Steric-Control for the Enantioselective Hydrolysis of Amino Acid Esters in Hybrid Membrane Systems.

Abstract

Stereoselective reactions have attracted considerable attention in connection with understanding the origins of the stereoselectivity observed with proteolytic enzymes. [1][2][3][4][5] In the course of our study on the stereoselective hydrolysis (deacylation) of amino acid and dipeptide esters with the functional molecular assemblies composed of synthetic surfactants and reactive species, we emphasized that the stereochemical (enantiomeric and diastereomeric substrates) control could be attained by changing the composition of the coaggregates [6][7][8] and regulating ionic strength 9) and temperature.10,11) Especially, an extremely high enantioselectivity was attained at the optimum composition of coaggregate. 6)However, the values of k t and k s (the first-order rate constants with and without the peptide catalyst, respectively) are fairly close for the enantioselective hydrolysis in the aggregate systems of synthetic surfactants. Then, it is impossible to disregard the errors of k t and k s values when we discuss catalytic activities of the artificial membranes bound enzyme on the basis of k a,obsd values.In this study, we employed native phospholipids instead of synthetic surfactants for the hydrolysis of amino acid esters12) with tripeptide (Z-PheHisLeu) 12) in the coaggregate systems. The advantage of this experiment should enable to evaluate of the catalytic activities directly on the basis of k a,obsd values because of the large difference of the k t and k s values, that is, the k t /k s ratios were above 30 and 4 folds for the L-and D-substrates, respectively.Recently, in the course of our study on artificial membranes composed of native phospholipids, very useful knowledges for medical engineering applications in vitro and in vivo have been obtained. For example, artificial lipid membranes were used as a drug carrier in drug delivery systems, such as 1,3-bis(2-chloroethyl)-1-nitrosourea encapsulated in hybrid liposomes (artificial membranes) which should be effective against malignant glioma in vitro and in vivo. 13)Moreover, the artificial membranes themselves were found to be useful as a new type of anticancer drug without side effects. 14,15) In this study, we report on the remarkable effects (composition of hybrid membranes and reaction temperature) on the enantioselective hydrolysis of C 12 -D(L)-Phe-PNP catalyzed by Z-PheHisLeu in the hybrid membranes composed of phospholipid (DMPC) and nonionic surfactant (C 12 (EO) 8 ) at pH 7.4. The relations between the enantioselectivity and the microenvironment of the reaction fields are discussed. Results and DiscussionThe composition dependence of coaggregates on the enantioselective hydrolysis of C 12 -D(L)-Phe-PNP catalyzed by Z-PheHisLeu was investigated in hybrid membranes of DMPC/C 12 (EO) 8 , as shown in Fig. 1. The concentration of C 12 (EO) 8 is expressed as mol% of total lipids. Both the second-order rate constants (k a,obsd ) and enantioselectivity (k ; 4-22-1 Ikeda, Kumamoto 860-0082, Japan. Received November 5, 2002; accepted November 26, 20...