ROLE OF MIXED FUNCTION OXIDASES IN KAURENE METABOLISM IN ECHINOCYSTIS MACROCARPA GREENE ENDOSPERM

Two of the initial reactions in the oxidative conversion of (-)-kaur-16-ene to the gibberellin plant growth hormones have been characterized in tissue homogenates of E. macrocarpa endosperm. The conversion of (-)-kaur-16-ene to (-) kaur-16-en-19-ol requires molecular oxygen and NADPH, and is inhibited by carbon monoxide. The photochemical action spectrum of light reversal of this inhibition shows a maximum at 450 mμ. The hydroxylation of (-)-kaur-16-ene under 18O2 results in a 78% incorporation of 18O into the product (-)-kaur-16-en-19-ol. The conversion of (-)-kaur-16-en-19-aldehyde to (-)-kaur-16-en-19-oic acid also requires oxygen and NADPH and is inhibited by carbon monoxide. The photochemical action spectrum for this reaction shows a maximum reversal at 450 mμ. The reaction of carbon monoxide with reduced hemoproteins of endosperm shows bands in the Soret region at 422 and 453 mμ. The latter peak is thought to be due to a cytochrome, similar to the cytochrome P-450 of mammalian systems, which participates directly in the oxidative reactions discussed above. These findings suggest that the pathways for the biosynthesis of the gibberellin hormones in higher plants and the steroid hormones in animals are analogous in that both involve an initial sequence of nonoxidative steps involving water-soluble substrates and soluble cytoplasmic enzymes followed by a secondary sequence of aerobic, oxidative steps involving hydrophobic substrates and membrane-bound enzymes. © 1969.