Intestinal absorption of bile salts

Bile acids are secreted from the liver into the duodenum where 
they aid in the digestion and absorption of dietary lipids. Absorption of bile acids 
occurs through both ionic and nonionic diffusion in the jejunum and colon and 
through an active sodium ion-dependent carrier mechanism in the ileum. The 
prima, y bile acids synthesized in the liver can be converted by intestinal bacteria 
into secondary and tertiary bile acids. Bile acids may also be conjugated with 
glycine or taurine which results in an increase in the hydrophilicity and solubility 
of these compounds at physiological pH. The amount of passive diffusion of bile 
acids that occurs across the brush border membrane along the length of the entire 
intestine depends upon the ratio of ionized to nonionized bile acids coupled with 
the bile salt concentration and the individual permeability coefficients of 
monomers. Active transport of both conjugated and nonconjugated species of 
bile acids depends upon the presence of a single negative charge on the side chain. 
Maximal transport rates for bile acids are related to the number of hydroxyl 
groups present while the Michaelis-Menten constant for transport is dependent 
upon whether or not the bile acid is conjugated. Although active uptake of bile 
acids from the ileum has been considered the major route for bile salt absorption 
in the small intestine, the mechanism may actually be responsible for only a small 
proportion of the total bile acid pool absorbed from the lumen.

T HE SECRETJON OF BILE FROM THE liver into the small intestine plays an important role in the efficient absorption of dietary lipids from the gut lumen.Bile is a complex mixture of bile acids, bile pigments, cholesterol, phospholipids, inorganic electrolytes and protein, which is secreted from the hepatocyte into the bile canaliculus and transported to the ga llbladder for storage.Following ingestion of a meal, bile is released into the duodenum where bile acids aid in the digestion and absorption of dietary lipids.The bile acids are absorbed either by ionic or nonionic diffusion in the small intestine and colon ( l ), and via a process mediated by a sod ium ion-dependent carrier in the terminal ileum (2).Although active absorption of bile acids by the ileum has trad itionally been regarded as t he main pathway of absorption, this mechanism may actually be responsible for only a small proportion of the total bile acid pool absorbed from the lumen.Following intestinal absorption, bile acids return to t he liver via the portal vein, where more than 95% are removed on the first pass though the liver.This enterohepatic circulation allows for conservation of the bile acids.Only 10% of the total bile acid pool is et a l'oeuvre clans !'ileum.Les acides biliaires primaires synrhetises clans le foie peuvent etre convertis en acides biliaires secondaires et tertiaires par des bacteries intestinales.Ils peuvent egalement se conjuguer avec le glycine ou la taurine, ce qui se traduit par l'hydrophilie et la solubilite accrues de ces composes au pH physiologique.Le degre de diffusion passive des acides biliaires qui se produit sur toute la longueur de l'intestin a travers la bordure en brosse, depend du rapport entre les acides ionises/non ionises, ainsi que de la concentration des sels biliaires et des coefficients de permeabilite individuels des monomeres.Le transport actif des types conjugues et non conjugues d'acides biliaires depend de la presence d'une seule charge negative de la chaine laterale.Les taux maximaux de transport des acides biliaires sont lies au nombre de groupes hydroxyle presents alors que le Kn varie selon que l'acide biliaire est conjugue ou pas.Bien que !'on ait estime que le captage actif des acides biliaires clans l'ileon constitue le voie principale d'absorption des sels biliaires clans l'intestin grele, ii se pourrait que le mecanisme ne soit en fait responsable que d'un petit pourcentage de la quantite totale du pool des acides biliaires absorbes a partir de la lumiere.lost in the feces per day.This daily loss is balanced by hepatic synthesis of new bile acids from cholesterol.The total bile acid pool in humans is 3 to 5 g, which circulates six to 10 times every day, resulting in over 18 g of bile acids entering the gut per day (3).

PHYSIOLOGICAL ROLE
Bi le sa lts consist of a steroid hydrophobic nucleus with hydrophilic hydroxyl litld amide linkages.At a concentration range of 0.6 to 10 mM (the critical micellar concentration), bile salt molecules aggregate with their hydrophobic sides together formi ng micelles ( 4 ).In this way, bile salt micelles can make soluble molecules that are otherwise water insoluble, eg, cholesterol, lecithin, lipovitamins and monoglycerides, to form mixed micelles ( 4).The critical micellar concentration varies depending on the individual bile salts present, temperature, pH, presence or absence of lipids, and ionic concentration of the solution (5) .Above the critical micellar concentration, bile salt monomers and micelles exhibit an equilibrium, with the monomeric concentration not exceeding the critical micellar concentration (5).hydrolysis (7).Mixed micelles consisting of fatty acids, monoglycerides, lecithin, lysolecithin, diglyceride, cholesterol and bile salts pass through the unstirred water layers to enterocytes where the digestion products passively enter the epithelial cell (8).
Bile secretion also provides a mechanism for the excretion of cholesterol and exogenous drugs (9).Not only are bile acids continually synthesized from cholesterol, but conjugated bile salts make cholesterol and lecithins soluble in mixed micelles (10).The secretion of these mixed micelles in bile provides for loss of cholesterol in feces.

