198 



METABOLIC HORMONES 



glucose. The optical isomers are unresponsive to insulin (Fig. 

 5-10). This suggests a point of further chemical attack upon the 

 problem of the nature of the hormone action in lowering blood- 

 sugars and making them available to the tissues ; but the relation 



Insulin responsive 



D- Glucose 



D- Galactose 



L-Arabinose 



Insiitin unresponsive 



! CHO j CH2OH I CHO i 



HO — C — H 0=0 HO — C — H 



__j. [ , I 



1 HO — C — H I I HO — C — H I H — C — OH 



H — C — OH H — C — OH H C — OH 



H— C — OH H — C — OH CH2OH 



CH2OH CH2OH 



D-Monnose D-Fructose D-Arobinose 



Fig. 5-10. The structure of some of the sugar molecules which 

 have been found to be responsive to insulin and to resemble 

 glucose in having the same side chains on the three terminal carbon 

 atoms ; and (below) three molecules of sugars which differ, however 

 slightly, from glucose, and are found to be unresponsive to insulin 

 (from Levine and Goldstein in Stetten and Bloom, 1955). 



of these facts to the known chemical structure of the insulin, which 

 is a protein, is not yet within sight. 



The rate of secretion of insulin is self-regulating, in that a high 

 level of blood-sugar increases insulin ; and, as this lowers the level, 

 so the insulin secretion itself is reduced. 



