Properties of a multifunctional 3′,5′-cyclic nucleotide phosphodiesterase from Lactuca cotyledons: Comparison with mammalian enzymes capable of hydrolysing pyrimidine cyclic nucleotides

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Hydrolysis of 3′,5′-cyclic nucleotides by this Lactuca multifunctional cyclic nucleotide phosphodiesterase yields the corresponding 5′-nucleotides. With 2′,3′-cyclic nucleotides, the point of cleavage is affected by the nature of the base. Equimolar amounts of 2′- and 3′-AMP are produced from 2′,3′-cyclic AMP but four times more 3′-CMP than 2′-CMP is produced from 2′,3′-cyclic CMP. With 2′,3′-cyclic GMP, 2′-GMP is the sole product. The presence of one cyclic nucleotide affects the rate of hydrolysis of another. With 3′,5′-cyclic nucleotide substrates, mixed-type inhibition was exhibited by other 2′,3′- and 3′,5′-cyclic nucleotides. During hydrolysis of3′,5′-cyclic AMP, Ki values of 16 and 6 μM were obtained with 3′,5′-cyclic GMP and 2′,3′-cyclic AMP, respectively. Coupled with the Km of 0.73 mM for 3′,5′-cyclic GMP and that of 1.12 mM for 2′,3′-cyclic AMP, the low Ki values suggest that more than one binding site is available for each nucleotide. Hill coefficients of 1.26, 0.88 and 0.91, for 3′,5′-cyclic AMP, 3′,5′-cyclic GMP and 3′,5′-cyclic CMP, respectively, together with the linear nature of the Hill plots obtained, indicate that when a single substrate is present, there is no cooperative effect between the sites. The properties of the Lactuca enzyme are compared with those of other plant and mammalian cyclic nucleotide phosphodiesterases and the significance of the occurrence of this enzyme in Lactuca is considered.