A nuclease from rat-liver nuclei with endo- and exonucleolytic activity

Generation of 8-hydroxydeoxyguanosine from DNA using rat liver homogenates

In relation to carcinogenesis, aging and other pathologic conditions, urinary 8-hydroxydeoxyguanosine (8OHdG) is widely used as a marker for evaluating the effect of oxidative stress on DNA. Because no reports have described how 8OHdG is generated from DNA in vivo or by biological materials, and how it is excreted into urine, the authors investigated the generation of 8OHdG from DNA, using rat liver homogenate. Oxidatively damaged DNA samples containing different levels of 8OHdG were prepared using ultraviolet irradiation with three different concentrations of riboflavin. Following incubation of damaged DNA samples with rat liver homogenates, the generation of 8OHdG from the DNA was determined using high-performance liquid chromatography with electrochemical detection after ultrafiltration of the incubation mixtures. The generation of 8OHdG was also tested with an anti-8OHdG antibody. The quantity of 8OHdG generated from the DNA by rat liver homogenates was dependent on the 8OHdG levels in the DNA: almost all 8OHdG in the DNA was released as 8OHdG by rat liver homogenates. Generation of 8OHdG correlated with the degradation of DNA. Interestingly, the generated 8OHdG was stable in the presence of rat liver homogenates, whereas deoxyguanosine (dG) rapidly disappeared in the same conditions. Less than 1/10,000 of dG was converted to 8OHdG when dG was incubated with rat liver homogenate. Incubation of 8-hydroxyguanine with rat liver homogenates did not generate 8OHdG. These findings suggest that most of the 8OHdG in DNA is released as 8OHdG during DNA degradation and that, because of its stability, 8OHdG is excreted into urine, thus providing a convenient measure of oxidative damage to DNA.

Endo-exonuclease of human leukaemic cells: evidence for a role in apoptosis

Inactive forms of endo-exonuclease, activated in vitro by treatment with trypsin, have been identified in human leukaemic CEM and MOLT-4 cells. They comprise over 95% of the total single-strand DNase activity in nuclei and are mainly bound to chromatin and the nuclear matrix. The activated enzyme had Mg2+(Mn2+)-dependent, Ca(2+)-stimulated activities with single- and double-strand DNAs and RNA (polyriboadenylic acid) and other properties characteristic of endo-exonucleases previously described. At least twice as much inactive endo-exonuclease has also been localised in extranuclear compartments of CEM and MOLT-4 cells, 85% bound to the membranes of the endoplasmic reticulum and 15% free in the cytosol. The soluble cytosolic trypsin-activatable endo-exonuclease was immunoprecipitated by antibodies raised independently to both Neurospora and monkey CV-1 cell endo-exonucleases. The free and bound enzymes of both nuclear and extranuclear compartments also cross-reacted on immunoblots with the antibody raised to Neurospora endo-exonuclease to reveal multiple polypeptides ranging in size from 18 to 145 kDa, many of which exhibited activity on DNA gels. The major species bound to the chromatin/matrix were in the 55–63 kDa range. Limited proteolysis of the large polypeptides to those of 18 to 46 kDa accompanied spontaneous chromatin DNA fragmentation to form DNA “ladders' in an isolated nuclei/cytosol system. When the leukaemic cells were treated in culture with either etoposide or podophyllotoxin to induce apoptosis, the largest polypeptides disappeared and smaller endo-exonuclease-related polypeptides of 18 to 46 kDa were detected in the nuclear extracts. The appearance of these polypeptides also correlated with extensive chromatin DNA fragmentation. In addition, there were correlations between the depletion of the major 55–63 kDa species bound to the membranes of the endoplasmic reticulum, depletion of the extranuclear trypsin-activatable activity and the onset and extent of chromatin DNA fragmentation in both cell lines. The extranuclear 55–63 kDa species may be precursors of the chromatin/matrix bound endo-exonuclease. The results indicate that endo-exonuclease plays a role in chromatin DNA degradation in mammalian cells during apoptosis.

Apoptosis and hepatobiliary disease

In this review we have attempted to summarize recent information on the role of apoptosis in normal liver biology and as a pathophysiological mechanism of cell death during hepatobiliary disease. Apoptosis is an important mechanism of cell injury and death in pathogenic settings as diverse as cell-mediated cytotoxicity, drug and chemical toxicity, carcinogenesis, viral disease, and autoimmune disease. Several factors account for the widespread attention and flurry of investigative activity into the phenomenon of apoptosis including the following: (1) realization that apoptosis is an important component of the behavior of all cells; (2) increasing evidence supporting a role for apoptotic cell death in diverse pathological settings; (3) elucidation of the biochemical pathways and insight into the genetic and molecular regulation of the process; and (4) development of new techniques to identify cells undergoing apoptosis. Recognition of the importance and role of apoptosis in hepatobiliary disease should stimulate interest and promote basic research toward understanding the mechanisms involved. Subsequent investigation into defining the cellular regulatory mechanisms and biochemical processes involved will be fertile ground for future research that may ultimately lead to novel therapeutic approaches for autoimmune diseases, malignancies, and viral disease.

Effect of hemorrhagic shock duration on the activity of nuclear Ca2+/Mg(2+)-dependent DNA-endonucleases in canine cerebral cortex

A direct correlation between the duration of arterial hypotension (40 mmHg) and the activity value of nuclear Ca2+/Mg(2+)-dependent DNA-endonucleases was revealed in dogs exposed to hemorrhagic shock. Thus, an increase in the activity of endonucleases was significant after 2 h of arterial hypotension. During that period there activity amounted to 4149 +/- 295 units/mg of protein instead of 2839 +/- 231 units/mg protein in the control (P < 0.01). With the prolonged hemorrhagic shock, the activity of DNA-endonucleases continued to increase. By 4 h of hemorrhagic shock it reached 5021 +/- 443 units/mg protein and by 6 h, 8661 +/- 338 units/mg protein which was three times the control values (P < 0.01). An electron-microscopic study of the structure of neuron nuclei karioplasm showed that by 2-4 h of hemorrhagic shock some neurons showed reduced chromatin content, and the nuclear matrix had clarified with the formation of electron-translucent empty zones. We think that the sharp increase in the nuclear endonuclease activity and the subsequent endonucleolysis can be regarded as a pathogenetic factor in the destructive processes taking place in the nuclei of cortex neurons during prolonged periods of hemorrhagic shock.