FIGURE 6.

Quiescent fibroblasts incorporate less labeled acetyl. Fibroblasts were cultured in [¹³C]glucose medium for 9 h and harvested while proliferating or after contact inhibition-induced quiescence. The relative abundance of isotopes for the H3 peptide K₉STGGK₁₄APR where Lys⁹ or Lys¹⁴ is acetylated is shown for proliferating (A) and quiescent (B) fibroblasts. The relative abundance of isotopes for the H4 peptide GRGK₅GGK₈GLGK₁₂GGAK₁₆R in which one lysine is acetylated is shown for proliferating (C) and quiescent (D) fibroblasts. E, the fraction of labeled acetyl groups after 9 h of incubation in [¹³C]glucose is plotted for several acetylations for proliferating and quiescent fibroblasts. F, the steady-state relative abundances (1 = 100%) for the H4 peptide (residues 4–17) when unmodified (unmod), singly acetylated, or doubly acetylated are plotted for proliferating and quiescent fibroblasts. Three independent experiments were performed, and the error bars represent S.E. A–D represent [M + 2H]⁺² spectra.