AFLATOXIN B-1 ACTIVATION IN HUMAN LUNG

Inhalation exposure to the carcinogen aflatoxin B-1 (AFB(1)) in certai n occupations is considerable. Because circumstantial epidemiological evidence suggests that AFB(1) inhalation may cause primary lung cancer , we investigated AFB(1) activation by human lung microsomes. Microsom es were incubated with [3]-AFB(1) (124 mu M), and activation to the AF B(1)-8,9-epoxide was measured as the AFB(1)-glutathione (AFB(1)-GSH) c onjugate by HPLC. The formation of AFB(1)-GSH was in the range of 0.05 -0.073 fmol/mg protein/min. The role of cytochrome P450 (CYP) 3A in th is activation was investigated by oxidation of nifedipine (a prototype substrate far CYP 3A), by immunoinhibition, and by immunoblot analysi s, Nifedipine oxidation varied from 0.2 to 19.2 pmol/mg protein/min in microsomes from different subjects, but did not correlate with AFB, a ctivation, Anti-human polyclonal CYP 3A4 IgG inhibited AFB(1) activati on. CYP 3A isoforms were immunoestimated to be in the range of 0.01-1. 90 pmol/mg protein. Neither CYP 1A2 nor associated activity was detect ed in the lung microsomes. These data indicate that human lung microso mes activate AFB(1) to form the exo-AFBt-8,9-epoxide and that CYP(s) o f the 3A subfamily may be responsible for this activity. The relativel y low amount of AFB, activation in human lung compared to that in huma n liver can be explained by the scarcity of CYP-containing cells in th e lung. In situ AFB(1) activation and resultant carcinogenic risk are distinctly possible in occupational settings where inhalation of AFB(1 )-contaminated dusts occurs. (C) 1997 Academic Press.