CEL (bile salt-dependent lipase) catalyzes the hydrolysis of extracellular cholesterol esters to yield cholesterol and a long-chain fatty acid. This reaction, in the lumen of the small intestine, is part of the process of digestion of dietary fats.
While alternative splicing gives rise to two CEL isoforms, only the longer one encodes all of the residues that form the active site of the enzyme (Reue et al. 1991). In vitro, monomeric CEL protein is active even in the absence of bile salts. Its activity is greatly increased when it is complexed with two molecules of cholate, chenodeoxycholate, or their glycine or taurine conjugates (Lombardo and Guy 1980), and the predominant form of the enzyme active on lipid micelles in the gut is a dimer of two such complexes (Aubert-Jousset et al. 2004).
CEL is synthesized in pancreatic acinar cells and released into the small intestine. It is also synthesized in the mammary gland and is a constituent of breast milk. The milk CEL is thought to play a role in digestion of milk fat in newborn infants, whose own pancreatic synthesis of the enzyme is low (Lombardo 2001; Bernback et al. 1990).