Biological aspects of bile salt chemistry an investigation of the roles and possible roles of the bile salts in the body and in health

Abstract

Roles and potential roles of bile salts in the human body and in health have been investigated. Measurement of the effect or bile salts upon pharmaceuticals and pharmaceutical anologues showed that, at the bile salt concentrations and in the pH range of the duodenum, bile salts were unlikely to significantly influence either the stability or the effectiveness of these compounds. Similarly, it seems that the conditions of the duodenum appear to have been selected by Nature so that non-enzymatic hydrolysis of food compounds is limited and our work indicates that most bile salts tend either not to affect ester hydrolysis or to restrict it even further. The bile salts, however, can have a disruptive effect upon biological membranes and it is speculated that bile salt induced destruction of bacterial membranes in the colon might be a source of colon cancer. A consequence of the finding that there was no interaction (or at best only a very small interaction) between bile salts and water soluble compounds was that it enabled the interactions of bile salts and substrates with Human Milk Lipase (HML) to be investigate separately. Bile salts stimulate this enzyme against water soluble substrates which contain an ester linkage and activate it against oil phase substrates. Variations of the nature of the bile salts, their degree of protonation and their concentration indicate that the enzyme - bile salt interaction is of a surfactant or hydrophobic-hydrophobic binding type, essentially involving only the steroidal skeletal portion of the bile salt molecule. Only one bile salt molecule is required per binding site in order to stimulate the enzyme. The degree of stimulation decreases with increasing hydrophilic character of either the steroidal skeleton of the side chain and by the formation of large bile salt aggregates or micelles. It has been shown, by use of suitable substrates and by measuring reactivity over a range of pH values, that the enzyme-esterolytic site contains a histidine residue, but that this site is not affected by bile salt stimulation of the enzyme. Similarly, by choosing a further variety of substrates, it has been shown that the enzyme possesses a specific acyl binding site, which is hydrophobic in nature and wide enough to accommodate one fatty acid chain, but not two. This binding side is also unaffected by bile salt stimulation of the enzyme. Further, it has been shown that the enzyme possesses an alkyl binding site, which is also hydrophobic in nature and wide enough to accommodate two fatty acid chains side by side – eg. a diacylglycerol residue. This binding site, however is affected by bile salt stimulation of the enzyme, which causes an increase in its hydrophobic nature. This increased hydrophobic may be an important factor in infant nutrition, because it would allow Human Milk Lipase to remove oil phase substrate molecules from their hydrophobic environment in an initial stages of the digestion of fats. Investigations of the hydrolysis behaviour of p-nitrophenylacetate at various pH values and in various buffer solutions have been made. At physiological pH, extensive acyl transfer to phosphate species occurs, and this pathway may serve as an analogue for physiological phosphorylations.