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= Lysosomal lipase = From Wikipedia, the free encyclopedia Lysosomal acid lipase (LAL) is encoded by the gene, LIPA, and is a form of lipase which functions intracellularly, in the lysosomes. LAL is also referred to as lipase A, acid cholesteryl ester hydrolase, acid cholesterol esterase, and acid cholesteryl esterase. LAL is crucial in lipoprotein lipid catabolism because it is the main enzyme responsible for hydrolysis of cholesterol esters (CE), which are derived from low-density lipoprotein (LDL) or modified LDL. LAL may also break down triglycerides, however this function is secondary to its catalytic activity involved in cholesterol homeostasis. Altered LAL activity may implicate pathologic lipid accumulation and progression of atherosclerosis.

Cholesterol Homeostasis
In the human body, cholesterol appears in two forms: free cholesterol (FC), or cholesterol esters (CE). FC is termed “free” because this form of cholesterol is not esterified, allowing it to be transported between the gastrointestinal lumen and intestinal enterocytes, or between the biliary tract and hepatocytes. This function enables the body to absorb or reabsorb cholesterol, and excrete excess amounts in order to maintain homeostasis. Cholesterol is stored in its inactive form, CE. It is a cholesterol molecule that has been esterified with the addition of a fatty acid. FC can be converted to CE by the enzyme intracellular acyl- (or acetyl) CoA cholesterol acyl transferase (ACAT). LAL catalyzes the reverse reaction, converting CE to FC by hydrolyzing and cleaving the fatty acid from the cholesterol molecule. LAL is the only known lysosomal lipase identified as having this function. Decreased LAL activity may lead to cellular accumulation of CE and triglycerides because cholesterol transport out of cells becomes impaired. The products formed by LAL activity, FC and fatty acids, additionally mediate cholesterol homeostasis. They interact with transcription factors that regulate the expression of genes that affect lipogenesis, and cholesterol uptake and synthesis.

Atherosclerosis
Research in human and animal in vivo models indicates that atherosclerosis greatly increases FC and CE in foam cell lysosomes, as well as CE storage in the cytoplasm. Early stages of disease suggest accumulation of CE droplets in the cytosol, whereas later stages show build up of FC and CE in lysosomes. It is thought that the unregulated uptake of LDL molecules into tunica intima cells increases cholesterol load which leads to overall loss of lysosomal functioning.

High FC concentrations impair LAL hydrolysis of CE, thus contributing to the production of fat-loaded foam cells in arterial plaque, in addition to abnormal lipid store distribution in cells. This proposed mechanism of disease characterizes atherosclerosis as a disorder of lysosomal storage. LAL expression and initial catalytic activity do not decrease with high cellular cholesterol load. As FC increases in lysosomal membrane over time, proton pumping ability of v-ATPases is dampened. Lysosomal pH increases, inhibiting activity of LAL and interfering with the metabolism of CE. The optimal pH for LAL activity is 3.5 – 4.5. This leads to CE accumulation in lysosomes and the cytosol as atherosclerosis progresses.

Extracellular LDL Modification
Atherosclerotic cells have LAL in their extracellular space. This LAL has been suggested to modify aggregated LDL and catalyze extracellular CE. Macrophages secrete lysosomal enzymes in the absence of stimuli, however the presence of inflammatory molecules increases this secretion. Atherosclerotic lesions have areas of varying acidity. Areas that contain higher levels of lipids and macrophages have locally increased acidity that may support LAL activity.

Clinical significance[edit | edit source]
A deficiency associated with Lysosomal Acid Lipase Deficiency, Wolman disease, and cholesteryl ester storage disease.

Chlorpromazine is an inhibitor of lysosomal lipase.[1]

A genome wide survey suggests that lysosomal lipase A (located at chromosome 10q23.31) is associated with coronary artery disease in humans.[2]