1997 Volume 12 Issue supplement Pages 110-111
7α- and 7β-OH-Δ8-THC MALCO activities in mouse liver were significantly enhanced by dexamethasone and phenobarbital. A cytochrome P450, named P450MDX-B, was purified from hepatic microsomes of dexamethasone-pretreated mice. Anti-P45OMDX-B antibody inhibited 7-OH-Δ8-THC MALCO activity in mouse liver. A cDNA clone encoding a P450 isozyme was isolated from a C57BL/6 mouse liver cDNA library. The nucleotide sequence of coding region of this cDNA was identical with that of the known mouse Cyp3al 1 and the deduced amino acid sequence had an N-terminal sequence identical to that of P45OMDX-B. Expression plasmid of Cyp3al1 was transformed into exponentially growing COS-7 cells by the method of electroporation. Immunologically related protein with P45OMDX-B which had the same molecular mass was expressed in the COS cells microsomes (COS-3a11). The COS-3a11 catalyzed the oxidation of 7-OH-Δ8-THC to 7-oxo-Δ8-THC. Oxygen-18 (18O) derived from atmospheric oxygen was incorporated into 7-oxo-Δ8-THC from 7α-OH-Δ8-THC by hepatic microsomes, P45OMDX-B and COS3a11. On the other hand, incorporation of the stable isotope into the oxidized metabolite from 7β-OH-Δ8-THC was negligible. However, when 7β-OH-Δ8-THC was incubated with mouse hepatic microsomes using cumene hydroperoxide instead of NADPH under 18O2, 18O was not incorporated into 7-oxo-Δ8-THC.
These results suggested that 7α-OH-Δ8-THC may be oxidized to the corresponding ketone by Cyp3all via a gemdiol pathway. On the other hand, 7β-OH-Δ8-THC may be converted to the ketone through a stereo selective dehydration of an enzyme-bound gem-diol rather than through a direct hydrogen extraction as a peroxy form of the enzyme.
