Nucleoside 3 ,5 -cyclic phosphate diesterase

G. Physiological Function of Nucleoside 3, 5 -Cyclic Phosphate Diesterase... 

Several analogs have been examined as substrates for nucleoside cyclic phosphate diesterase purified from a variety of tissues. In particular, Nb-monobutyryl-cAMP and DBcAMP are not hydrolyzed by cyclic nucleotide phosphodiesterase from heart 6, liver O,/ adipose tissue23,26,78 gr brain... 

A suggestion that such a system functions in the breakdown of ribosomal RNA in vivo has been provided by Maruyama and Mizuno (28a). They showed that the degradation of ribosomal RNA induced by phosphorus starvation was catalyzed by the primary attack of RNase on ribosomes followed by the accumulation of oligonucleotides with 2, 3 -cyclic phosphate end groups and these were subsequently converted to nucleosides and inorganic phosphate presumably by cyclic phosphate diesterase. 

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. 

F. Possibility of Other Nucleoside Cyclic 3, 5 -Phosphate Diesterases... 

Other Ribonudeases. A diesterase more specific than that of venom was found in spleen. This enzyme attacks substituted nucleotide 3 -phosphates. Both purine and pyrimidine nucleotides serve as substrates. This enzyme neither forms nor attacks cyclic diesters, and was thus useful in establishing that the 3, not the 2, position is esterified in purine as well as pyrimidine nucleoside components of RNA. Synthetic 2 -nucleotide phosphate diesters are not split by this enzyme. 

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