Surface chemistry of the monoglyceride-bile salt system: Its relationship to the function of bile salts in fat absorption

Abstract

Ingested triglyceride is hydrolyzed by pancreatic lipase to yield monoglycerides and fatty acids, which are dispersed by bile salts into aggregates of micellar size. To determine the molecular arrangement of these aggregates, the surface interaction of appropriate fatty acids, glycerides, and their analogs with synthetically prepared, pure bile salts varying in conjugation, number, and position of hydroxyl groups and type of A/B ring juncture has been characterized. Monoolein was readily penetrated by bile salts. The degree of penetration was proportional to the intrinsic surface activity of the bile salt, and this correlated with the number and position of hydroxyl substituents: taurocholate <tauroallodeoxycholate (5α) < taurodeoxycholate ≦ taurochenodeoxycholate < taurolithocholate. Curves of π A and Δ V A indicated that at high areas the bile salt nucleus lies parallel to the interface; as the area was reduced, the nucleus became perpendicular and the ionic head of the bile salt was forced into the aqueous phase. With further compression, dihydroxy and trihydroxy bile salts separated from the monolayer and passed into the bulk phase; monohydroxy bile salts remained associated with the film, reflecting their greater surface activity. Penetration of monoglycerides, monoglyceride analogs, diglycerides, and triglycerides of oleyl and stearyl homologs indicated that the interaction of bile salts with monoglycerides was much stronger than that with higher glycerides, fatty acids, or propylene glycol monoesters. At 25°C, penetration did not occur unless spacing between the polar heads of monoglycerides was present. The mechanism of penetration was considered to be hydrogen bonding between the hydroxyl groups of the bile salt nucleus and those of the glycerol moiety of the monoglyceride. Monoglycerides form a mesomophic phase in water. Bile salts adsorb to the presumably lamellar or cylindrical arrays and round them off into smaller aggregates; a spherical model of the bile salt-monoglyceride-fatty acid aggregate is proposed. In this model, bile salts function as wetting agents, rather than as detergents, since they adsorb to the interface and do not penetrate the hydrocarbon chains. The ability of bile salts to penetrate films of monoglycerides plus the existence of a mesomorphic phase in the water-poor region of the ternary composition phase diagram of the bile salt-water monoglyceride system suggests that a mesomorphic phase of bile salt-water-monoglyceride and fatty acid may occur during pancreatic lipolysis at the oil/water interface. Thus the mechanism proposed by Lawrence for spontaneous detergence may well occur in biological systems.