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J. Biol. Chem., Vol. 265, Issue 15, 8484-8489, May, 1990
F Echevarria, RA Norton, WD Nes and Y Lange
Zymosterol (5 alpha-cholesta-8(9),24-dien-3 beta-ol) comprised a negligible
fraction of the mass of sterol in cultured human fibroblasts but was well
labeled biosynthetically with radioactive acetate. Treatment of cells with
triparanol, a potent inhibitor of sterol delta 24-reductase, led to a
marked increase in labeled zymosterol while its mass rose to 1 mol% of
total sterol. All of this sterol could be chased into cholesterol.
Furthermore, cell homogenates converted exogenous radiolabeled zymosterol
to cholesterol. Three lines of evidence suggested that biosynthetically
labeled zymosterol was associated with the plasma membrane. 1) About 80% of
radiolabeled zymosterol was oxidized by the impermeant enzyme, cholesterol
oxidase, in glutaraldehyde-fixed intact cells. 2) Sucrose density gradient
analysis of homogenates showed that the equilibrium buoyant density profile
of newly synthesized zymosterol was identical with that of the plasma
membrane. 3) Newly synthesized zymosterol was transferred as readily from
fixed intact fibroblasts to exogenous acceptors as was cholesterol. Given
that cholesterol is synthesized within the cell, it is unclear why most of
the zymosterol is in the plasma membrane. The pathway of cholesterol
biosynthesis may compel zymosterol to flux through the plasma membrane.
Alternatively, plasma membrane zymosterol may represent a separate pool, in
equilibrium with the zymosterol in the intracellular biosynthetic pool.
Zymosterol is located in the plasma membrane of cultured human fibroblasts
Department of Pathology, Rush Medical College, Chicago, Illinois 60612.
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