The metabolism of 4-aminobiphenyl in rat. II. Reaction of N-hydroxy-4-aminobiphenyl with rat blood in vitro

Abstract

1. N-Hydroxy-4-aminobiphenyl (N-hydroxy-ABP) reacts with HbFe2+ of rat blood in vitro at a molar ratio of 1:47 to produce 20% HbFe3+ within 1 min; N-hydroxy-ABP oxidized 9.4 equiv. of HbFe2+. N-hydroxy-ABP rapidly disappeared and HbFe3+ was reduced at a rate of 44 microM/min. 2. On titration of rat blood in vitro with N-hydroxy-ABP up to 0.81 mM, 4-nitrosobiphenyl (nitroso-BP) disappeared within 5 min; with concn of N-hydroxy-ABP greater than 0.81 mM, N-hydroxy-ABP was present also as nitroso-BP, indicating saturation of reactive binding sites. When N-hydroxy-ABP reacted with HbFe2+ at a molar ratio of 1:103 to 1:1.9, 13 to 1.3 equiv. of HbFe3+ were formed per mol of N-hydroxy-ABP in 5 min, indicating that with increasing N-hydroxy-ABP concn side-reactions increased. 3. After incubation of N-hydroxy-ABP (1.72 mM) with rat Hb (7.66 mM HbFe2+), nitroso-BP disappeared with a half-life of 1 min, maximal HbFe3+ of 72% occurred at 47 min, and the concn of 4-aminobiphenyl (ABP) increased at a rate of 51 nmol/ml per h. 4. In rats injected with 0.24 mmol/kg ABP, HbFe3+ concn plateaued at 56% after 75 min, indicating an equilibrium between HbFe3+ formation and HbFe3+ reduction. Such equilibrium was simulated by titrating rat blood in vitro with N-hydroxy-ABP for 1 h. 5. The long-lasting HbFe3+ formation by ABP in rat results from a cycle of activation of ABP to N-hydroxy-ABP, its rapid co-oxidation with HbFe2+ to form HbFe3+ and nitroso-BP, and binding of nitroso-BP to erythrocyte thiol groups. ABP is released from the Hb adduct and enters a new cycle of activation and inactivation, until terminated by ring-hydroxylation.

Drug-Induced Methaemoglobinaemia Treatment Issues

“…[56.57J Many aromatic amines, such as aniline and especially 4-arrunobiphenyl are potent methaemoglobin formers, as they are also bioactivated to hydroxylamines. [58] A number of other compounds, including flutamide, [59] phenazopyridine [60] and metoclopramide [61] are thought to be metabolised to anilinelike derivatives and, especially in overdose, may cause substantial methaemoglobinaemia. Aniline itself may be oxidised by erythrocytic haemoglobin, as well as by the Iiver.l 62 ] Exposure to aromatic arrunes will cause methaemoglobin to escalate after the agent has been cleared from the stomach, as compound taken up by the liver will be metabolised and the hydroxylamines released continuously.…”

Quantification of Hemoglobin and White Blood Cell DNA Adducts of the Tobacco Carcinogens 2-Amino-9H-pyrido[2,3-b]indole and 4-Aminobiphenyl Formed in Humans by Nanoflow Liquid Chromatography/Ion Trap Multistage Mass Spectrometry

“…AαC undergoes extensive metabolism by primary human hepatocytes to produce multiple phase I and phase II metabolites, whereas the pathways of metabolism of 4-ABP in human hepatocytes are not well characterized . On the basis of our understanding of 4-ABP metabolism in rodent and canine models, ,− we surmise that the amount of HONH-4-ABP circulating in blood and delivered to the erythrocyte is far greater than that of HONH-AαC, resulting in proportionately higher levels of 4-ABP-Hb adducts. , The N -hydroxylated metabolites of 4-ABP and AαC form adducts with human or rat serum albumin. ,− In the case of 4-ABP, the binding of its N-oxidized metabolite is 250 fold-greater for Hb than that for albumin in the rat model; , however, the comparative levels of HONH-AαC binding to Hb and albumin have not been measured in vivo in rodents or humans. The covalent (or noncovalent) binding of HONH-AαC to drug binding sites of albumin may be favored because of the planar nature of its fused 3-ring system, as opposed to the nonplanar 4-ABP, and impact the relative levels of adduct formation between these two blood proteins. , Further studies on the relative covalent binding of N-oxidized AαC to Hb and albumin are warranted.…”