Human milk contains many enzymes and other biologically active proteins. One of the enzymes, the bile salt-stimulated lipase, constitutes as much as 1% of the milk proteins. Its importance for efficient utilization of milk lipids by the breast-fed infant is now well established. However, whether the lipase protein is a product of protein synthesis within the mammary gland has up till now been an unanswered question. Using biopsy material from lactating human mammary gland we have now demonstrated that the enzyme is synthesized within the gland. This was done by immunoprecipitation of [35S]methionine-labelled protein from tissue pieces. By activity determination we could also determine the amount of enzyme stored in the gland. It was concluded that bile salt-stimulated lipase accounted for 1.3 μg/mg tissue protein. Finally, from this figure it could be calculated that about 10–15% of the total protein present in the tissue was milk protein.
