Ca/Mg-dependent endonuclease from porcine liver. Purification, properties, and sequence specificity.

A rapid and reproducible method for the purification of the Ca/Mg-endonuclease from porcine and rat liver and for the stabilization of the enzyme activity is presented. The optimum conditions for enzyme activity were determined. The enzyme degrades double-stranded DNA endonucleolytically. In the course of digestion of form I closed circular SV 40 DNA, the form II nicked circular DNA is the prominent intermediate product. Digestion of hen erythrocyte nuclei with added endonuclease produces a ladder of mono- and oligonucleosomal fragments similar to that generated by micrococcal nuclease digestion. Determination of the 5'-terminal nucleotides indicates the absence of a significant base specificity. Analyzing the cleavage pattern of end-labeled pBR322 restriction fragments on sequencing gels shows that the enzyme exhibits a weak preference for dinucleotides with A in the 5'-position; dinucleotides with 5%-C are less readily cleaved. Digestion of end-labeled pBR322 DNA, followed by electrophoresis in agarose gels produces a "smear"-like fragmentation pattern with weak superimposed bands, while micrococcal nuclease generates a different and much more distinct pattern. These data show that the sequence specificity of the enzyme is less pronounced than that of micrococcal nuclease and that the sites preferentially cleaved are not the same.