PRIMARY
CHEMISTRY OF BILE ACIDS Bile acids consist of a family of molecules formed from cholesterol (Figure 1).Two of the bile salts, known as primary bile salts, are synthesized in the liver in humans -cholic acid and chenodeoxycholic acid (11).Intestinal bacteria are able to convert primary bile acids into secondary and tertiary bile acids via three major mechanisms: deconjugation ( thereby releasing unconjugated bile acids); 7-dehydroxylation converting cholic acid to deoxycholic acid and chenodeoxycholic acid to lithocholic acid; and 7dehydrogenation converting chenodeoxycholate to 7-oxolithocholate, which can be epimerized to ursodeoxycholate ( 12).While bile normally contains only very small amounts of secondary and tertiary bile acids ( less than 2 to 5 M), feces and urine contain a large number of different fonns of bile acids (13,14 ).This is mainly due to the inefficient absorption of uncommon bile acids from the colon ( where most are produced by bacterial action) and the efficient conversion of uncommon bile acids to common forms by reduction, hydroxylation, and epimerization in the liver (15 ).
After synthesis, bile acids are conjugated with glycine or taurine via the Cs branched side chain (16).Conjugation serves to increase the hydro-

SECONDARY TERTIARY
Bile salt micelles form a one-to-one complex with colipase (6).In turn, colipase binds to lipase in a one-to-one ratio (6).This binding interaction of micelles to colipase to lipase serves to facilitate removal of the lipolytic end products into the mixed micelles and prevent feedback inhibition of philicity and solubility of bile ac ids at physiological pH , thereby decreasing their ability to cross plasma membranes (16).Taurine conjugates (pKa 2.0) are more water soluble than are glycine conjugates (pKa 3 to 4) and generally trihydroxy bile salts a re more water soluble t han arc dihydroxy bile salts (12).At physiological pH, taurinc anJ glycine conjugates are complete ly ionized and water soluble, leading co the term 'bile salt'.'Bile acids' generall y refer to undissociated bile salts, con-Jugated or unconJugated, that a re only mildly water soluble.Bile acids and salts differ in several of their physicochemical properties, resulting in different cransport propenies (17).

