On the source of bile salt-stimulated lipase in human milk: a study based on serum concentrations as determined by sandwich enzyme-linked immunosorbent assay technique.

Human milk contains a bile salt-stimulated lipase (BSSL) which is considered to contribute significantly to lipid digestion in newborns. Human pancreatic juice contains an enzyme, carboxyl ester hydrolase (CEH), that is immunochemically and functionally identical to the milk enzyme. It has not been clear, however, whether the milk enzyme is synthesized within the mammary gland or whether the pancreatic enzyme is transported to the mammary gland for secretion. In this study we describe an enzyme-linked immunosorbent assay (ELISA) for determinations of BSSL and CEH in serum. Serum samples from healthy adults of both sexes contain significant amounts of enzyme (3.2 +/- 1.7 micrograms/L). Lactating women did not have higher serum levels, which would be expected if the milk enzyme were a product of the pancreas, and it was calculated that the concentrations found could not sufficiently account for the amounts secreted in the milk. We therefore conclude that BSSL is synthesized within the mammary gland. This conclusion is further strengthened by our findings that serum of breast-fed newborns contains very little if any enzyme. Since macromolecular absorption is high early in life, and witch's milk contains high enzyme levels, again one would expect high serum levels if BSSL were a pancreatic product.