Bile salt-stimulated lipase activation

Warded, J.M., Wright, A.J., Bardsley, W.G. and D Souza, S.W. (1984) Bile salt-stimulated lipase and esterase activity in human milk after collection, storage, and heating nutritional implications. Pediatr. Res. 18, 382-386. 

Pancreatic carboxytester lipase, secreted by the pancreas as an active enzyme without proteolytic activation, displays broad substrate specificity and has therefore received many names in the literature carboxylesteraae, bile salt-stimulated (or activated or dependent) lipaae (due to its absolute requirement for bile salts to hydrolyze insoluble substrates), carbaxylester lipase or hydrolase, cholesterol... 

Human milk differs from cows milk in that it contains, in addition to an LPL similar to that in cows milk, a bile salt-stimulated lipase (BSSL), which appears to have no counterpart in cows milk (Hernell and Blackberg, 1994). In addition, a third lipase known as platelet-activating factor acet-ylhydrolase activity (PAF-AH) has been demonstrated in human milk the activity is absent, or extremely low in cows milk (Furukawa, et al., 1994). 

Cows milk LPL has phospholipase Ai activity (Scow and Egelrud, 1976), but its action on milk phospholipids has not been recorded. Freshly secreted goats milk has been shown to have phospholipase A activity (Long and Patton, 1978) but it is not known whether this can be attributed to the LPL of that milk. Human milk contains an acid sphingomylinase C, as well as ceramidase activity provided by the bile salt-stimulated lipase present (Nyberg et al., 1998). 

Other factors. A cow s hormonal balance can affect the susceptibility of her milk to spontaneous lipolysis (Fredeen et al., 1951 Kastli et al., 1967 Bachman et al., 1988). The oestrus cycle appears to have little effect on spontaneous lipolysis (Fredeen et al, 1951) but may affect lipase activity in the milk (Kelly, 1945). In contrast, treatment of cows with oestradiol and progesterone has been shown to lead to increased lipolysis in the milk (Bachman, 1982 Heo, 1983 Bachmann eta/., 1985) but no change (Bachman, 1982) or a transient increase (Bachmann et al., 1985) in total lipase activity. It appears that the increased lipolysis in milk following hormonal treatment, or in milk from cows with ovarian cysts, may not be typical spontaneous lipolysis as cooling is not needed to initiate it (Bachman, 1982) a lipase other than lipoprotein lipase, possibly a bile salt-stimulated lipase, may be responsible for such lipolysis (Heo, 1983 Bachmann et al., 1985). Treatment of cows with bovine somatotropin has been reported to have no significant effect on milk lipoprotein lipase activity (Azzara et al., 1987). 

Fredrikzon, B., Hernell, O., Blackberg, L., Olivecrona, T. 1978. Bile salt-stimulated lipase in human milk evidence of activity in vivo and of a role in the digestion of milk retinol esters. Pediatr. Res. 12, 1048-1052. 

Dietary retinyl esters, retinol, and provitamin A carotenoids are dispersed and emulsified in the stomach during the gastric phase of lipid digestion. They then enter the lumen of the duodenum where extensive hydrolysis of retinyl esters takes place. In infants, the bile salt-stimulated lipase in human milk may play a role in retinyl ester hydrolysis in the duodenum (Fredrikzon et al., 1978). The major retinyl ester hydrolase activity that acts in the intestines is, however, derived from the pancreas. 

Several studies have been conducted on calcium-fat interactions in human infants (64-70). Low synthesis of bile salts and low pancreatic lipase activity may be responsible for poorer fat utilization in infants than in adults (63,71). Fat from infant formulas may be lower than that from human milk because of the lack of a bile-stimulated lipase in the former (72). In infants, fat absorption tends to decrease with increase in fatty acid length, with lower degree of saturation, and with increase of total fat (3). Triglyceride structure may also influence fat absorption in the infant and, thus, indirectly, might also affect calcium absorption in the infant. 

For pancreatic lipase to be active, an additional protein, termed colipase, is required (B26). Pure pancreatic lipase is inhibited by bile salts in concentrations exceeding their critical micellar concentrations (B27). The fiinc-tion of coUpase is to restore lipase activity in the presence of bile salts. Although colipase by itself has no lipolytic activity (B27), defective fot digestion and absorption occur if either lipase or colipase activity is low in the small intestine. Patients with steatorrhea due to either isolated lipase deficiency (F4) or isolated cohpase deficiency (H16) have been reported. A lipase which requires bile acids for activity is human milk lipase (Ol). This enzyme comprises 1% of the protein of human milk, but is inactive i ainst milk fots until its activity is stimulated by bile acids in the small intestine. 

Willstatter ef al. 196, 301) found that activation could be observed in an alkaline medium but not in an acid medium. Inhibition of tributyrin hydrolysis by bile salts at pH values below 7.0 was also observed by Click and King (297), and Wills (302) found that, although triolein hydrolysis by lipase was stimulated by sodium taurocholate in an alkaline medium, it was inhibited at pH values below 7.0. The effect of the bile salt may also depend on the length of fatty acid chain of the triglyceride under investigation. Thus Click and King (297) showed that, at pH 7.0, sodium taurocholate inhibited tributyrin hydrolysis but stimulated triolein hydrolysis. Similar results were obtained by Wills (302), who found that triacetin hydrolysis was inhibited and triolein hydrolysis stimulated by equivalent concentrations of sodium taurocholate. 

Various bile salts have effects on the hydrolysis rate that do not parallel their effect on the lowering of surface tension. Thus RotUin and Schalch (303) found the order of stimulation of lipase hydrolysis was cholic acid > taurocholic acid > deoxycholic acid, but the order for maximum decrease of surface tension was deoxycholic acid > taurocholic acid > cholic acid. Kawashima (304) found that chohc acid increased the synthetic activity of pancreatic lipase to a greater extent than did deoxycholic acid, and that bile itself was far more effective than chohe acid. The special efficiency of bile was assumed to be due to the presence of amino acids. 

Phylloquinone is highly lipophilic however, at low concentrations it is transported by a saturable, energy-dependent transport system across the gut wall, mainly in the upper small intestine. Phylloquinone in foods consisting of plant tissues is much less readily bioavailable for absorption than the pure vitamin since it is tightly bound to the thylakoid membranes of the chloroplasts, and the absorption of vitamin K from plant foods is considerably improved by including additional fat in the meal. Its absorption also depends on the stimulation of bile salt and pancreatic lipase secretions. The long-chain menaquinones, which are even more lipophilic, are only passively absorbed and are much less bioavailable for absorption than phylloquinone. However, if given by injection (e.g., intracardially), they can be even more functionally active than phylloquinone. 

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