<p>Deoxyribonuclease I (DNase I) (<db_xref db="EC" dbkey="3.1.21.1"/>) [<cite idref="PUB00003994"/>] is a vertebrate enzyme which catalyzes the endonucleolytic cleavage of double-stranded DNA to 5'- phosphodinucleotide and 5'-phosphooligonucleotide end-products. DNase I is an enzyme involved in DNA degradation; it is normally secreted outside of the cell but seems to be able to gain access to the nucleus where it is involved in cell death by apoptosis [<cite idref="PUB00001239"/>].</p><p>As shown in the following schematic representation, DNase I is a glycoprotein of about 260 residues with two conserved disulphide bonds.<pre> +-+ +--------+ | | | | xxxxxxxxxxxxxxxxx#xxxxxxCxCxxxxx#xxxxxxxxxCxxxxxxxxCxxxxxxxxxxxxx'C': conserved cysteine involved in a disulphide bond.'#': active site residue.</pre> </p><p>DNase I has a pH-optimum around 7.5 and requires calcium and magnesium for full activity. It causes single strand nicks in duplex DNA. A proton acceptor-donor chain composed of an histidine and a glutamic acid produce a nucleophilic hydroxyl ion from water, which cleaves the 3'-P-O bond [<cite idref="PUB00004018"/>].</p><p>DNase I forms a 1:1 complex with G-actin, resulting in the inhibition of DNase activity and loss of the ability of G-actin to polymerise into fibres [<cite idref="PUB00004077"/>].</p><p>DNase I has been used in the treatment of lung problems in patients with cystic fibrosis: here it acts by degrading DNA found in purulent lung secretions, reducing their viscosity and making it easier for the patient to breathe [<cite idref="PUB00004719"/>].</p><p>The sequence of DNase I is evolutionary related to that of human muscle-specific DNase-like protein and human proteins DHP1 and DHP2. However, the first disulphide bond of DNase I is not conserved in these proteins.</p><p>This entry represents DNaseI and related proteins such as DNase gamma.</p>