Which enzyme initiates hydrolysis of triglycerides in circulating chylomicrons?

• A. Hepatic lipase
• B. Lipoprotein lipase
• C. Endothelial lipase
• D. Cholesteryl ester transfer protein

Answer: (B)

Understanding how triglycerides are broken down in the bloodstream relies on where the initial lipolysis happens. In circulating chylomicrons, the enzyme anchored to the capillary endothelium—lipoprotein lipase—initiates the hydrolysis of triglycerides. It uses apolipoprotein C-II on the surface of the chylomicron to become activated, then cleaves triglycerides into free fatty acids and glycerol, releasing them for uptake by nearby tissues such as muscle or adipose tissue.

Hepatic lipase acts later, helping process triglyceride-rich remnants (like IDL) and remodeling lipoproteins, but it does not start the hydrolysis in circulating chylomicrons. Endothelial lipase mainly targets HDL phospholipids and contributes to HDL remodeling rather than initiating TG hydrolysis in chylomicrons. Cholesteryl ester transfer protein is not an enzyme that hydrolyzes triglycerides; it transfers cholesteryl esters and triglycerides between lipoproteins.

So, the enzyme that begins the breakdown of triglycerides in circulating chylomicrons is lipoprotein lipase.