Bile salt-stimulated lipase (BSSL) distribution in rat, mouse and transgenic mouse expressing human BSSL.

In some species, including man and mouse, bile salt-stimulated lipase (BSSL) in milk catalyzes the hydrolysis of triacylglycerides into glycerol and free fatty acids, a reaction that is of particular importance during suckling. The enzyme is also secreted by the pancreas (referred to as carboxyl-ester hydrolase, CEH). We wished to localize sources and storage sites for BSSL/CEH in rats, in wild-type mice, and in transgenic mice producing recombinant human BSSL in milk. Immunoreactivity against several BSSL fragments was strong in the pancreatic acinar cells and moderate in the absorptive cells of the small intestine and in salivary duct cells of the mice, as well as in rats. Sections from lactating mammary glands of mouse, but not rat, also showed immunoreactivity for BSSL; the signal was strongest in the transgenic mice. Radioactive riboprobes for BSSL mRNA hybridized on sections of rat and mouse pancreatic acinar cells, and mouse mammary glands (both wild-type and transgenic). Using RT-PCR, it was possible to amplify BSSL mRNA from wild-type mouse pancreas and mammary gland, from rat submandibular glands, and, in a few cases, from rat liver. In transgenic mice, the BSSL mRNA was highly expressed only in lactating mammary gland, but could be detected in a few other organs as well.

Naturally occurring variants of human milk bile salt-stimulated lipase.

Analysis of milk samples from a number of lactating women revealed molecular variants of bile salt-stimulated lipase (BSSL) of both lower and higher molecular mass than that commonly occurring. In contrast to previous observations, we report on individuals having only a variant of lower mass, both one of lower and one of common mass, or both one of lower and one of higher mass of the lipase. From two individuals we purified the lower molecular mass BSSL variant and characterized it. The amount of lipase in the milk of these two individuals was considerably less than average (mean of 10 women with BSSL of the most common molecular mass). The BSSL variant of lower mass showed the same bile salt activation, pH dependency, temperature stability as those most commonly occurring. We could localize the difference in mass to the large O-glycosylated repeat sequence close to the C-terminus of the protein. With respect to all characteristics studied, the BSSL variant of higher mass was also similar to that most commonly ocurring. Again, the difference in mass could be localized to the repeat region of the protein. Hence, it appears as if the repeat region, normally carrying 16 repeats of 11 amino acids each, varies in size between individuals.

Molecular cloning and characterization of the mouse carboxyl ester lipase gene and evidence for expression in the lactating mammary gland.

DNA hybridization was used to isolate a 2.04-kb cDNA encoding carboxyl ester lipase (CEL) from a mouse lactating mammary gland, lambda gt10 cDNA library. The cDNA sequence translated into a protein of 599 amino acids, including 20 amino acids of a putative signal peptide. Comparison of the deduced amino acid sequence of the mouse CEL with CEL from five other species revealed that there is a high degree of homology between the different species. The mouse CEL gene was also isolated and found to span approximately 7.2 kb and to include 11 exons. This organization is similar to those of the recently reported human and rat CEL genes. We have also analyzed expression of the CEL gene in the mammary glands from other species by performing a Northern blot analysis with RNA from goat and cow. The results show that the gene is expressed in both species.