Plant sterol biosynthesis: 7-oxo-obtusifoliol analogues as potential selective inhibitors of cytochrome P-450 dependent obtusifoliol 14 alpha-demethylase.

A series of 7-oxo-obtusifoliol analogues have been synthetized and investigated as potential inhibitors of cytochrome P-450 dependent obtusifoliol 14 alpha-demethylase (P-450OBT.14DM) from higher plant microsomes. 7-Oxo-24 xi(24')-dihydro-obtusifoliol and 7-oxo-24(25)-dihydro-29-nor-lanosterol were potent competitive inhibitors for P-450OBT.14DM, binding 125-200 times more tightly than the substrates obtusifoliol and 24(25)-dihydro-29-nor-lanosterol. Inhibition of P-450OBT.14DM by these analogues showed strict structural requirements including the 8-en-7-one system which was compulsory for binding. 7-Oxo-24(25)-dihydro-lanosterol possessing an additional 4 beta-methyl substituent, did not have such inhibitory effects. Treatment of cultures of suspended bramble cells with 7-oxo-24(25)-dihydro-29-nor-lanosterol resulted in a strong decrease of [14C]acetate incorporation into the demethylsterols fraction and in an accumulation of [14C]obtusifoliol. This confirms that P-450OBT.14DM is the main in vivo target of 7-oxo-24(25)-dihydro-29-nor-lanosterol in the sterol-biosynthetic pathway.