Principal Polypeptide Target of Carcinogen at the Beginning of Liver Carcinogenesis by Three Carcinogens

A cytosolic 14,000-dalton polypeptide in rats was previously reported to be the principal target protein of a tracer dose of the carcinogen N-2-fluorenylacetamide in normal liver and liver at the start of hepatocarcinogenesis by three types of carcinogens. Short-term ingestion of any of the carcinogens, the aromatic amide N-2-fluorenylacetamide the aminoazo dye 3’-methyl-4-dimethylaminoazobenzene, or the amino acid analog ethionine, causes a marked loss of the ability to make the labeled fluorenyl carcinogen:polypeptide complex and a great reduction in the content of the target polypeptide itself. The present report describes the purification, partial characterization, and species and organ distributions of the target polypeptide. A 3-step procedure was devised for the purification of the target polypeptide from normal rat liver cytosol based on the charge and size of the ¹⁴C-carcinogen:polypeptide complex from normal rats given N-[9-¹⁴C]-2-fluorenylacetamide. The purified target polypeptide was homogeneous according to molecular size and immunoreactivity following sodium dodecyl sulfate:gel electrophoresis. The polypeptide is basic with an isoelectric pH of 8.3. Nevertheless, the polypeptide has 1.6-fold more acidic than basic amino acids, suggestive of the prevalence of amidated glutamic and aspartic residues. The molecule has no detectable tryptophan; one cysteine; two residues of arginine, histidine, tyrosine and proline; and three alanines. An abundance of five residues of methionine and the scarcity of other known potential biological nucleophilic amino acids (zero tryptophan, one cysteine, two tyrosine, two histidine) makes methionine the most likely target of activated carcinogen in the covalent interaction that is apparently involved in the formation of the carcinogen:polypeptide complex. Bound carbohydrate was not detected. Rabbit antisera against the purified target polypeptide reacted specifically with both the polypeptide and its complex with carcinogen. Using sensitive autoradiographic detection of its immunoreactivity, the target polypeptide was found to be present among the liver cytosolic proteins of normal rats, mice, and hamsters of both sexes following sodium dodecyl sulfate:gel electrophoresis, but not in similarly resolved cytosols of their many other organs, indicative that the polypeptide functions mainly (but not exclusively) in liver. Thus far, all of the four known principal target.