Lipoprotein metabolism.

Hyperlipidaemias are important risk factors for coronary heart disease. To prevent and treat the different forms of hyperlipidaemias it is important to understand lipoprotein metabolism. The current knowledge on lipoprotein metabolism is based on many different biochemical and metabolic studies which are summarized here for a clinically orientated overview. Exogenous fat is transported in chylomicrons from the intestine to the liver. After entry in the blood stream the chylomicrons are hydrolyzed by the endothelial-bound lipoprotein lipase. The chylomicron remnants are rapidly taken up into the liver via the LDL receptor and the LDL receptor-related protein. Apolipoprotein E and lipoprotein lipase are the recognition signals for these receptors. The liver utilizes the exogenous fat and can release surplus lipids via VLDL into the blood. The VLDL are another substrate for lipoprotein lipase. The remaining VLDL remnants can either be taken up into the liver or are hydrolyzed to LDL. LDL delivers cholesterol to all body cells via the LDL receptor. The level of LDL cholesterol is regulated by the amount of LDL receptor, and defects in the LDL receptor molecule lead to hypercholesterolaemia. The level of triglyceride-rich remnants is regulated by the amount and activity of lipoprotein lipase and defects in this enzyme cause mixed hyperlipidaemias. Both these forms of hyperlipoproteinaemias are the most frequent and represent major risk factors for arteriosclerosis.