A deoxyribonuclease is any enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 that catalyzes the hydrolytic cleavage

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

 of phosphodiester linkages in the DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 backbone. Thus, deoxyribonucleases are one type of nuclease

Nuclease

A nuclease is an enzyme capable of cleaving the phosphodiester bonds between the nucleotide subunits of nucleic acids. Older publications may use terms such as "polynucleotidase" or "nucleodepolymerase"....

. A wide variety of deoxyribonucleases are known, which differ in their substrate

Substrate (biochemistry)

In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...

 specificities, chemical mechanisms, and biological functions.

Modes of action

Some DNases cleave only residues at the ends of DNA molecules (exodeoxyribonucleases, a type of exonuclease

Exonuclease

Exonucleases are enzymes that work by cleaving nucleotides one at a time from the end of a polynucleotide chain. A hydrolyzing reaction that breaks phosphodiester bonds at either the 3’ or the 5’ end occurs. Its close relative is the endonuclease, which cleaves phosphodiester bonds in the middle ...

). Others cleave anywhere along the chain (endodeoxyribonuclease

Endodeoxyribonuclease

Endodeoxyribonuclease is a endonuclease ribonuclease.Restriction enzymes are a type of endodeoxyribonuclease.More examples: Deoxyribonuclease I, Deoxyribonuclease II, Deoxyribonuclease IV, UvrABC endonuclease...

s, a subset of endonuclease

Endonuclease

Endonucleases are enzymes that cleave the phosphodiester bond within a polynucleotide chain, in contrast to exonucleases, which cleave phosphodiester bonds at the end of a polynucleotide chain. Typically, a restriction site will be a palindromic sequence four to six nucleotides long. Most...

s).

Some are fairly indiscriminate about the DNA sequence

DNA sequence

The sequence or primary structure of a nucleic acid is the composition of atoms that make up the nucleic acid and the chemical bonds that bond those atoms. Because nucleic acids, such as DNA and RNA, are unbranched polymers, this specification is equivalent to specifying the sequence of...

 at which they cut, while others, including restriction enzyme

Restriction enzyme

A Restriction Enzyme is an enzyme that cuts double-stranded DNA at specific recognition nucleotide sequences known as restriction sites. Such enzymes, found in bacteria and archaea, are thought to have evolved to provide a defense mechanism against invading viruses...

s, 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

Cystic fibrosis

Cystic fibrosis is a recessive genetic disease affecting most critically the lungs, and also the pancreas, liver, and intestine...

 sufferers. DNase enzymes help because white blood cells accumulate in the mucus, and, when they break down, they release DNA, which adds to the 'stickiness' of the mucus. DNase enzymes break down 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

Deoxyribonuclease I

Deoxyribonuclease I , is an endonuclease coded by the human gene DNASE1.DNase I is a nuclease that cleaves DNA preferentially at phosphodiester linkages adjacent to a pyrimidine nucleotide, yielding 5'-phosphate-terminated polynucleotides with a free hydroxyl group on position 3', on average...

 and deoxyribonuclease II

Deoxyribonuclease II

Deoxyribonuclease II, or Acid DNase, hydrolyzes deoxyribonucleotide linkages in native and denatured DNA, yielding products with 3'-phosphates. As the name implies, it is more effective at acid pH.There are several known DNases II, including:...

.

Other types of DNase include Micrococcal nuclease

Micrococcal nuclease

Micrococcal Nuclease is an endo-exonuclease that preferentially digests single-stranded nucleic acids.The rate of cleavage is 30 times greater at the 5' side of A or T than at G or C and results in the production of mononucleotides and oligonucleotides with terminal 3'-phosphates...

.

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 as 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 ºC 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.