Abstract
Chloroplasts, mitochondria and microsomes were isolated from a cell-free extract of green and etiolated leaves of bean, by differential and sucrose or ficoll gradient centrifugation. The sterols and sterol esters of whole leaves and subcellular fractions were compared. The following sterols were identified from the leaves: cholesterol (cholest-5-en-3β-ol), Δ 7-cholestenol (cholest-7-en-3β-ol), campesterol ( cholest-5-en-(24R)-24- methyl-3β-ol ), stigmasterol ( stigmast-5,Z-22- dien-3β-ol ), sitosterol (stigmast-5-en-3β-ol) and isofucosterol ( stigmast-5,Z-24(28)- dien-3β-ol ). These sterols are present in all subcellular fractions but their concentrations differ significantly; in particular the chloroplasts and the mitochondria are much richer in cholesterol (24%) compared to the leaves (1%). The amount of sterols per mg of proteins in the fractions varied as follows: chloroplasts < mitochondria < endoplasmic reticulum. Sterol esters were generally enriched in cholesterol (25–75%) and poor in stigmasterol (10%). These results are discussed in terms of the role of the sterols in the structure and function of plant cell membranes.
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