Sterol distribution in intracellular organelles isolated from tobacco leaves.

All membrane-containing fractions isolated from tobacco leaves contained free sterols, sterol glycosides, and sterol esters. The three sterol forms increased, on a dry weight basis, with a decrease in particle size. The supernatant fraction contained only trace amounts of sterol. The major sterols in all cellular fractions, in the order of decreasing amounts, were: stigmasterol, beta-sitosterol, campesterol, and cholesterol. The 500g pellet contained the largest percentage of free sterol, while the 46,000g pellet contained the largest percentage of esterified sterol. The individual sterol composition of the free sterol and sterol glycoside fraction was very similar; however, the composition of the sterol ester fraction varied widely among intracellular fraction. The intracellular distribution pattern of cholesterol-(14)C added to the isolation medium provided evidence that the intracellular sterol distribution pattern is not an artifact. These results support the suggestion that sterols in plant cells may have a physiological function associated with membranes.