Melting calorimetry of rat liver nuclei in the presence of magnesium ions

Abstract

Differential scanning calorimetry was used to determine thermodynamic parameters of decondensation of intranuclear rat liver chromatin was induced by a decrease in the concentration of magnesium ions from 5 mM to 0 mM. The process of chromatin melting in the temperature range of 70-100°C occurs in the following order: melting of core-histones, melting of relaxed DNA, and melting of topologically constrained DNA. It was found that Tm and Д H of individual peaks also depend on the concentration of Mg2+ ions in the buffer. In nuclei with condensed chromatin, Mg2+ ions at a concentration of 5 mM increased significantly the Tm of core histones (by ~7°C), as compared to that in unfolded chromatin but at the same time lowered the Tm of nuclear DNA both in the relaxed and constrained state (by ~2.5°С and ~7.5°С, respectively). In the presence of Mg2+ ions, melting enthalpy for peaks increased significantly. At the same time, a decrease in molecular weights of intranuclear DNA levels out a stabilizing effect of Mg2+ ions on core histones. A rise in the concentration of Mg2+ ions above 5 mM leads to the appearance of a new peak with Tm above 100°С, which probably reflects the thermal behavior of some Mg-induced aggregates. Possible mechanisms underlying thermal behavior of chromatin inside the nucleus are discussed.