The nucleic acids, nucleotide and protein levels in rat bone marrow were followed after the administration of the aromatic nitrogen mustard Merophan alone and with cysteine pre-treatment. The DNA dose response curve was similar to the mortality curve, which suggests that the DNA concentration of the bone marrow can be used as a sensitive index of toxicity. Based on this index the dose reduction factor for bone marrow toxicity given by cysteine pre-treatment agreed with that for whole body protection. A direct relationship was found between the rise in free SH in the bone marrow after administration of various doses of cysteine and the protection afforded to this tissue. The mechanism of the protective action thus appears to be that of an intracellular reaction between a thiol and the nitrogen mustard.
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