Alternative pathways of sterol synthesis in yeast

Yeast produce traces of aberrant sterols by minor alternative pathways, which can become significant when normal metabolism is blocked by inhibitors or mutations. We studied sterols generated in the absence of the Δ8–Δ7 isomerase (Erg2p) or Δ5 desaturase (Erg3p) by incubating three mutant strains of Saccharomyces cerevisiae with 5α-cholest-8-en-3β-ol, 8-dehydrocholesterol (Δ5,8 sterol), or isodehydrocholesterol (Δ6,8 sterol), together with the corresponding 3α-3H isotopomer. Nine different incubations gave altogether 16 sterol metabolites, including seven Δ22E sterols formed by action of the yeast C-22 desaturase (Erg5p). These products were separated by silver-ion high performance liquid chromatography (Ag+-HPLC) and identified by gas chromatography–mass spectrometry, nuclear magnetic resonance spectroscopy, and radio-Ag+-HPLC. When Δ8–Δ7 isomerization was blocked, exogenous Δ8 sterol underwent desaturation to Δ5,8, Δ6,8, and Δ8,14 sterols. Formation of Δ5,8 sterol was strongly favored over Δ6,8 sterol, but both pathways are essentially dormant under normal conditions of sterol synthesis. The Δ5,8 sterol was metabolically almost inert except for Δ22 desaturation, whereas the Δ6,8 sterol was readily converted to Δ5,7, Δ5,7,9(11), and Δ7,9(11) sterols. The combined results indicate aberrant metabolic pathways similar to those in mammalian systems. However, Δ5,7 sterol undergoes only slight isomerization or desaturation in yeast, an observation that accounts for the lower levels of Δ5,8 and Δ5,7,9(11) sterols in wild-type yeast compared to Smith–Lemli–Opitz individuals.