Human milk bile salt-stimulated lipase: functional and molecular aspects

In breast-fed infants, digestion of milk triglycerides, the major source of energy and long-chain polyunsaturated fatty acids, is catalyzed by a concerted action of gastric lipase, colipase-dependent pancreatic lipase, and bile salt-stimulated lipase (BSSL). The major part of BSSL is present in the milk and the lesser part originates in the infant's exocrine pancreas. Gastric lipase is important in initiating digestion of milk fat globule triglycerides in the stomach. BSSL shifts the final products of triglyceride digestion from monoglyceride and free fatty acid (the products of colipase-dependent pancreatic lipase) to glycerol and free fatty acid, which may promote efficient absorption. Moreover, BSSL is likely to promote efficient use of milk cholesteryl- and fat-soluble vitaminesters and long-chain polyunsaturated fatty acids (> C18). The cDNA sequence has shown that BSSL has a unique primary structure. The N-terminal half is highly conserved between species and shows striking homology to typical esterases, for example, acetylcholine esterase. In contrast, the C-terminal half, containing 16 proline-rich repeats of 11 amino acid residues, is unique to BSSL. Using several recombinant variants of BSSL, we have found that these unique repeats and the glycosylation are completely dispensable for activity. Thus all typical properties of BSSL reside in the N-terminal half of the molecule.