Microspectrophotometric estimates of non-histone proteins in cell nuclei displaying differing degrees of chromatin condensation.

Nuclei isolated from mouse thymus, kidney, and liver were fixed in ethanol-acetic acetic acid; treated with dilute acid to extract histones; stained by three protein end-group procedures; and measured by scanning, integrating microspectrophotometry. Measurements were also made of nuclei isolated from the same organs and stained by the Feulgen procedure for DNA. Protein end-group procedures included pH 2.8 Biebrich scarlet (for basic groups), mercury orange (for sulfhydryl groups), and mercury orange after thioglycolate reduction (for the sum of sulfhydryl and disulfide groups). With the exception of the comparison between Feulgen-stained 2C liver and kidney nuclei, the integrated extinction values obtained for nuclei of a given organ differed significantly from the measurements of nuclei from other organs, regardless of the staining procedure. Furthermore, the integrated extinction values for 2C nuclei were highest in larger, more vesicular nuclei (from liver and kidney) and lowest in condensed thymocyte nuclei, except in the case of measurements of the disulfide content of the nuclei. In this instance, the values of integrated extinction were highest in condensed thymocyte nuclei, intermediate in kidney nuclei, and lowest in 2C liver nuclei. When 2C, 4C, and 8C liver nuclei were compared, the integrated extinction values of 4C nuclei were found to be approximately twice those of 2C nuclei whose disulfide and Feulgen values were, respectively, higher and lower than expected. The greater disulfide values and reduced Feulgen values obtained in thymocyte and 8C liver nuclei might be related to a greater degree of chromatin condensation in these nuclei, and therefore, to a reduction or selective restriction of their RNA transcriptional capacities.