Studies on the Specificity and Site of Action of alpha-Cyclopropyl-alpha-[p-methoxyphenyl]-5-pyrimidine Methyl Alcohol (Ancymidol), a Plant Growth Regulator

α-Cyclopropyl-α-p-methoxyphenyl]-5-pyrimidine methyl alcohol (ancymidol) is an inhibitor of ent-kaur-16-ene oxidation in microsomal preparations from the liquid endosperm of immature Marah macrocarpus seeds. The K_i for this inhibitor is about 2 × 10⁻⁹ m. Ancymidol also blocks ent-kaur-16-en-19-ol and ent-kaur-16-en-19-al oxidation by the same preparations with a similar efficiency, but does not significantly inhibit ent-kaur-16-en-19-oic acid oxidation. Ancymidol appears to be specific for this series of oxidations in higher plant tissues. It does not inhibit the oxidation of kaurene nor kaurenoic acid in rat liver microsomes and has no significant effect on the oxidation of cinnamic acid in microsomal preparations from Sorghum bicolor seedlings. Ancymidol also does not inhibit kaurene oxidation in vitro nor in vivo in cultures of the fungus Fusarium moniliforme. The presence of ancymidol did not significantly alter the activities of NADPH-cytochrome c reductase, NADH-cytochrome c reductase, or NADH-cytochrome b₅ reductase. The addition of ancymidol to suspensions of oxidized M. macrocarpus endosperm led to a difference spectrum with an absorption maximum at 427 nm and a minimum at 410 nm.