Deoxyribonuclease substrate specificity

In the body, enzymes are compartmentalized and function under highly restricted conditions. Some enzymes (e.g., proteinases) are not substrate-specific. When present in active form in an inappropriate part of the body, they act indiscriminately and cause considerable damage to the tissue. Inhibitors inactivate these enzymes at sites where their action is not desired. Proteinase inhibitors, which are themselves proteins, are widely distributed in intracellular and extracellular fluids. Protein inhibitors of enzymes other than proteinases are relatively rare. Such inhibitors are available for a-amylases, deoxyribonuclease I, phospholipase A, and protein kinases. 

Reaction of Deoxyribonuclease. DNAase -acts on polymers of various lengths and forms a mixture of oligonucleotides with very little mononucleotide. Dinucleotides containing purines, pyrimidines, and both have been isolated from digests. The substrate specificity is obviously not so restricted as that of pancreatic RNAase, but has yet to be defined completely. The mechanism of action is also obscure. The linkage of phosphate to the 3 position is split by pancreatic DNAase the monoesters found in the products are all 5 -phosphates. From analogy with ribonucleic acid metabolism, it may be anticipated that DNAase with different specificities will be found, and that some will be found to split the 5 ester bonds. 

This final acid phosphatase preparation had a specific activity of 468 and represented an approximately 1900-fold purification of the acid phosphatase in the starting crude spleen nuclease II. It contained no acid deoxyribonuclease, acid ribonuclease, exonuclease, and phosphodiesterase activities that could be detected in a 0.1-ml sample after 2 hours of incubation with the appropriate substrate. The relative rates of hydrolysis of various substrates were as follows p-nitrophenyl phosphate, 100 5 -AMP, 63 j8-glycerophosphate, 60 ATP, 0. With p-nitrophenyl phosphate as substrate, the pH optimum was broad and lay between pH 3.0 and pH 4.8. The Michaelis constant at 37°C was 7.25 X 10" mM. Phosphate and chloride ions acted as competitive inhibitors. 

Whether the interaction between histone and DNA is specific and occurs in vivo to repress DNA is not certain. However, it cannot be excluded that the histones interfere with the polymerase reaction simply by making the primer unavailable in a fashion common to all basic polymers. Histones and protamines interfere with the hydrolysis of DNA by deoxyribonuclease, and it thus can be established that the interference results from the precipitation of substrate. Injected, histones have been claimed to interfere with DNA biosynthesis in regenerating liver. Such intriguing observations undoubtedly have to be confirmed in several laboratories before their full impact can be appreciated. In conclusion, modern studies suggest that histones may be important in the transfer of genetic information from DNA to messenger RNA, but the molecular details remain to be elucidated. 

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