HUMAN LYSOSOMAL ACID LIPASE CHOLESTERYL ESTER HYDROLASE AND HUMAN GASTRIC LIPASE - IDENTIFICATION OF THE CATALYTICALLY ACTIVE SERINE, ASPARTIC ACID, AND HISTIDINE RESIDUES

摘要

Human lysosomal acid lipase/cholesteryl ester hydrolase (HLAL), human gastric lipase (HGL), and rat lingual lipase (RLL) constitute a family of mammalian lipases characterized by an acidic pH optimum. HGL and RLL are secreted by the chief cells of the stomach and by the serous von Ebner's glands of the tongue, respectively, and hydrolyze dietary long-chain triglycerides in the gastrointestinal tract. HLAL, in contrast, catalyzes the intralysosomal degradation of both triglycerides and cholesteryl esters in virtually all cells except erythrocytes. All three enzymes are proposed to be serine esterases with a catalytic Ser-Asp-His triad similar to other lipases, despite their sensitivity towards sulfhydryl modifying reagents. To investigate the role of conserved serine, aspartic acid, and histidine residues in HLAL and HGL, we constructed 24 mutant lipases with single amino acid substitutions using the site-directed mutagenesis approach. Our combined data strongly support the conclusion that Ser(153), Asp(324), and His(353) are components of the catalytic triad of HLAL and HGL. Structural integrity of the conserved His-Gly dipeptide of lipases also appears to be important for neutral lipid hydrolysis, as replacement of His(65) by glutamine abolished HLAL and HGL enzymic activity. Substitution of HLAL residues Asp(93), Asp(130), and Asp(328) with glycine, in contrast, had a more pronounced impact on cholesteryl oleate hydrolysis than on triglyceride hydrolysis. These results provide new insights into the structural basis of HLAL and HGL function. [References: 56]