Abstract
The effect of a chemical carcinogen, 4‐nitroquinoline 1‐oxide (4NQO), on two mammalian cell strains of tissue culture was investigated with special reference to its binding to macromolecular constituents of cells by the use of cell strains L.P3, a substrain of L‐929 mouse fibroblasts, and JTC‐25.P3, a substrain of rat liver cells transformed by Nagisa culture. Both substrains have been cultivated in a protein‐free synthetic medium. Population‐dependent inhibition of growth by 4NQO was detected. This was due to the population‐dependent incorporation of the carcinogen into cells, i.e. the lower the population density in a culture vessel, the higher was the amount of carcinogen incorporated into cells, resulting in the amplification of the inhibitory effect. On addition to culture medium, 4NQO‐^3^H was readily incorporated into the cold acid‐soluble pool of cells attaining the maximum within 30 min. This labelling value, however, gradually decreased during a few hours, more than 90% being metabolized and excreted into the medium. On the other hand, the binding of 4NQO‐^3^H to the cold acid‐insoluble macro‐molecular fraction rapidly reached the maximum within 30 min and was kept unreleased for several hours. On removal of the carcinogen from the medium, the value of the bound label decreased with a half‐life time of about 5 h. In binding to cellular proteins the carcinogen was found to have bound uniformly to every fraction of various soluble proteins but it was released from each fraction with a half‐life of approximately 5 h, as revealed by the fractionation on Sephadex G‐100 columns. Three successive additions of 4NQO‐^3^H to the medium brought about the binding of the carcinogen to tRNA, DNA and rRNA, molar ratios of the carcinogen to 10,000 nucleotides of each nucleic acid fraction being 2.6, 3.3, and 5.3, respectively. In 24 h of recovery incubation, specific activities in these nucleic acid fractions decreased to 27.4%, 39.1% and 22.0%, respectively.