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Crystals of DNase protein.

A deoxyribonuclease (DNase, for short) is any enzyme that catalyzes the hydrolytic cleavage of phosphodiester linkages in the DNA backbone. Deoxyribonucleases are thus one type of nuclease. A wide variety of deoxyribonucleases are known, which differ in their substrate specificities, chemical mechanisms, and biological functions.

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Modes of action

Some DNases cleave only residues at the ends of DNA molecules (exodeoxyribonucleases, a type of exonuclease). Others cleave anywhere along the chain (endodeoxyribonucleases, a subset of endonucleases).

Some are fairly indiscriminate about the DNA sequence at which they cut, while others, including restriction enzymes, are very sequence-specific.

Some cleave only double-stranded DNA, others are specific for single-stranded molecules, and still others are active toward both.

DNase enzymes can be inhaled using a nebuliser by cystic fibrosis sufferers. DNase enzymes help because white blood cells accumulate in the mucus and when they breakdown they release DNA which adds to the 'stickiness' of the mucus. DNase enzymes breakdown the DNA and the mucus is much easier to clear from the lungs.

Types of deoxyribonucleases

The two main types of DNase found in metazoans are known as deoxyribonuclease I and deoxyribonuclease II.

Other types of DNase include Micrococcal nuclease.

Assay of deoxyribonucleases

DNA absorbs UV light with a wavelength of maximal absorbance near 260 nm. This absorption is due to the pi electrons in the aromatic bases of the DNA. In dsDNA, or even regions of RNA where double stranded structure occurs, the bases are stacked parallel to each other and the overlap of the base molecular orbitals leads to a decrease in absorbance of UV light. This phenomenon is called the hyperchromic effect. When DNAse liberates nucleotides from dsDNA, the bases are no longer stacked like they are in dsDNA, so that orbital overlap is minimized and UV absorbance increases. This increase in absorbance underlies the basis of Kunitz unit of DNAse activity. One Kunitz unit is defined as the amount of enzyme that causes an increase in absorbance at 260 nm of 0.001 per mL when acting upon highly polymerized DNA at 25 C and pH 5.0 under specified conditions. A standard enzyme preparation should be run in parallel with an unknown because standardization of DNA preparations and their degree of polymerization in solution is not possible.

References

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