Presence and measurement of sample histidine in the ames test: Quantification and possible elimination of a source of false‐positive mutagenicity test results

Both histidine and dipeptides that can be converted to histidine can potentially interfere with the Ames test by increasing the number of spontaneous revertants. Such interference might be especially evident when urine and other biological samples are studied in this assay. We have developed a turbidimetric bioassay that utilizes a nonrevertible Salmonella typhimurium histidine auxotroph, NS1135. The assay is linear with histidine over at least a 300-fold range (0.015-5 micrograms/ml of L-histidine.HCl.H2O). Data indicate that several histidine-containing dipeptides can be utilized as efficiently as free histidine. Our data suggest that this assay may be used to measure biological samples accurately for their histidine content and thereby permit an adjustment for sample histidine during the setup of Ames assays, thus eliminating increased reversion caused by sample histidine.