Everything You Want to Know About Lipoprotein Lipase

Lipoprotein lipase (LPL) is a glycoprotein synthesized and secreted by parenchymal cells such as adipocytes, cardiomyocytes, skeletal muscle cells, mammary cells, and macrophages, with a molecular weight of 60ku and containing 3-8% carbohydrates. The physiological function of LPL is to catalyze the breakdown of TG in the core of CM and VLDL into fatty acids and monoglycerides for tissue oxidative energy and storage. ApoCII is an essential cofactor for LPL, in which amino acids 61-79 at the C-terminus have a role in activating LPL. The enzyme protein level structure of LPL in mammals such as bovine, murine and porcine has 87%-94% homology, which indicates that LPL is highly conserved in evolutionary process. It is hypothesized that all three may originate from the same gene family and have a common mechanism of action.

The LPL gene is located on the short arm of chromosome 8, 8p22, and is approximately 35 kb long. It consists of 10 exons and 9 introns and encodes a protein with 475 amino acid residues. There are polymorphisms in the LPL gene loci, mainly in the intron and flanking sequences of the LPL gene, in which the PVU II polymorphic site in intron 6 and the Hind III polymorphic site in intron 8 are associated with hyperlipidemia and provide genetic markers for the family linkage analysis of hyperlipidemia.

LPL is synthesized in the rough endoplasmic reticulum of parenchymal cells, and the newly synthesized LPL remains in the perinuclear endoplasmic reticulum, which is an inactive enzyme, and the inactive LPL synthesized by mRNA translation, which is called the enzyme precursor, is then converted into active LPL after glycosylation. The second is regulated secretion, in which newly synthesized LPL is stored in the secretory ducts of some cells and is secreted once the cells are exposed to a suitable pro-secretory stimulus, which is often greater than synthesis. All cells have the basic type of secretion, and only a few cells have both forms of secretion. Heparinsulphateproteoglycans (HSPG) present on the outer surface of the cell membrane keep the enzyme in an inactive and concentrated state and are then secreted by heparin through an as yet unexplained mechanism, i.e. post-heparin stimulation of the plasma to obtain activated LPL, which is distributed among the triglyceride-containing lipoproteins, mainly catabolizing CM and VLDL triglycerides, and binds and attaches to these lipoprotein remnants, possibly as a signal for hepatic uptake of these particles.

The physiological function of LPL is currently thought to be to catabolize triglycerides, the nuclear component of lipoproteins, and also phospholipids such as lecithin and phosphatidylethanolamine, and to facilitate the transfer of cholesterol, phospholipids and apolipoproteins between lipoproteins, whose metabolites, free fatty acids, provide energy to tissues or are re-esterified to TG and stored in adipose tissue. In addition, LPL has the ability to increase the binding of CM residues to the LPL receptor and promote CM residue uptake.

When measuring plasma LPL activity, heparin must be injected intravenously because of the high affinity of LPL for heparin. Intravenous injection of heparin, which releases LPL from the surface of endothelial cells into the blood, is a necessary operation for the determination of LPL activity in blood. Usually, 10 units per kilogram of body weight are injected intravenously, and plasma is collected 10 minutes later to obtain LPL activity. Generally, 1/3 of the total plasma lipase activity after intravenous heparin injection is LPL, and almost all of the remainder is hepatic lipase (HTGL). LPL activity can also be measured by selective inhibition of LPL activity with high concentrations of hydrochloric acid or fisetin. Recently, it has been reported that activity can also be assayed with antibodies to LPL or HTGT.

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