The influence of chromatin compactness on the stoichiometry of the Feulgen-Schiff procedure studied in model films. II. Investigations on films containing condensed or swollen chicken erythrocyte nuclei.

As models for different states of chromatin compactness, nuclei from chicken erythrocytes were isolated and either osmotically swollen or kept as condensed as possible. Both types of nuclei were then fixed and incorporated into polyacrylamide films. Hydrolysis with 5 N HCl and staining with Schiff's reagent of these model films were studied using several parameters. The phosphate content of the films was analyzed as a parameter for the depolymerization losses and the staining with Schiff's reagent as a parameter for the apurinic acid (APA) content. The loss of ultraviolet absorbance from the films and the accumulation of ultraviolet absorbing substances in the hydrolyzing acid were monitored as parameters for the progress of hydrolysis. Conversion of the generated aldehyde groups to APA-Schiff chromophore is shown to take place with the same stoichiometry for both types of nuclei as well as for DNA in model films. It is further shown that the nuclei- and DNA-films are suitable models for investigating the influence of chromatin compactness on the course of the Feulgen-Schiff reaction. For the most compact form of chromatin studied, a very high reduction in staining intensity of up to 40% could be demonstrated after certain normally applied hydrolysis times. This is due primarily to a decrease with a factor of 2.3 of the depurination rate constants of these models (from 0.030/min to 0.013/min). Therefore prolonged hydrolysis periods are required to obtain the same APA concentrations, but then depolymerization processes cause losses of nuclear material. The differences in depurination rates could be explained by a decrease in [H3O]+ in the neighborhood of the purine-sugar linkages, caused by the presence of fixed positive charges form the protein components of the chromatin. These findings may explain the cytophotometrically determined differences in chromophore yield of 10-20% found in the nuclei of cells with different states of compactness of their chromatin. The descending part of the Feulgen hydrolysis curve represents the depolymerization of APA and loss by diffusion of the reaction products. In the Appendix, cytophotometric data of cells have been analyzed to show that this part of the hydrolysis curve may be used to estimate the acid stability of chromatin complexes. The depurination and depolymerization rates found closely correspond with the data obtained from the model films.