Immunochemical analysis of a cytochrome P-450IA1 homologue in human lung microsomes

The monoclonal antibody MAb 1-7-1, which specifically binds to cytochromes P-450IA1 and P-450IA2 in 3-methylcholanthrene-induced rat liver microsomes, was used to identify a cytochrome P-450IA1 homologue in human lung microsomes. Although MAb 1-7-1 had similar affinity constants for human and rat microsomes, the amount bound to human lung microsomes was severalfold lower than that bound to microsomes from untreated rat or rabbit lung and much lower than the amount bound to 3-methylcholanthrene-induced rat lung or liver microsomes. The amount bound to untreated baboon lung microsomes was similar to that bound to human lung microsomes. Three cytochrome P-450IA1-catalyzed activities, 7-ethoxyresorufin O-deethylase, 7-ethoxycoumarin, O-deethylase, and aryl hydrocarbon hydroxylase, were measurable in human lung microsomes, but the cytochrome P-450IA2-dependent activity acetanilide 4-hydroxylase was not. MAb 1-7-1 inhibited, and its binding correlated strongly with, 7-ethoxyresorufin O-deethylase activity (r = 0.92, p less than 0.01) in human lung microsomes. 7-Ethoxyresorufin O-deethylase activities in human lung were similar to those measured in untreated baboon lung but considerably lower than those present in untreated rabbit lung, untreated or 3-methylcholanthrene-induced rat lung and liver, or human liver. We conclude that MAb 1-7-1 recognizes a cytochrome P-450IA1 homologue in human lung and that no cytochrome P-450IA2 homologue is detected. Cytochrome P-450IA1 is expressed in human lung at relatively low levels, similar to those observed in untreated primate (baboon) lung. The majority of the 19 human lung samples examined do not exhibit a permanent polycyclic aromatic hydrocarbon-induced state with respect to this isozyme.