Initial velocity and product inhibition kinetics of the histone acetyltransferase (EC 126.96.36.199) reaction indicate that the rat liver nuclear enzyme operates under a rapid equilibrium ordered bireactant mechanism. Histone adds first to the enzyme, and under the conditions of the experiment Ka = 0 as acetyl coenzyme A. (CoA) concentration approaches saturating conditions. The Km for acetyl-CoA was 2.10 ± 0.48 µM. Inhibition with acetyllysine resulted in a Ka for the enzyme-acetyllysine complex of 1.96 ± 0.30 mM. Inhibition with CoA yielded Kiq for the ternary complex of 3.19 ± 0.48 µM. These results indicate that the enzyme activity is comparatively independent of histone concentration, and, since the enzyme is sensitive only to acetyl-CoA and CoA concentrations, the enzyme will tend to maintain histones in the acetylated state.
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