Studies in bile salt solutions: XVII. The effect of proteins on bile-salt-stimulated human milk lipase activity against 4-nitrophenylacetate

Abstract

Bovine serum albumin, porcine pancreatic lipase, and horse heart myoglobin, but not any of bee venom melittin, human milk lysozyme, human milk lactotransferrin, human milk α-lactalbumin, human colostrum immunoglobulin A (IgA), or chicken egg white lysozyme, were found to catalyze the decomposition of 4-nitrophenylacetate (PNPA) in 0.1 M Tris, pH 7.5, 298 K, in the presence (2 m M) and absence of sodium taurocholate. The effect of each of these proteins (except bovine serum albumin) on the activity of bile salt-stimulated human milk lipase against PNPA was tested under the same conditions. In the absence of bile salt, human milk lysozyme had no effect, horse heart myoglobin was catalytic, and the other proteins were inhibitory. In the presence of bile salt, porcine pancreatic lipase and human milk lactotransferrin were slightly inhibitory, chicken egg white lysozyme was slightly catalytic, and the other proteins had no effect. A mechanism is proposed in which the bile salt competes with the enzyme for the protein interface. It is postulated that the binary bile salt-enzyme complex is active, whereas the binary protein-enzyme complex is not.