4-Aminobiphenyl

General mechanism

4-Aminobiphenyl causes DNA damage, which is thought to be mediated by formation of DNA adducts. In this process, 4-aminobiphenyl is oxidized in the liver, giving the N-hydroxy derivative (4-aminobiphenyl-(NHOH)) by an isozyme of cytochrome P450. The final products of this metabolism are aryl nitrenium ions, which form DNA adducts.^[10] During this process reactive oxygen species might also be produced and lead to oxidative DNA damage, which might also play a role in carcinogenesis. (The N-hydroxy derivative causes oxidative DNA damage dramatically enhanced by NADH, which leads to oxidation of 4-aminobiphenyl to a hydronitroxide radical).^[10] A linear correlation was found between adduct levels and the occurrence of liver tumors in female mice by comparing DNA adducts and tumorigenesis.

4-ABP leading to mutation in p53 gene

One mechanism by which 4-ABP causes bladder cancer is a mutation in the p53 gene, which is seen in thirty to sixty percent of bladder cancer cases. The p53 gene codes for the tumor suppressor p53 proteins. A mutation in this gene can lead to formation of tumors. Five p53 hotspots are known for bladder cancer. These are three CpG sites that are common hotspots in several human cancers, which are on codons 175, 248 and 273. The other two codons, 280 and 285, do not have CpG sites. These sites are unique hotspots for mutation in bladder cancer and other urinary tract cancers, in which the chemistry is not yet fully understood.^[11]

Metabolism process in humans

Cytochrome P450 1A2 oxidizes 4-aminobiphenyl to N-hydroxy-4-aminobiphenyl. Following O-acetylation, the latter can form DNA adducts. O-Acetylation reactions are catalyzed by NAT, N-acetyltransferase; and UDP-glucuronosyltransferase (UGT) enzymes.^[12] Two different enzymes can catalyze this reaction, NAT1 and NAT2. These enzymes can also N-acetylate 4-aminobiphenyl. N-Acetylated products are difficult to oxidize, and because of this acetylation is considered a detoxification step for aromatic amines.^{[citation needed]}

Glucuronidation also represents a major metabolic pathway for carcinogenic aromatic amines. A certain human UGT catalyzes the formation of the N-glucuronide of 4-aminobiphenyl. Glucuronidation results in inactivation and excretion, therefore N-glucuronidation also competes with N-oxidation. 4-aminobiphenyl is proposed to initiate bladder cancer by a mechanism involving hepatic N-oxidation and subsequent N-glucuronidation. The N-hydroxy aryl amine N-glucuronide conjugate is thought to be excreted from the liver and to build up in the bladder lumen. N-glucuronides of 4-aminobiphenyl and N-hydroxy-4-aminobiphenyl can be hydrolyzed by acidic urine to their corresponding arylamines, they can in turn enter the bladder epithelium and undergo further metabolism by peroxidation and/or O-acetylation to form DNA adducts.^[12]

Human toxicity

Toxic fumes arise from this compound when heated to decomposition.^[13] Excessive inhalation exposure of 4-aminobiphenyl may induce acute toxicity such as headache, lethargy, cyanosis and burning sensations mainly in the urinary tract.^[14]

4-Aminobiphenyl is a human carcinogen, specifically to the tissues involving the urinary system, i.e., the bladder, ureter, and renal pelvis. In one study, out of 171 workers in a plant manufacturing 4-aminobiphenyl, 11% of them developed bladder tumors.^[13] Tumors appeared on subjects which were exposed to 4-aminobiphenyl in a range of duration from 1.5 to 19 years. The compound can be metabolized by humans which the product may form adducts with DNA in human urothelial mucosa and bladder tumor tissues. Levels of these adducts in smokers of blond and black tobacco were found to be proportional to bladder cancer risk.^[13]

Animal toxicity

The LD₅₀ (dogs, oral) is 25 mg/kg.^[15] The oral LD₅₀ for rats is 500 mg/kg body weight and for rabbits is 690 mg/kg body weight.^[16] Repeated oral administration of a 25% 4-aminobiphenyl solution in olive oil led rabbits to weight loss, anemia, decrease in the number of lymphocytes, increase of granulocytes or the rod neutrophilic granulocyte and to a pronounced hematuria or hemoglobinuria.^[14]