Figure 1.

Cholesterol oxidase reaction and proposed pathway for cholesterol oxidase-dependent sterol metabolism in transgenic tobacco plants. Oxidation and isomerization steps are catalyzed directly by cholesterol oxidase. Reduction of the 3-ketosteroids to stanols occurs by an uncharacterized endogenous plant pathway. The aliphatic groups in the side box show the side chain structures of the four corresponding 3-hydroxysterols detected in this study. Stanol derivatives with side chain structures corresponding to cholesterol (cholestanol), sitosterol (stigmastanol), and campesterol (campestanol) were observed in transgenic tobacco expressing cholesterol oxidase. No steroid was observed having both the unsaturated stigmasterol side chain and a saturated ring nucleus. Complete reduction of stigmasterol leads to a structure with a side chain identical to that found in sitosterol and stigmastanol.