the relative affinities of the enzyme for the two nucleotides.