INTESTINAL ABSORPTION OF
BILE ACIDS AND SALTS Bile salts differ greatly in polarity and have a t least three physiological routes of the entcrohepatic circulation that differ in rate, locanon, and species transported.These include the jejuna!passive route (fast}, the ilea!active and passive routes (intermediate), and the colonic passive route (slow).A large percentage of g lycine-conJugated dihydroxy bile acids are absorbed passively in the jejunum (18,19), while caurine and glycine conjugates of the dihydroxy and trihydroxy bile salts are absorbed actively in the ileum (l, 18) and unconjugated bile acids are absorbed from the colon (1,20).Passive diffusion: Bile acids, both conjugated and unconjugated, are absorbed passively at all levels of the intestine.This passive movement is characterized by movement down existing concentration gradients (lumen to portal blood); an increasing con centration of bile acids resulting in increased absorption; no saturation kinetics; no competitive inhibition between bile ac ids; and no effect of metabolic inhibitors on absorption (1) .
There a re two barriers which have to be crossed in order for the bile acid to enter the enterocyte -these are the unstirred water layer and the brush border membrane.Either one of these may be rate limiting.
Size and weight are two main facto rs that influence the rate of diffusion through the unstirred water layer, the diffusion coefficient being inversely proportional to size (21 ).Fo r t his reason, micelles exhibit a much slower rate of diffusion through the unstirred water layer than do b ile salt monomers, resulting in a slower rate of uptake from higher concentrations (greater than the critical m1cellar concentration) of bile acids (22).However, because micelles and ionized and nonionized monomers are m equi librium with each other above the critical micellar concentration, all three species will diffuse independently through the water layer and contribute to total absorption (23).
The acidic microclimate (pH 5.1 to 6.3) that exists at t he surface of the enterocytes (23) will also affect uptake of bile acids, as it is the luminal pH which determines the proportion of bile acids in ionized or nonionized fonn.Also, micelles have been shown to dissoc iate at a pH of 5 co 6 (24).The acidic microclimate would therefore favour micelle dissociation and the release of monomers to the epithelial surface for absorption.
In general, it 1s the ratio of ionized to nomomzed bile acids, as determined by incraluminal pH, coupled with the bile salt concentration and individual permeability coefficients of monomers, that will affect the relative proportion of bile acids absorbed passively.Passive nonionic diffusion: In the lumen of the small intestine, the pH varies from 5.5 to 6.5, causing about 50% of unconjugated bile salts (pKa 5 to 6.5) co be nonionized, while a very small percentage of t he glycine-conjugated bile sales (pKa 3.5 co 5.2) and no taurine-conjugaced bile salts (pKa less than 1) are non ionized.Under normal physiological conditions, only conjugated bile salts are present in the jejunum (25).Therefore, the major bile acids absorbed in the jejunum are glycine conjugates.However, decon-jugat1on of bile salts occurs to a large extent in the terminal ileum and colon, and colonic passive absorption of unconj ugaced bile acids occurs continuously througho ut the day (26).Nonionized species can readily diffuse through epithelial cell me mbra nes driven by a concentratio n gradient only.The rate of diffusion of bile acids across membranes has been found to be dependent on the number of hydroxyl groups present and the size of che group conjugated at C24 (27).T he passive permeability coefficients of monomers have been shown to decrease by factors of 3.4, 6.8 and 8.1, with the addition of a hydroxyl, glycine, or taurine, respectively ( I).Therefore, for nonionized bile acids, diffusion through the unstirred water layer becomes rate limiting, especially fo r micelle solut ions (28).Bile acid absorption from the micelle at t he membrane is believed to occur only for monomers present in the aqueous phase that are in equilibrium with t heir aggregates within the mice lie (28).There is no evidence chat t he entire micelle is taken up intact by the cell membrane (22).Ionized diffusion: The driving force for ionized diffusion is the electrochemical gradient across the membrane.However, biological membranes are relative I y impe rmeable to c h a rged molecules; therefore, the diffusion rate of negatively charged conjugated bile acids is very low (29).Based on an intraluminal activity of l mM, ionized bile salts will theoretically passively diffuse at the rate of 400 nmol/min/cm gut, compared to nonionizcd movement of 2000 nmol/mm/cm gut (30).Further evidence to support a low rate of diffusion for ion ized bile salts comes from nuclear magnetic resonance spectroscopy studies.These have shown chat at a high pH ( when bile acids are ionized), there is no appreciable movement of cho lic acid, ch enodeoxycho lic acid or deoxycholic acid across a model bilayer membrane (31).However, fo llowing proconation, a rapid equ ilibrium between the inner and outer monolayers of the membrane was observed (31).The presence of more than one type of bile acid in the same bilayer membrane was found co have no effect on the movement of any ocher type (3 1 ).
The relative contribution of ionic versus nonionic diffusion will depend on the intraluminal pH, the pKa of the bile acid, and t h e permeability and partition coefficients of the ionic and nonionic species.Active transport of bile acids: Bile salts are transpo rte<l in the ileum via a sod ium ion-depe nde nt bile acid cotransport system (29).Energy for active transport is provided by the extracellular tO in trace I lular sodium ion gradie nt ac ross the brush border membrane, that is maintained by Na+,-K+-ATPase activity on the basolateral membrane.Active transport is evidenced by the following observations: absorption can occur aga inst a concentration gradient (32,33); absorption is blocked by metabolic inhibito rs or a n aerobios is (29,32); absorption of bile acids follows saturation kinetics ( 18,32,34 ); individual bile acids inhibit absorption of ocher bile acids ( 18); absorption is dependent on the presence of sodium ions (33, 35,36); and transport can occur when transmural potential <lifference is zero (37) .
All natura ll y occurring bile salts h ave been s h ow n co be act ively transported (38).There appears to be no abso lut e require me nt fo r the presence of a hydroxyl group for a bile salt to be transported (38).However, while both conj ugated and nonconjugated species are t ransported , a single n egativ c harge on the side chain is required (38).The presence of two negative c harges results in very limited transport, and the introduction of a positive charge onto the substrate results in no transport at all (38).These structure-activity studies indicate that the initial recognition site for bile ac id transport must involve three elements: interaction between the scerol nucleus of the bile acid aml the carrier; a coulo mbic interaction between the negatively charged side chain and a posit ive ly charged s ite o n the transporter; and interaction between sodium ions and an anionic site on the transporter (35,39).
Photoaffinity labelling of jejuna! and ileal brush border me mbrane vesicles with phocolabile bile salt derivatives identified a 99,000 dalton polypeptide that was unique to the ileum, leading to the supposition chat it is this protein that is the sodium iondependent bile sale transporter ( 40).This 99,000 dalton protein is also labelled in the proximal tubules of the rat kidney ( 11) -an area that exhibits similar sodium ion-dependent bile salt transport (28), but not in liver ( 41 ), a nothe r site of sodium ion-dependent bile salt uptake ( 42).These studies suggest that a single me mbrane protein is involved in both renal and ilea! bile salt cotransport, and that sodium ion-dependent transport across the sinusoidal membrane of hepatocyces is mediated by a different membrane protein.
Several studies with brush border membrane vesicles have demonstrated that bile acid uptake is sodium ion specific (32,35).H owever, conflicting data exists as to whether or not sodium ion -dependent bile sa lt tra nsport is electroneutral ( 43) or electrogenic (36,44).While Lucke et al (36) and Wilson et al (44) observed a relationship between various diffusion potentials across brush bo rder membrane vesicles and the rate of sodium ion-dependent taurocholate uptake, Rouse et al ( 43) showed no effect of interchanging chloride, thiocyanate, or sulphate ions.Further studies will have to be done to determine clearly whe ther sodium ion-dependent bile salt transport is electrogenic or electrone ucral.
The Mic haelis-Menten constant (Km) of bile acid transport appears to be related to whether or not the bile acid is conjugated, as unconjugated species show a higher Km than do glycine or taurine conjugates (45).However , max imum transport races Om) are independent of conjugation, but instead are related to the number of hydroxyl gro ups (trihydroxyl>dihydroxyl> mon ohydroxyl) (45).It appears at this point that a single species o f bile transporter in the small intestine is responsible for the transport of all bile acids.This is evide nced by the inhibition kinetics exhibited between different pairs of bile acids (46).

