ROLE OF BILE IN LIPID ABSORPTION 109 



the synthesis of choleic acid complexes. However, there are several con- 

 siderations which prompt the question as to whether or not such a mecha- 

 nism can have a direct application in vivo. 



In the first place, although desoxycholic and apocholic acids do readily 

 form coordination compounds with a wide variety of lipids acting as the 

 acholic components, these are the only two bile acids in which such an 

 in vitro reaction can be demonstrated. Actually, desoxycholic acid is a 

 minor constituent in the bile of most animals, and apocholic acid is a syn- 

 thetic product never found as such in normal bile. Cholic acid, which has 

 a wide distribution in the bile of various species and which, quantitatively, 

 is usually the most important of the bile acids, is completely impotent in 

 forming choleic acids. 



Secondly, while free desoxycholic acid can combine with fatty acids and 

 other lipids in vitro to form the choleic acids, conjugated desoxycholic 

 acids do not react in this way. Wieland and Stender 653 reported, in 1919, 

 that taurodesoxycholic acid lacked the capacity of choleic acid formation, 

 while Cortese and Bauman 550 later noted the inability of glycodesoxy- 

 cholic acid to participate in such a reaction. 



On the other hand, Verzar and McDougall, 654 in their monograph on 

 absorption from the intestine, emphasized the fact that the conjugated 

 cholic acids facilitate the solution of fatty acids. This property is referred 

 to as hypertrophism. Hypertrophic substances are defined as compounds 

 which can render water-insoluble substances water-soluble. 555 Their action 

 cannot be explained by any influences on membrane permeability. 



Using drop formation as measured with the Traube stalagmometer as the 

 criterion, Verzar and Kuthy 556 have shown that oleic, palmitic, and stearic 

 acid solutions in glycocholic acid have a markedly depressed surface tension 

 within the physiological range of pH 8 to 6. The effect on surface tension 

 is not observed from pH 8 to 11, where the fatty acids are present as the 

 alkaline soaps, or below pH 6 where the complex is again broken down to 

 free fatty acids and the bile salt. Since the smaller the difference in surface 

 tension occurring at the interface of two liquids, the higher is the solubility 

 of one in the other, it is believed that "water-insoluble substances can be 

 dissolved in an aqueous solution of a hydrotropic substance, owing to the 

 surface force between them being remarkably decreased." 554 The effect 



563 H. Wieland and H. Stender, Z. physiol. Cheni., 106, 181-189 (1919). 

 664 F. Verzar and E. J. McDougall, Absorption from the Intestine, Longmans, Green, 

 London and New York, 1936. 



566 C. Neuberg, Biochem. Z., 76, 107-176 (1916). 



556 F. Verzar and A. Kuthy, Biochem. Z., 210, 281-285 (1929). 



