ENZYME VARIABILITY 



485 



this prediction is borne out. But there are other instances where unex- 

 plainable specific effects are directed towards certain tissues, such as the 

 selective pancreatic effect of alloxan. 



ENZYME VARIABILITY 



It is generally accepted that the enzymes of different species or of dif- 

 ferent tissues of the same species often differ with respect to certain prop- 

 erties, even though the enzymes are all included under the same name. 

 The susceptibility of these enzymes from different sources also varies. But 

 recently a further type of variability has been recognized (Kalow and Da- 



Table 9-2 

 Inhibition of Human Serum Cholinesterases " 



" From Kalow and Davics (1958) 



vies, 1958). Human sera were selected on a genetic basis and postulated 

 to contain either the usual serum cholinesterase, an atypical variety of 

 cholinesterase, or both enzymes. Inhibition-concentration curves for six- 

 teen different inhibitors showed that generally both enzymes occurred 

 but in quite different proportions. These two cholinesterases differ very 

 markedly in their susceptibility to inhibitors (Table 9-2). The curves for 

 certain sera and inhibitors showed very clearly the superposition of two 

 curves representing the effect on each enzyme. The active centers of these 

 enzymes must differ significantly. If the occurrence of two different varie- 

 ties of enzyme were not recognized, inhibition experiments on serum might 

 lead to some very erroneous conclusions as to the x^roperties of the assumed 

 single enzyme. Furthermore, results on the sera from different individuals 



