The identification of distinct populations of acetylated histone.

We have utilized sodium butyrate to inhibit histone deacetylases in order to study the rates of histone acetylation in hepatoma tissue culture cells. In this manner, we have been able to observe two rates of hypermodification of acetylated core histone. By selectively radolabeling acetylated histone fractions based upon differences in their acetate exchange rates, we have identified the rate of histone acetate hydrolysis and the rate of hyperacetylation in 50 mM sodium butyrate for two distinct populations of acetylated histone. One population, comprising no more than 15% of each of the non-H1 histones, is characterized by rapid hyperacetylation (t 1/2 congruent to 7 min for monoacetylated H4) and the rapid (t 1/2 congruent to 3 to 7 min) removal of this modification. A second population is deacetylated with t 1/2 congruent to 30 min and is hypermodified much less vigorously in 50 mM sodium butyrate (t 1/2 congruent 200 to 300 min for monoacetylated H4). Unlike the rapidly metabolized group, the fraction of total histone in this slow population varies between the four core histones. In addition, there appears to be no interconversion of histone between these populations.