Digestion of human milk lipids: physiologic significance of sn-2 monoacylglycerol hydrolysis by bile salt-stimulated lipase.

The bile salt-stimulated lipase secreted with human milk was found to be devoid of positional specificity, i.e., it hydrolyzed emulsified triacylglycerols to glycerol and fatty acids. It also hydrolyzed micellar sn-2 monoacylglycerols. This is in contrast to pancreatic lipase which has a pronounced preference for hydrolysis of sn-1 and sn-3 ester bonds. When the two enzymes were operating together, as in the intestine of the infant fed raw human milk, the sn-2 monoacylglycerols formed by pancreatic lipase served as an excellent substrate for bile salt-stimulated lipase. Thus, the end products of triacylglycerol hydrolysis became glycerol and fatty acids and not sn-2 monoacylglycerol and fatty acids. The bile salt-stimulated lipase also catalyzed incorporation of fatty acids into acylglycerols to a much lesser extent than did pancreatic lipase. Together these two effects of bile salt-stimulated lipase have a promoting effect on the overall process of intraluminal lipolysis. In newborn infants, with low intraduodenal bile salt concentrations, glycerol and fatty acids also should be more readily absorbed than monoacylglycerol and fatty acids. Thus, by serving as a complement to pancreatic lipase, bile salt-stimulated lipase can ensure efficient utilization of milk lipids also in infants with immature endogenous mechanisms for fat digestion and absorption.