CELLULAR BIOCHEMISTRY 



insulin is able to exert its effect, i.e., by increasing permeability 

 of the cell membrane for glucose or by displacing an inhibitor. 

 Animal hexokinase, in contrast to that of yeast, is noncompeti- 

 tively inhibited by glucose-6-phosphate, and competitively 

 inhibited (with respect to ATP) by ADP (4). If (3) were the 

 necessary condition, insulin would accelerate an enzymatic step 

 other than the first, or in some other way counteract the product 

 inhibition. 



Thyroxin 



The effect of thyroxin and triiodothyronine in uncoupling 

 aerobic phosphorylation in isolated mitochondria is obtained not 

 only with dinitrophenol but also with a number of other unre- 

 lated substances, none of which can replace thyroxin in thyroid 

 deficiency. It may be difficult to show that this is the primary 

 effect in the intact animal. The greatly increased oxygen con- 

 sumption in the hyperthyroid animal may well more than com- 

 pensate for the decreased efficiency of aerobic phosphorylation 

 and could be the primary effect. 



This discussion about the action of epinephrine, insulin, and 

 thyroxin makes it clear how difficult a problem it is to unravel 

 the mechanism of their actions, not to speak of a number of other 

 hormones the study of which has just begun. The task is made 

 somewhat easier if one adopts the point of view that hormones 

 are specific in their action, that they influence only one kind of 

 reaction, and that the apparent multiplicity of their effects is 

 secondary to a primary effect. The metabolic hormones which 

 have been most extensively studied from a biochemical point of 

 view have more or less conformed to this pattern. 



References 



1. Ashomore, I., A. B. Hastings, and F. B. Nesbitt, Proc. Natl. Acad. Sci. 

 U.S., 40, 673 (1954). 



2. Beam, A. B., B. H. Billing, and S. Sherlock, Cib a Foundation Colloquia on 

 Endocrinol., VI, 250 (1953). 



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