Activation of human milk lipase.

The presence of a serum-activated lipase (lipoprotein lipase) in bovine milk was first reported by Quigley et al. (1958). A similar lipolytic system has been studied in milk of various species where it is assumed to be the predominant lipid-hydrolysing enzyme (Korn, 1962). It was found to be unable to hydrolyse milk triglycerides; its presence in milk has been explained as a result of a penetration of the mammary-gland lipoprotein lipase into the milk (Korn, 1962). Beside the serum-activated lipase, human milk contains another active lipolytic system which has only recently been studied (Olivecrona et al., 1973; Hernell & Olivecrona, 1974). This enzyme is not activated by serum but its activity is stimulated by bile salts. Its concentration in human milk was reported to be several times higher than in cow's milk (Tarassuk et al., 1964). We have studied this enzyme and found that the lipolytic activity in freshly drawn human milk is low and free fatty acids can be detected only after incubation for 3-4h at 37°C (Table 1). However, in the presence of bile salts an active lipolysis begins as soon as milk is added to the incubation system and continues for more than 5h with zero-order kinetics prevailing for 1-2h (Table 1). The optimal activation of the lipolysis was found to occur in a 2 m ~ solution of bile salts, which is the critical micellar concentration of bile acids in the conditions similar to those in the intestinal lumen (Signer et al., 1974). This