Age changes of the isoelectric points of non-histone chromosomal proteins from rat liver in the pH range 5 to 8.

Non-histone chromosomal proteins were extracted from purified nuclei of young (2-3 months) and old (24-28 months) rat liver by a two-phase partition method (chloroform:-isoamyl alcohol, 24:1) after histone depletion with 0.25N HCl. The proteins were subjected to analytical isoelectric focusing in the pH range 5-8. On the densitograms derived from Coomasie brilliant blue R-250 stained gels, there were formally four areas separated having the following pH ranges: I) 5.80-6.10; II) 6.25-6.42; III) 6.80-7.00; IV) 7.20-7.35 for young rats, and I) absent; II) 6.35-6.75; III) 6.80-7.00; IV) 7.12-7.40 for old rats. Hence, the main result is the absence of a group of proteins from old rats having the pH range 5.80-6.10. The factors involved in protein separation by isoelectric focusing, like time-dependent post-translational charge modifications: phosphorylation, acetylation, ADP-ribosylation, deamidation, terminal peptide cleavage, glycosylation, and DNA-like contaminants are discussed.