Modification of L1210 Cell Nuclear Proteins by lMethyl-l-nitrosourea and 1-Methyl-3-nitro-l -nitrosoguanidine1

1-Methyl-i -nitmosourea (MNU) and 1-methyl-3-n itro-1-ni trosoguanidine (MNNG) are potent carcinogens which al kylate nucleic acids. Little is known about their reactions with nuclear proteins. Protein binding occurs via two mech anisms for each compound: alkylation (both), carbamoyla tion (MNU), or guanidination (MNNG). MNU and MNNG were obtained with 14Clabels to investigate these reactions. Nuclear proteins were isolated from L1210 cells following one hr of incubation at 37° with the labeled carcinogens in concentrations from 0.038 to 3.8 mM. Separation of the proteins by sodium dodecyl sulfate gel electrophoresis and Triton-acetic acid-urea gel electrophoresis yielded compa rable results. All histones and many nonhistones were modified. At 0.38 mM, binding ranged from the lowest detectable level of 0.1 pmol of [carbony!-14C]MNU per nmol of H4 (carbamoylation) to 13.1 pmol [methyl-14C]MNNG per nmoh of H2A. MNNG is approximately 12 times as potent as is MNU at that concentration and shows more binding than does MNU at all doses for both types of reactions. Hi is more strongly guanidinated than are the other histones (11.0 pmol/nmol at 0.38 mM) whereas H2B and H3 are the major targets of cambamoylation (0.9 and 1.8 pmol/nmol, respectively, at 0.38 mM). Hi and H2A are most strikingly methylated by both drugs. Generally, there is twice as much binding to acid-soluble nonhistones. This study shows that nuclear protein modification occurs with exposure to MNU and MNNG.