Reconstitution of intercalator-induced DNA scission by an active component from nuclear extracts

Treatment with intercalating agents causes formation of protein-associated DNA breaks in mammalian cells in culture and in the nuclei isolated from these cells. We found that this effect, when induced by the intercalator m-AMSA, required a component which could be dissociated from nuclei by 0.3 M NaCl. The effect was restored by combining the extracted nuclei with the nuclear extract. The active component of the extract eluted in gel filtration at a point corresponding to a molecular weight of 800 000. During its reaction with DNA, DNA-protein links and DNA breaks appeared in approximately equal frequencies. In this respect the reaction stimulated by m-AMSA resembled the reaction of a topoisomerase with DNA. However, intercalator-stimulated formation of protein-associated DNA breaks differed from the activity of the nuclear topoisomerase I in that there was a different optimum salt concentration and a different apparent molecular weight.

Interaction of serum lipoproteins with the intestine. Evidence for specific high density lipoprotein-binding sites on isolated rat intestinal mucosal cells

To determine if plasma lipoproteins interact and therefore possibly regulate intestinal lipoprotein metabolism, we investigated the binding, internalization, and degradation of 125I-labeled low density lipoprotein (LDL) and high density lipoprotein (HDL) by enzyme-dispersed rat intestinal mucosal cells. Both human and rat LDL and HDL were bound, internalized, and degraded in a concentration-dependent manner with calculated half-saturation occurring at approximately 30, 35, 35, and 15 micrograms/ml for human LDL, rat LDL, human HDL, and rat HDL, respectively. Isolated brush border membranes had no saturable or specific binding sites for 125I-labeled HDL or LDL, suggesting that lipoproteins may be bound to receptors on lateral or basal membranes of mucosal cells. Compared with HDL, LDL binding was characterized by a large non-specific component. LDL of human and the rat were not only displaced by excess LDL but at least as effectively by excess HDL of their own species. Labeled HDL was displaced by corresponding unlabeled lipoproteins, but human LDL could produce only minor displacement of human HDL3. ApoE-deficient rat HDL, separated by heparin-Sepharose affinity chromatography also showed highly specific saturable binding to intestinal cells. Thus, apparently two different lipoprotein binding sites exist in intestinal plasma membranes, one recognizing B and/or E apoproteins present in human and rat LDL and rat HDL while another binds human HDL3 and apoE-deficient rat HDL which contain A apoproteins as major components.

Topoisomerase I activities in L-1210 cell nuclei

Topoisomerase was extracted from L-1210 nuclei by 0.35 M sodium chloride Extract fractionation by molecular weight revealed two activities. The activities were quantitatively determined using a technique based on the electrophoretic separation of 3H-labelled DNA product from substrate. This technique enables the determination of topoisomerase activity in extracts where nuclease activity is also present.

Specific, saturable binding and uptake of rat chylomicron remnants by rat skin fibroblasts

To investigate the possible contribution of chylomicron remnants to the accumulation of cholesterol in non-hepatic tissues, rat chylomicron remnants were incubated with rat skin fibroblasts. The binding of remnants was saturable and specific. Native, undegraded chylomicrons were almost as effective as unlabeled remnants in displacing the uptake of labeled remnants. Rat low density and high density lipoproteins were relatively ineffective in displacing the uptake of labeled remnants. Accumulation of radioactive unesterified fatty acids occurred in proportion to the uptake of labeled remnants, indicating probable internalization and degradation of the particles after binding. The incorporation of added [14C]acetate into cell non-saponifiable lipids was not significantly suppressed by added remnants, indicating an apparent lack of feedback regulation of cholesterol biosynthesis after remnant uptake. Our results show that the physiological mechanism underlying uptake of remnants by hepatic parenchymal cells might not be accounted for by tissue or cellular specificity, but may perhaps arise because of the lower capillary permeability of extra-hepatic sites compared with the hepatic sinusoid.