Regulation of Nucleoside Cyclic 3′:5′-Monophosphate Phosphodiesterase Activity from Rat Brain by a Modulator and Ca 2+

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Abstract
Gel filtration of the 40,000 rpm supernatant fraction of a homogenate of rat cerebral cortex on a Sepharose 6B column yielded two fractions: fraction II with the "Ca(2+) plus Mg(2+)-dependent" phosphodiesterase activity and fraction III containing its modulator. The activity of fraction II was stimulated by micromolar concentrations of Ca(2+) and the modulator when present together; the modulator stimulated the activity of fraction II only when the Ca(2+) concentration was above a threshold value (about 2 muM with 0.4-1 muM substrate), and the stimulatory effect of Ca(2+) was dependent upon the presence of the modulator. A possibility is discussed that the modulator may reversibly bind to the enzyme, which by itself is inactive, to form an active enzyme-modulator complex and that Ca(2+) stimulates the activity of phosphodiesterase by shifting the equilibrium between these three species towards the formation of the active enzyme-modulator complex. Although fraction II hydrolyzed both cyclic AMP and cyclic GMP, hydrolysis of the latter was more significantly influenced by Ca(2+) and the modulator than that of the former, and the "Ca(2+) plus Mg(2+)-dependent" phosphodiesterase is likely to be a cyclic GMP enzyme. This conclusion is based on the following evidence: (a) Ca(2+) stimulated hydrolysis of cyclic GMP by fraction II more than that of cyclic AMP. (b) In the presence of Ca(2+) and the modulator, fraction II hydrolyzed cyclic GMP about 8 times faster than cyclic AMP when incubated with 0.4 muM substrate. (c) Half-maximal stimulation of hydrolysis of cyclic GMP was attained at a lower concentration of Ca(2+) (4 muM) than that of cAMP (8 muM). (d) Increase in the concentration of Ca(2+) from 0.06 muM to 12 muM in the presence of the modulator caused a decrease in the K(m) value of cyclic GMP hydrolysis by fraction II from 20 muM to 2 muM accompanied by 4-fold increase in the V(max) value. Under similar conditions, there was only a slight decrease in the K(m) value of cylic AMP hydrolysis (90 muM --> 50 muM), although the V(max) value increased 7-fold. The anomalous shape of the kinetic plot of cyclic GMP hydrolysis became linear when the Ca(2+) concentration was increased in the presence of the modulator. The modulator seems to be a protein, but it is heat stable. It is probably identical to the protein activator of phosphodiesterase first described by Cheung.