Stereoselectivity of lipases. II. Stereoselective hydrolysis of triglycerides by gastric and pancreatic lipases

In the present study, porcine pancreatic lipase, rabbit gastric lipase, and human gastric lipase stereospecificity toward chemically alike, but sterically nonequivalent ester groups within one single triglyceride molecule was investigated. Lipolysis reactions were carried out on synthetic trioctanoin or triolein, which are homogenous, prochiral triglycerides, chosen as models for physiological lipase substrates. Diglyceride mixtures resulting from lipolysis were derivatized with optically active R-(+)-1-phenylethylisocyanate, to give diastereomeric carbamate mixtures, which were further separated by high performance liquid chromatography. Resolution of diastereomeric carbamates gave enantiomeric excess values, which reflect the lipases stereobias and clearly demonstrate the existence of a stereopreference by both gastric lipases for the sn-3 position. The stereoselectivity of human and rabbit gastric lipases, expressed as the enantiomeric excess percentage, was 54% and 70% for trioctanoin and 74% and 47% for triolein, respectively. The corresponding values with porcine pancreatic lipase were 3% in the case of trioctanoin and 8% in that of triolein. It is worth noting that rabbit gastric lipase, unlike human gastric lipase, became more stereoselective for the triglyceride with shorter acyl chains (trioctanoin). This is one of the most striking catalytic differences observed between these two gastric lipases.