Stereoselectivity of lipases: esterification reactions of octadecylglycerol

Stereoselectivity of several triacylglycerol lipases (EC 3.1.1.3) has been investigated in the enzymatic esterification of rac-1-O-octadecylglycerol with oleic acid in the presence of organic solvents, such as hexane. X-1(3)-O-Octadecylmonooleoylglycerols were the only products formed with most lipases; considerable proportions of X-1(3)-O-octadecyldioleoylglycerols were also formed with the lipase from Candida cylindracea. The mixtures of unesterified enantiomeric substrates, i.e., X-1(3)-O-octadecylglycerols were converted to their 3,5-dinitrophenylurethane derivatives and subsequently resolved into sn-1 and sn-3 enantiomers by HPLC on a chiral stationary phase (Sumichiral OA 2100). The data on enantiomeric excess (ee) and enantiomeric ratio (E) in the unesterified substrate revealed for the lipases from porcine pancreas, Rhizopus sp., Pseudomonas sp., Candida cylindracea, Chromobacterium viscosum and Penicillium cyclopium a distinct preference for 1-O-octadecyl-sn-glycerol over its enantiomer indicating stereoselectivity for the sn-3 position. For the lipase from Rhizomucor miehei a slight stereoselectivity for the sn-1 position was observed. Solvents, such as diethyl ether and dichloromethane, strongly inhibited the esterification reaction, but the enzymatic activity could be restored upon removal of such solvents by washing with hexane indicating reversible inhibition.