Studies on bile-salt-stimulated lipolytic activity in human milk. II Demonstration of two groups of milk with different activities.

Human milk samples were assayed for bile-salt-stimulated lipolytic activity in the presence of an unpurified and purified preparation of glycoconjugated bile salts and two patterns of activity were obtained. One group of milks (high activity) had similar activities (mean +/- 1 S.D.) of 16.9 +/- 3.5 (purified bile salts) and 17.2 +/- 3.5 mumole . ml-1 . h-1 (unpurified bile salts) whereas another group (low activity) had a lower activity with the purified (4.2 +/- 2.3 mumole . ml-1 . h-1) than with the unpurified bile salt preparation (19.9 +/- 4.0 mumole . ml-1 . h-1). Activities of the low activity milks (assayed with the purified bile salts) could be increased following storage at -20 degrees C, sonication, or the preparation of an acetone ether powder and, in some cases, reached the maximal activity obtained with the unpurified bile salt preparation. These procedures had no effect on the activity of the high activity milks. Recombination experiments, whereby the milk serum from a low activity milk was added to the milk fat of a high activity milk and vice versa, and also kinetic studies suggest that the differences in activities between the two groups result from differences in availability of the enzyme and are independent of the source of milk fat.