Comparative utilization of sterols by the Oomycetes Lagenidium giganteum and Lagenidium callinectes

The mosquito-parasitizing Oomycete Lagenidium giganteum requires exogenous sterols before it can produce the infective zoospores. In contrast, L. callinectes will synthesize small quantities of cholesterol and produce zoospores. Since the former is a promising biological control agent it was important to delineate the metabolic role(s) and fate(s) of these sterols in zoosporogenesis. Twenty-three different sterols were administered to growing cultures of both organisms. These compounds were reisolated after 7 days, along with any metabolites produced from them. This approach was designed so as to lead to the outline of some of the metabolic sequences which were present. The data show that both organisms possess very similar patterns of sterol transformation. The apparent metabolic route leading to cholesterol is cycloartenol →→ fucosterol →→ cholesterol, which is similar to the overall metabolic sequence observed in many algae. Some transformations that were indicative of obvious intermediate biosynthetic steps were apparent. Both Lagenidium spp. transformed 24-alkylidene but not 24-alkyl sterols to cholesterol. The organisms reduced cholesta-5,7-dienol to cholesterol, metabolized cholestanol to cholesterol, and transformed 3-ketosteroids to 3β-hydroxysterols. Coprostanol was changed to cholestanol.