UPTAKE AND RELEASE OF 3H‐MITOMYCIN C IN MOUSE P‐388 CELL CULTURES

Mouse P-388 cells were grown in suspension culture and exposed to 3H-labelled mitomycin C for 1 hour. With concentrations from 0.5 to 50 μg of 3H-mitomycin C per ml cell culture, about 0.01 per cent of the radioactivity added was found in washed cells. Most of the intracellular antibiotic was present as cold acid soluble material, but significant amounts of tritium were found in the nucleic acid- and protein fractions. The ratio of 3H per weight unit was of the same order of magnitude for DNA and RNA, but substantially lower for cell protein. Double labelling experiments using 14C-thymidine and 3H-mitomycin C were carried out to study the specific loss of antibiotic from the cells during post-treatment incubation. Cold TCA soluble 3H-mitomycin C was rapidly released from the cells during the first hours after they had been transferred to non-radioactive growth medium. Loss of 3H-radioactivity from the RNA- and protein fractions roughly paralleled the loss of 14C radioactivity from the DNA fraction over a period of 3 days. The content of 3H-radioactivity in the DNA containing cell fraction increased during the first day of post-treatment incubation, followed by a substantial drop during the second day. This drop was far more extensive than the corresponding loss of 14C radioactivity. During the third day after exposure to mitomycin C the drops in 3H and 14C in the DNA fraction were not significantly different.