CONTRIBUTION OF PASSIVE VERSUS ACTIVE TRANSPORT
The active uptake of bile salts from the ileum h as been considered quantitatively to be the major mechanism for bile salt absorption from the small intestine.This con cept has arisen from several different studies.First, the concentration of bile salts is maintained throughout the jej unum, and only begins to fall in the d istal ileum (29).Second, resection of the terminal ileum (4 7,48) but not the jej unum ( 47) results in extensive fecal bile salt loss.Finally, an efficient active transport mechanism operates in the ileum only ( 44 ).
The report by Dietschy (29) that bile salt concentration does nor fall except in the terminal ileum does nor necessarily mean that bile salt absorption has not occurred.C han ges in the volume of luminal contents by absorption of water wi 11 affect con centration measurements.If the concentration of bile salts remains constant when water is being absorbed, absorption of bile salts must be occurring concomitantly.If not, the bile concentration would increase.
Resection of the terminal ileum often involves removal of the ileocecal valve, allowing colo nic bacteria to mig rate proxima ll y (49).G reater colonization of bacteria may result in greater bile salt loss due to an increase in deconjugation of bile salts.
With extensive passive absorption of bile acids occurring in the jejunum, it would be expected that a lower concentration of bile aci<ls would reach the ileum.Therefore, an efficient active transport system occurring at low concentrations would be necessary in the ileum in order to conserve effectively any remaining bile acids after jejuna!passage.Studies that have shown distal absorption as outweighing proximal absorption have used low concentrations of bile salts, for which the active ileal pump would be expected to dominate (24).
McClintock and S hiau (24 ), using kinetic data derived from taurocholate active transport in the rat, calculated t hat active ileal absorption was not sufficient to recover the percentage of bile salts secreted.Passive absorption was shown to account for the maj ority 9( total bile salt conservation.Passive absorption was also shown to account for a significant portion of bile salt uptake in the pig (50) and the killifish (34), supporting the view that it is the passive jejuna!uptake of bile acids that is of most importance quantitatively in bile salt conservation.Also, Dowling et al (51) have demonstrated that ileectomized monkeys are able to reabsorb from 38 to 58% of the bile salts entering the sma ll intestine, suggesting a large quantitative role for the jejunum tn btle salt conservation.
It therefore appears that while the distal ileum is more efficient at ahsorbing bile salts Jue ro the presence of a

Figure l )
Figure l) Primary, secondary and tertiary bile salts of humans showing sites of synthesis and metabolism