Some Functional Aspects of the Plasma Apolipoproteins
Human plasma very low density lipoprotein (VLDL) and Chylomicrons (Chyl) contain the same protein as low density lipoprotein (LDL) and an additional group of small apolipoproteins of approximately 10,000 Molecular Weight. These smaller apolipoproteins constitute approximately 55% of the protein in VLDL and over 90% of the protein in Chyl. The two major constituents of this latter group of apolipoproteins are designated by their carboxyl terminal amino acids as apoLp-Glu (glutamic acid) and apoLp-Ala (alanine). Both have marked effects on the triglyceride lipase (TGL) of lipoprotein lipase. Using milk lipoprotein lipase, 1to 2 ug of apoLp-Glu produces a ten-fold activation of TGL activity. ApoLp-Ala inhibits both the milk and plasma TGL activities at levels above 50 ug/ml. The inhibition occurs at levels of apoLp-Ala greater than 2% of the substrate mass. No differences in these properties have been observed with apoproteins purified from normal subjects, or Type III and Type V hyperlipoproteinemics. The inhibition of apoLp-Ala of lipoprotein lipase may be blocked in vitro by certain lipid factors. The properties of these lipids which block apoLp-Ala inhibition are presently under study in our laboratory.