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The cholesteremia induced in rabbits by a diet of the usual rabbit food plus 1 % cholesterol is manifest in very high levels of serum very low density lipoproteins (VLDL, d < 1.006 g/cm3). The low density lipoproteins are moderately elevated and the high density lipoproteins are decreased. The

Lipoprotein lipase forms an enzyme-substrate complex with fat emulsions in the presence of serum lipoproteins. Lipoproteins of very low density and high density have this property, but the former are much more active per unit weight of protein. In this investigation, the activity, expressed as quantity giving half-maximal rate of production of free fatty acids, of specific glycopeptides isolated from very low density and high density lipoproteins was tested in an incubation mixture containing lipoprotein lipase from cows' milk and 1.8 mg triglyceride per ml. The two major polypeptides of high density lipoproteins were virtually inactive in amounts up to 100 fig per ml. Activity of the unfractionated apoproteins of very low density lipoprotein was similar to that of the native lipoprotein (about 4 jag/ml). Two of its polypeptides were active: one with carboxyl-terminal glutamic acid at 0.45 to 0.60 fig/ml and one with carboxyl-terminal alanine at 1.8-2.8 ju,g/ml. Some preparations of the latter peptide were less active and inhibited at high levels. Three other glycopeptides from very low density lipoprotein were inactive. Low density lipoprotein from subjects with primary biliary cirrhosis and a lipoprotein of density 1.04 to 1.06 from a subject with specific elevation of this fraction, both containing the active glycopeptides, had considerable activity (5 to 11 fig/ml). These studies indicate that specific glycopeptides are required for the action of lipoprotein lipase on emulsified triglycerides and suggest that they are important components of the mechanism for extra-hepatic utilization of plasma triglycerides.ADDITIONAL KEY WORDS triglycerides phospholipids polypeptides primary biliary cirrhosis fat emulsions• Lipoprotein lipase is necessary for the utilization of triglycerides in chylomicrons and very low density lipoproteins (1). It will also catalyze hydrolysis of triglycerides in artificial fat emulsion when serum lipoproteins are added. The lipoproteins associate with the emulsion particles to produce an active substrate for the enzyme.

The protein moieties of high-density lipoproteins of human serum contain comparable quantities of two polypeptide subunits of different amino acid sequence. These peptides in urea solutions were separated by polyacrylamide gel electrophoresis and by chromatography on DEAE-cellulose. The carboxylterminal sequences -Thr-Gln and probably -Lys-Tyr-Lys-Asn-Leu-Thr were elucidated by the actions of carboxypeptidases A and B on the lipid-free protein

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Two different polypeptides separated from the protein moiety of high-density lipoproteins of human serum were found to be similar in molecular weight but very different in amino acid composition. One of the peptides, characterized by carboxyl-terminal glutamine, contains no histidine, arginine, tryptophan, or cysteine. Its amino acid composition is: