SN5
SHORT NOTE
5M
Describe lipoprotein lipase with its site, activator and significance.
Definition
Lipoprotein lipase (LPL) = the enzyme on the luminal surface of capillary endothelium that hydrolyses triglyceride in chylomicrons and VLDL, releasing free fatty acids for uptake by tissues.
Site and Properties
| Feature | Detail |
|---|---|
| Location | Anchored by heparan sulphate to capillary endothelium |
| Tissues | Adipose tissue, skeletal and cardiac muscle, lactating mammary gland |
| Activator | Apo C-II — essential |
| Inhibitor | Apo C-III |
| Released by | Heparin — hence the old name "clearing factor lipase" |
| Induced by | Insulin (adipose tissue) |
Tissue-specific KM — How One Enzyme Does Two Jobs
| Tissue | Km | Consequence |
|---|---|---|
| Adipose tissue | High | Active only when triglyceride is plentiful, as after a meal → fat is stored |
| Cardiac and skeletal muscle | Low | Active even when triglyceride is scarce, as in fasting → fat is used for energy |
- The same enzyme therefore directs fat to storage after a meal and to oxidation during fasting, purely through its kinetic properties
- No hormone is needed to make the switch — the Km difference does it automatically
Related Lipases
| Enzyme | Site | Substrate | Regulation |
|---|---|---|---|
| Lipoprotein lipase | Capillary endothelium | Chylomicron and VLDL triglyceride | Insulin ↑; apo C-II |
| Hormone-sensitive lipase | Inside the adipocyte | Stored triglyceride | Glucagon and adrenaline ↑ (phosphorylation); insulin ↓ |
| Hepatic lipase | Liver sinusoids | IDL and HDL triglyceride | — |
| Pancreatic lipase | Intestinal lumen | Dietary triglyceride | Needs colipase and bile salts |
CLINICAL PEARL
Do not confuse the two adipose enzymes: lipoprotein lipase brings fat IN to the fat cell and is stimulated by insulin; hormone-sensitive lipase sends fat out and is inhibited by insulin.
Applied Aspects
- Type I hyperlipoproteinaemia — deficiency of lipoprotein lipase or apo C-II → massive chylomicronaemia, eruptive xanthomas, lipaemia retinalis and recurrent acute pancreatitis
- Post-heparin lipolytic activity is measured to diagnose LPL deficiency
- In uncontrolled diabetes low insulin means low LPL activity → chylomicrons and VLDL are cleared poorly → hypertriglyceridaemia
- Fibrates lower triglyceride partly by inducing lipoprotein lipase through PPAR-α
All questions in Lipid Metabolism
- Q1Describe the β-oxidation of fatty acids. Explain the carnitine shuttle, the energetics for palmitate, and the regulation of the pathway. ★
- Q2Describe the synthesis and utilisation of ketone bodies. Explain the causes of ketosis and the biochemical basis of diabetic ketoacidosis. ★
- Q3Describe the de novo synthesis of fatty acids. Explain the citrate shuttle, the role of acetyl-CoA carboxylase, and compare it with β-oxidation. ★
- Q4Describe the biosynthesis and regulation of cholesterol. Give its functions, transport and fate. Add a note on hypercholesterolaemia. ★
- Q5Classify the plasma lipoproteins. Describe their metabolism and the role of apolipoproteins. Add a note on the hyperlipoproteinaemias. ★
- SN1Describe the carnitine shuttle and its regulation. ★
- SN2Define fatty liver. Describe its causes and mechanisms. Add a note on lipotropic factors. ★
- SN3Define essential fatty acids. Give their sources, functions and the features of deficiency. ★
- SN4Describe the synthesis and actions of the eicosanoids. Explain the action of aspirin. ★
- SN5Describe lipoprotein lipase with its site, activator and significance.
- SN6Classify phospholipids and describe their functions. Add a note on lung surfactant. ★
- SN7Describe the biochemical basis of atherosclerosis and the lipid risk factors. ★