PHYSIOLOGY AND BIOCHEMISTRY 



antithrombic action of heparin because heparinized blood contains 

 the normal amount of prothrombin. Since, however, the nature of 

 the interaction of prothrombin, thromboplastin, calcium, and other 

 factors is at present unknown, it is not possible to indicate the particular 

 reaction in clotting prevented by heparin. 



It can be seen that many questions remain unanswered in the 

 heparin field, the solution of the problems depending upon the develop- 

 ment of new methods and experimental approaches. 



The Lipotropic Factors 



The recognition of the significance of choline as a dietary factor, 

 and the rapid development of our knowledge of other lipotropic agents 

 such as betaine, methionine, and inositol, have opened a new chapter 

 in physiology and have stimulated almost as much experimental work 

 as did the isolation of insulin. The presence of choline or methionine 

 in the diet is essential for the survival of young animals of various 

 species. Death is apparently due to a failure of liver or kidney func- 

 tion. Three of the main problems demanding investigation are: 

 (a) the mechanism of action of lipotropic agents; (b) the physiological 

 significance of the lipotropic factors; and (c) the clinical significance 

 of the lipotropic factors. Choline apparently acts by virtue of its 

 incorporation as an intact unit into the molecule of phosphatidyl 

 choline (lecithin) ; and, as a result of this step, the rate of regeneration 

 of the choline-containing lipids of liver, kidney, blood, and perhaps 

 other tissue is increased. Betaine acts as a donator of one of its methyl 

 groups which, by methylating ethanolamine, forms choline. Me- 

 thionine is also a methyl donor; while it may also exert dramatic 

 efTects by providing a source of organic sulfur or perhaps by virtue of 

 its intact molecule, its lipotropic action is accomplished by donation 

 of its methyl group to ethanolamine, i. e., it acts as a precursor of 

 choline. Inositol, which has been well established as a lipotropic 

 factor under certain conditions, has been reported, without adequate 

 evidence, to aflfect particularly the cholesterol fatty liver produced by 

 various procedures. It may stimulate the regeneration of inositol- 

 containing phospholipids but this phospholipid has not, as yet, been 

 isolated from liver, the only tissue on which inositol has been shown to 

 exert a lipotropic action. 



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