Hydrolysis of stereoisomeric alpha-tocopheryl acetates catalyzed by bovine cholesterol esterase

The kinetics of the bovine cholesterol esterase-catalyzed hydrolysis of three stereoisomers of alpha-tocopheryl acetate (alpha T-Ac) have been examined in vitro at 37 degrees C in the presence of dimyristoylphosphatidylcholine and sodium cholate. In contrast to in vivo results obtained earlier in rats (Ingold, K.U., Burton, G.W., Foster, D.O., Hughes, L., Lindsay, D.A. and Webb, A. (1987) Lipids 22, 163-172), 2R,4'R,8'R-alpha T-Ac (RRR-alpha T-Ac) is hydrolyzed (to form 'natural' vitamin E) more slowly (by a factor of approx. 7) than SRR- (and SSS-)alpha T-Ac. It is concluded that chirality at position 2 plays the dominant role in determining Vmax. The Km values show that RRR-alpha T-Ac is 2.1- and 2.7-times more strongly bound to the enzyme than are the SRR- and SSS-alpha T-Ac, respectively. The reaction is subject to competitive inhibition by the product with RRR-alpha T being 2.3-times as powerful an inhibitor as SRR-alpha T.