An endonuclease was extracted from intact rat liver mitochondria with 0.4 M NaCl, and partially purified. A zymographic assay in SDS-polyacrylamide gel containing single-stranded DNA revealed that the enzyme has an apparent molecular mass of 55 kDa. It was different from the molecular mass of the major endonuclease of bovine heart mitochondria (a homodimer of a 29-kDa peptide), that was recently shown to be identical to the endonuclease G. The purified 55-kDa enzyme degraded both DNA and RNA, preferring RNA and single-stranded DNA at a weak alkaline pH, required Mg(2+) and Mn(2+) but not Ca(2+) for activity, and was strongly inhibited by monovalent cations. Nicks generated by the enzyme were resealable with T4 DNA ligase, indicating that the enzyme produces 5'-p and 3'-OH ends. The 55-kDa enzyme, like endonuclease G, displayed a strong preference to nick within a (dG)n.(dC)n sequence tract.
