INTERMEDIARY METABOLISM 



unchanged isotope concentration; if ^-tyrosine oi- (-/-glutamic acid 

 liad been formed in a normal synthetic process, it should have mixed 

 with the administered isotopic compound, and hence the excreted 

 amino acid should have had a lower isotope concentration. 



As a great variety of foreign substances, e. ff., the odd-numbered 

 fatty acids, are not only modified but completely metabolized by the 

 animal cell, it must be inferred that the organism is al)le to handle 

 substances "to which it is ordinarily accustomed" as well as many 

 others which do or do not bear a structural relationship to natu- 

 rally occurring compounds. Conversely, compounds of established 

 biological activity, such as the lipids, are found to be unaffected by 

 isolated tissues. In vitro, the higher fatty acids, cholesterol, and other 

 steroids fail to show any evidence of synthetic or degradative reactions 

 involving carbon-carbon bonds. Limitations of solubility and cell 

 permeability must, to some extent, impair in vitro activity toward 

 lipids and may be responsible for their apparent metabolic inertia. 



In view of our recent experiences with isotopes, another aspect 

 must be considered for the interpretation of these findings. The studies 

 of Schoenheimer and Rittenberg with isotopic test substances have 

 made it clear that the constancy of tissue composition in the normal 

 adult animal is maintained as a result of balanced synthesis and 

 degradation, and have provided an experimental basis for the concept 

 of dynamic equilibrium of the tissue constituents. It is conceivable 

 that a similar situation would obtain in isolated tissues so that a reaction 

 proceeds without over-all change of the substrate concentration. Evi- 

 dence that the dynamic equilibrium may obscure the recognition that 

 an in vitro reaction is proceeding may be seen in the following findings. 

 The utilization of acetic acid for cholesterol synthesis, first demon- 

 strated in intact animals with the aid of isotopic acetate (4), has re- 

 cently been observed to occur also in isolated tissue (unpublished re- 

 sults of Bloch, Borek, and Rittenberg). When liver slices were in- 

 cubated with deuterioacetic acid, deuteriocholesterol was formed at a 

 rate similar to that in the liver of intact animals. As the total amount 

 of cholesterol does not change noticeably under these conditions, it 

 is clear that a quantity equivalent to that synthesized had simultane- 

 ously been degraded. The sensitivity of the isotope method is such 

 that syntheses corresponding to as little as 0.1% of the cholesterol 

 present can be detected. Experiments in which liver slices were in- 



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