lipase

Meaning: Lipase is a hydrolase that breaks the ester bonds of triglycerides, releasing free fatty acids and monoglycerides or glycerol. Pancreatic lipase does most dietary fat digestion in the small intestine. It acts at the surface of fat droplets, so bile salts (made in the liver, stored in the gallbladder) first emulsify fat, and colipase helps lipase bind. Products enter intestinal cells, are rebuilt into triglycerides, packaged into chylomicrons, and enter lacteals (lymph) rather than blood capillaries. Other lipases act elsewhere: hormone-sensitive lipase in adipocytes mobilizes stored fat (stimulated by epinephrine and glucagon, inhibited by insulin), and lipoprotein lipase on capillary walls frees fatty acids from chylomicrons and VLDL.

On the MCAT: Passages ask what happens when the bile duct or pancreatic duct is blocked (poor fat digestion, fatty stools, reduced absorption of fat-soluble vitamins), or which organ secretes which digestive agent. Questions also test fed versus fasting hormone control of fat breakdown.

Common confusion: Bile is not an enzyme. Bile salts emulsify fat physically, increasing its surface area, but they do not hydrolyze it. Lipase does the chemical cleavage, and it works far better on emulsified fat.