Plays an important role in fat metabolism. It preferentially splits the esters of long-chain fatty acids at positions 1 and 3, producing mainly 2-monoacylglycerol and free fatty acids, and shows considerably higher activity against insoluble emulsified substrates than against soluble ones (By similarity). Plays a role in hibernation as a key enzyme that shows high activity at low temperatures. When expressed in the hibernating heart it liberates fatty acids from triglycerides at temperatures as low as 0 degrees Celsius. a triacylglycerol + H2O = a diacylglycerol + a fatty acid + H(+) 1,2,3-tributanoylglycerol + H2O = butanoate + dibutanoylglycerol + H(+) 1,2,3-tri-(9Z-octadecenoyl)-glycerol + H2O = (9Z)-octadecenoate + di-(9Z)-octadecenoylglycerol + H(+) all-trans-retinyl hexadecanoate + H2O = all-trans-retinol + H(+) + hexadecanoate 1,2-di-(9Z-octadecenoyl)-glycerol + H2O = (9Z)-octadecenoate + (9Z-octadecenoyl)-glycerol + H(+) Inhibited by bile salts, is reactivated by (pro)colipase/CLPS. Forms a 1:1 stoichiometric complex with (pro)colipase/CLPS. Expressed in many tissues with highest expression in liver. During hibernation there is a significant increases in expression in heart, white adipose tissue (WAT), and testis; but not in pancreas. Up-regulated during hibernation. Belongs to the AB hydrolase superfamily. Lipase family.