590 G. SCHMIDT 



relatively weakly active phosphatase preparations, whereas the action of 

 intestinal phosphatase was studied on highly active phosphatase samples. 

 Owing to the relatively slow rate of the enzymic depolymerization of PNA 

 and of deoxypolyniicleotides, the diesterase action of weakly active phos- 

 phatase preparation might have been overlooked. It seems that the prob- 

 lem of nucleophosphodiesterase action of alkaline phosphatases requires 

 clarification by further investigations. 



yH Optimum. The pH optimum of intestinal nucleophosphodiesterase 

 has been reported to be in the alkaline range ; since all available observations 

 are based on the liberation of inorganic phosphate from the substrate, it is 

 possible that these data describe the behavior of the phosphomono- rather 

 than that of the phosphodiesterase activity. 



Assay. Approximate figures for the activities of intestinal phosphodi- 

 esterases are usually obtained by the determination of inorganic phosphate 

 or of phenols which originate from the intermediary phosphomonoesters by 

 the action of the phosphomonoesterase which is always present. In the case 

 of other nucleophosphodiesterases, it might be preferable to determine the 

 organic acid-soluble phosphorus compounds formed, particularly for studies 

 of the effects of enzyme inhibitors. 



III. Enzymes Catalyzing the Hydrolytic Cleavage of the Phosphoryl 

 Groups of Mononucleotides 



1, Phosphatases of Low Degrees of Specificity 



Mononucleotides are rapidly dephosphorylated by many phosphatases 

 such as intestinal phosphatase, bone phosphomonoesterase, acid prostatic 

 phosphatase, almond phosphatase. Since these phosphatases are active 

 toward many other phosphoric acid monoesters, their detailed description 

 is beyond the scope of this book. It should be mentioned, however, that in 

 some cases the hydrolysis of nucleotides shows "kinetic differences in com- 

 parison with that of other substrates. The rates of hydrolysis of 3 '-nucleo- 

 tides by prostate phosphatase follow the course of a unimolecular reaction, 

 at least up to degrees of 70 % hydrolysis, whereas those of the hydrolysis of 

 many other substrates (in particular, of the glycerophosphates) fall much 

 more rapidly even in the initial stages." 



The phosphatases just mentioned hydrolyze all mononucleotides regard- 

 less of the position of the phosphoryl groups. In some cases, however, the 

 rates of hydrolysis catalyzed by one enzyme are very markedly influenced 

 by the position of the phosphoryl groups in the nucleoside moiety. For 

 example, the hydrolysis of 5'-adenylic acid by prostatic phosphatase is 

 approximately one-third of that of 2'- or of 3'-adenylic acid." 



