The occurrence of sterol-ester hydrolase activity in roots of white mustard seedlings

Abstract

Roots of white mustard ( Sinapis alba) seedlings contain sterol-ester hydrolase activity. Enzyme assays with [4- 14C]cholesteryl palmitate as the substrate show that the hydrolytic activity is located in cell membrane structures but can be easily solubilized with 0.1 % Triton X-100. During gel filtration on Sepharose 6B in the presence of Triton X-100 and sodium chloride the hydrolytic activity is eluted as a single peak. This peak also contains hydrolytic activities toward some other esters such as p-nitrophenyl palmitate, tripalmitoylglycerol or n-hexadecanyl palmitate. However, the rate of hydrolysis of [4- 14C]cholesteryl palmitate is not significantly affected by a large excess of these esters in the incubation mixtures, suggesting that different enzyme proteins are involved in the hydrolysis of steryl ester and the other esters tested. Sterol-ester hydrolase from white mustard seedlings exhibits a marked specificity with respect to the length of the acyl chain bound to sterol. For a series of steryl esters containing saturated fatty acids (from C 2 to C 22) the rate of hydrolysis is the highest for esters of C 14-C 18 acids. Steryl acetate (C 2), butyrate (C 4) and behenate (C 22) are very poor substrates.