Lipoprotein lipase domain function.
Human lipoprotein lipase (LPL) monomer consists of two domains, a larger NH2-terminal domain that contains catalytic residues and a smaller COOH-terminal domain that modulates substrate specificity and is a major determinant of heparin binding. Analyses of NH2-terminal domain function were performed after site-directed mutagenesis of the putative active-site serine residue, while COOH-terminal domain function was assessed following reaction with a monoclonal antibody. The native enzyme and mutant LPL in which serine 132 was replaced with alanine, cysteine, or glycine were transiently expressed in COS-7 cells. Mutant proteins were synthesized and secreted at levels comparable to native LPL; however, none of the mutants retained enzymatic activity. The mutant with alanine replacing serine 132 was purified and shown to be inactive with both esterase and lipase substrates; however, binding to a 1,2-didodecanoyl-sn-glycero-3-phosphatidylcholine monolayer was comparable to native LPL. These results are consistent with a catalytic, and not a lipid binding, role for serine 132. To investigate the function of the smaller COOH-terminal domain, LPL lipolytic and esterolytic activities as well as heparin binding properties were determined after reaction with a monoclonal antibody specific for this domain. Lipolytic activity was inhibited by the monoclonal antibody, whereas esterolytic activity was only marginally affected, indicating that the LPL COOH-terminal domain is required for lipolysis, perhaps by promoting interaction with insoluble substrates. Also, the affinity of antibody-reacted LPL for heparin was not significantly different from that of LPL alone, suggesting that (i) the heparin-binding site is physically distinct from the COOH-terminal domain region required for lipolysis and (ii) binding of antibody did not cause dimer dissociation. A model is proposed for the two LPL domains fulfilling different roles in the lipolytic process.
Wong
H
H
Lipid Research Laboratory, Veterans Administration Wadsworth Medical Center, Los Angeles, California 90073.
Davis
R C
RC
Thuren
T
T
Goers
J W
JW
Nikazy
J
J
Waite
M
M
Schotz
M C
MC
D000818:Animals; D001665:Binding Sites; D002417:Cattle; D002478:Cells, Cultured; D018076:DNA, Complementary; D006493:Heparin / Q000378:metabolism; D006801:Humans; D008071:Lipoprotein Lipase / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D012694:Serine / Q000378:metabolism; D013379:Substrate Specificity
D018076:DNA, Complementary; D012694:Serine; D006493:Heparin; D008071:Lipoprotein Lipase