3 , 5 -cyclic phosphate diesterase and

Early studies revealed that the 3, 5 -cyclic phosphate diesterase is present in all mammalian tissues (38, 33, 36), being most active in cerebral cortex (36, 37). It has also been identified in extracts of liver fluke (Fasciola hepatica), the common earthworm (Lumbricus terrestris), and fly larvae (36) and it has been studied in marine organisms (38), the cellular slime mold Dictyostelium discovdeum (39, AO), and in E. coli (Al). The enzyme has been partially purified from beef heart (30), dog heart, (A8) and bovine brain (37, A3). No highly purified preparations have yet been obtained and most studies have been performed with relatively crude preparations. 

In addition to the proposed regulatory role of ATP and pyrophosphate, some possibility exists that 3, 5 -cyclic phosphate diesterase is under physiological control. Such ideas arose through observations of Cheung (43, 62) that the partially purified enzyme from beef brain was markedly activated by snake venom. The stimulatory factor was labile at extreme pH it was not dialyzable and appeared to be a protein. A similar activating factor is also present in brain tissue (63) and is removed during purification of the diesterase. It seems to interact stoichiometrically with the enzyme. The activator is destroyed by trypsin and is not proteolytic itself. The precise role of this protein in regulating the phosphodiesterase in vivo is not yet established, however. 

It must be emphasized that much of the information on the enzyme to date comes from partially purified preparations. Even though it seems uniquely specific for nucleoside 3, 5 -cyclic phosphates, the possibility cannot be excluded that in some tissues more than one diesterase may be present. In fact, Hardman and Sutherland (64) have shown that a second 3, 5 -cyclic phosphate diesterase exists in heart muscle which is primarily specific for uridine 3, 5 -cyclic phosphate. This enzyme was found to be more sensitive to inhibition by theophylline and to activation by imidazole. Further studies are needed to clarify its precise status other such enzymes may be discovered in the future. 

Although in no case has the enzyme been purified to homogeneity, much evidence exists that the ribonudeoside 2, 3 -cyclic phosphate diesterase activity and the 3 -nucleotidase activity reside in the same protein. Thus, in all cases the ratio of the two activities remained constant throughout purification which has varied from 130-fold for the P. mirabilis enzyme (4) to 2000-fold for the enzyme from V. alginolyticus (6). Anraku (S) found that both activities from E. coli B had the same optimal pH, both showed the same behavior to activators such as Co2+, and to inhibitors [Zn2+, Cu2+, ethylenediaminetetraacetate (EDTA)], both were activated simultaneously by heating at 55° for 5 min and... 

The enzyme was originally found to be membrane bound and resisted solubilization and purification (26). Lundblad and Moore (27), however, have reported solubilizing it using dilute (5 mM) sodium borate buffer at pH 9 after 16 hr at 37°. Studies on regional and subcellular distribution using density gradient techniques have revealed that the 2, 3 -cyclic phosphate diesterase concentrates in those fractions containing myelin (28, 29), and the conclusion has been reached that the enzyme is localized in the myelin sheath or intimately associated structures. Kurihara and... 

Caffeine and puromycin also stimulate the conversion of phosphorylase (a) to phosphorylase (b), probably by interfering with the breakdown of the 3, 5 -cyclic adenosine nucleotide. A diesterase capable of splitting the cyclic phosphate of the 3, 5 -cyclic adenosine phosphate has been found in beef heart. The effect of epinephrine on phosphorylase is important because it constituted the first explanation of a hormone s mechanism of action in molecular terms. 

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