Niemann-Pick disease results from deficiency of sphingomyelinase, leading to accumulation of sphingomyelin in lysosomes, particularly affecting the nervous system and liver.

ApoE acts as a ligand recognized by hepatic receptors (LDL receptor and LDL receptor-related protein), mediating the uptake of chylomicron remnants and VLDL remnants.

LCAT esterifies free cholesterol on lipoprotein surfaces, forming cholesterol esters that move to the hydrophobic core. This process is essential for lipoprotein maturation.

During fasting, the liver increases synthesis and secretion of VLDL to transport endogenous triglycerides produced from gluconeogenesis and fatty acid oxidation.

Familial hypercholesterolemia results from mutations in the LDL receptor gene, leading to impaired LDL uptake and elevated plasma LDL cholesterol levels.

Lipoproteins are organized with a hydrophobic lipid core (cholesterol esters and triglycerides) surrounded by a hydrophilic shell of proteins, free cholesterol, and phospholipids.

Cholesterol molecules insert between phospholipids in the bilayer, regulating membrane fluidity. At high temperatures, it reduces fluidity; at low temperatures, it increases it.

HMG-CoA reductase catalyzes the conversion of HMG-CoA to mevalonate and is the rate-limiting step in cholesterol biosynthesis. It is inhibited by statins.

Triglycerides are the primary storage form of lipids in adipose tissue, accounting for >95% of stored lipids. They are mobilized during energy deficit states.

CYP27A1 mutations cause cerebrotendinous xanthomatosis (CTX), characterized by elevated cholestanol and cholestane triol precursors due to impaired side-chain oxidation of cholesterol. This leads to progressive neurological deterioration, cataracts, and diarrhea. Chenodeoxycholic acid therapy is beneficial.