For studying the role of Ser132 in the putative catalytic site of human lipoprotein lipase (LPL), mutant LPL cDNAs expressing LPLs with amino acid substitutions of Gly or Asn for Ser132 were obtained by site-directed mutagenesis, and were expressed in COS-1 cells. Considerable amounts of LPL enzyme protein mass were detected in the culture medium of COS-1 cells transfected with wild-type LPL, LPL-Gly132, or LPL-Asn132. LPL-Gly132 hydrolyzed Triton X-100-triolein and tributyrin as effectively as wild-type LPL, whereas LPL-Asn132 showed no activity. LPL-Asn132 bound to very low density lipoproteins as effectively as wild-type LPL.