Monitoring exposure to 4-aminobiphenyl via blood protein adducts

The feasibility of monitoring exposure to 4-aminobiphenyl (4-ABP) was determined by measuring the formation of covalent blood protein adducts in rats following administration of the amine. A single major adduct was obtained from serum albumin, which was formed from 4-acetylaminobiphenyl and the single tryptophan residue of albumin. The adduct was isolated as part of a tetrapeptide, following Pronase digestion and reverse-phase high-performance liquid chromatography purification, and was identified by amino acid analysis, 1H-nuclear magnetic resonance and mass spectrometry. This adduct was cleared in vivo and showed a half-life of 4.7 days, essentially identical to that of albumin. Similarly, 4-ABP yielded a single major adduct with haemoglobin. This adduct showed a stability in vivo that was comparable to that of haemoglobin, and was shown to accumulate, with chronic dosing, to a level 30 times higher than that resulting from a single dose. Treatment of adducted haemoglobin, with 0.1 N hydrochloric acid in acetone, caused hydrolysis and release of 4-ABP. The formation of the haemoglobin adduct involves 4-nitrosobiphenyl as an intermediate product and, by implication, 4-hydroxyaminobiphenyl. Thus, the haemoglobin adduct is a measure of the fraction of 4-ABP that is N-hydroxylated directly, whereas the albumin adduct reflects that portion which is acetylated prior to N-hydroxylation. Approximately 0.02% of a dose of 4-ABP was bound to the tryptophan in albumin (dose of 2-80 mg/kg) and 5% was bound to haemoglobin as an acid-labile adduct (dose of 0.5-5000 micrograms/kg).