Characterization of detergent dispersed cytidylate cyclase of rat brain

Cytidylate cyclase was demonstrated to be distributed in various tissues of rat, with the highest activity in brain, and it was shown to be a membrane-bound type enzyme. The enzyme was effectively dispersed from the membrane fraction of rat brain with 0.3% (w/v) Triton X-100. The dispersed cytidylate cyclase had an optimal pH of 9.4 and the activity at a physiological pH of 7.5 was less than 20% of the maximum value. This enzyme completely lost its activity in the absence of divalent cation such as Mn2+ and Mg2+. The Km value for CTP was calculated as 0.0156 mM, by Lineweaver-Burk analysis. It was also found that activity of dispersed enzyme was inhibited by ATP, but not GTP. Both forskolin and lanthanum chloride, which affect adenylate cyclase, showed no effect on cytidylate cyclase. These results indicate that cytidylate cyclase is a unique membrane-bound enzyme distinct from purine nucleotide cyclases, adenylate cyclase and guanylate cyclase.