Thermostable Compounds Opposing Toxicity of the Carcinogen 4‐Nitroquinoline N‐Oxide to Flagellates and Bacteria*

SYNOPSIS. In defined media growth inhibition of the flagellates Ochromonas danica and Euglena gracilis , the photosyn thetic bacterium Rhodopseudomonas palustris , and Coryne bacterium bovis was overcome for each organism, for concentrations of 4‐nitroquinoline N ‐oxide (4‐NQO) to ca. 5 μg/ml, by a combination of L‐tryptophan, riboflavin, nicotinic acid, L‐tyrosine, and less sharply, thymine and naphtho quinones. Tryptophan used alone was effective only in rather high concentrations— ca. 0.2 mg/ml for 4‐NQO 2 μg/ml; it was competitive with 4‐NQO through this range. At least for Euglena , gramine and 5‐methyl‐DL‐tryptophan were about as effective. 4‐NQO uptake may therefore be via the trypto phan transport system. 4‐NQO may be a uniquely stable, accessible counterpart of the more polar, more carcinogenic compounds into which mammals oxidatively convert the azodye carcinogens by N‐hydroxylation and the hydrocarbon carcinogens by hydroxylation and ketonization.