A new method for the measurement of lipoprotein lipase in postheparin plasma using sodium dodecyl sulfate for the inactivation of hepatic triglyceride I ipasel

Lipoprotein lipase (LPL) and hepatic triglyceride lipase (H-TGL) are lipolytic activities found in postheparin plasma. A simple and precise method for the direct determination of LPL in postheparin plasma is described. Preincubation of this plasma (45-60 min at 26°C) with sodium dodecyl sulfate (35-50 mM) in 0.2 M Tris-HC1 buffer, pH 8.2, results in the inactivation of H-TGL, while leaving LPL fully active. Direct determination of H-TGL is done in a separate aliquot of the same postheparin plasma sample using previously reported assay conditions that do not measure LPL. The sodium dodecyl sulfate-resistant lipolytic activity has the characteristics of LPL as judged by a) its activation by serum and by apoEpoproteinC-11; b) its inactivation (over 90%) by 0.75 M NaCl; and c) its inactivation by a specific antiserum. No sodium dodecyl sulfateresistant activity was found in postheparin plasma from a patient with LPL deficiency (primary type I hyperlipoproteinemia). An excellent correlation of values was obtained ( r = 0.99) for 30 samples assayed after sodium dodecyl sulfate treatment and after immuno-inactivation of H-TGL. The intra-assay coefficient of variation was & 11 % and 4% before and after normalization of values, respectively. Baginsky, M. L., and W. V. Brown. A new method for the measurement of lipoprotein lipase in postheparin plasma using sodium dodecyl sulfate for the inactivation of hepatic triglyceride lipase. J. Lipid Res. 1979. 20: 548-556. Supplementary key words anti-lipoprotein lipase serum . apoC-I1 Lipoprotein lipase (LPL) (glycerol ester hydrolase, EC 3.1.1.3), an enzyme present in several extrahepatic tissues and released into the blood stream by heparin ( l ) , is thought to play a key role in the removal of triglycerides from both chylomicrons and very low density lipoproteins (25). A triglyceride hydrolase believed to originate in hepatic tissues is also released by heparin into the circulation (61 l), making a significant contribution to the total postheparin lipolytic activity. The function of this hepatic triglyceride lipase (H-TGL) in the clearance of plasma triglycerides has not yet been elucidated. In 1955 Korn (12) observed that lipoprotein lipase from several tissues was inhibited by 1 M NaCl and by protamine sulfate. These inhibitory effects are not found with the triglyceride lipase released into plasma from liver by heparin (6, 13). These properties provide the basis of an assay (8, 14) in which the hepatic triglyceride lipase activity of postheparin plasma is determined after inhibition of most of the LPL activity by protamine sulfate. LPL is then estimated as the difference in activity determined in assays with or without protamine sulfate. Problems related to the indirect nature of this method for LPL determination make it less desirable than methods assaying LPL directly. Independent determinations of both LPL and H-TGL activities in postheparin plasma have been possible after their separation and partial purification by column chromatography on Sepharose gel containing covalently bound heparin (15). This permits the assay of each enzyme under its optimal assay conditions, but the method is laborious and involves significant losses during chromatography. Selective inhibition of H-TGL or LPL by antisera prepared against the purified enzymes has provided a rapid and precise method for the determination of each of these enzymes in human postheparin plasma (PHP) samples (16, 17). However, such specific antisera are not widely