LIPASE STEREOSELECTIVITY AND REGIOSELECTIVITY TOWARD 3 ISOMERS OF DICAPRIN - A KINETIC-STUDY BY THE MONOMOLECULAR FILM TECHNIQUE

Here we present a kinetic study on the steroselectivity and regioselec tivity of 23 purified lipases of animal and microbial origin. This wor k, concerning a general problem of the mechanism of lipase-substrate m olecular recognition, was performed using pure dicaprin isomers: 1,2-s n-dicaprin, 2,3-sn-dicaprin, and 1,3-sn-dicaprin spread as monomolecul ar films at the air-water interface. The first two isomers are optical ly active antipodes (enantiomers), forming stable films up to 40 mN m( -1), while the last is a prochiral compound, with a surface pressure o f collapse of 32 mN m(-1). To our knowledge, this is the first report on the use of three diglyceride isomers as lipase substrates under ide ntical and controlled physicochemical conditions. The lipases tested d isplay a typical behaviour, characteristic of each enzyme, which allow ed us to classify the lipases in groups according to (1) the profiles of enzyme velocity as a function of surface pressure, (2) their prefer ences for a given diglyceride isomer, quantified using new parameters termed steroselectivity index (S.L), vicinity index (V.I), and surface pressure threshold (S.P.T.). The general observation, true for all th e enzymes tested, is that the three substrates are well differentiated , and the differentiation is more pronounced at high interfacial energ y (low surface pressure). This observation supports our hypothesis tha t lipase conformational changes, resulting from the enzyme-surface int eraction, affect the enzymes' specificities. Generally speaking, the s tereopreference for either sn-1 or sn-3 position on glycerides is main tained both in the case of di- and tri-glycerides. (C) 1995 Wiley-Liss , Inc.