BIOCHEMICAL EVOLUTION 



343 



of ureotelic Vertebrates (p. 309). The new function appears as the result 

 of a speciaHzation in which arginase has been added to the enzyme system 

 bringing about the synthesis of arginine (p. 244). 



(c) Specialization by Loss of Constituents 



An enzyme system may be specialized not only by the acquisition of 

 new enzymes, but also by the loss of certain existing ones. An example is 

 the enzyme system of uricolysis. The most complete form of this system is 

 found in marine Crustaceans and is shown in Fig. 91. 



Most Insects only carry uricolysis to the stage of uric acid. The form 

 of the uricolytic system in Insects compared to the most primitive form of 

 the Crustaceans, is characterized by the disappearance of urease, allan- 

 toicase, allantoinase and uricase. The enzyme system for purine break- 

 down consists of uricase, allantoinase and allantoicase in the Batracians, it 

 consists of only uricase in the Mammals, with the exception of the 

 Primates who have lost the complete system of enzymes as likewise have 

 the terrestrial Reptiles and Birds. 



{d) Specialization by Introduction of a Constituent of a Primitive System 



into a more Modern System 



Rhodopsfn 



Vitamin Ax + Opsin"^ 



Vitamin Aj of 



the pigmented 



epithelium and 



of the circulation 



DPNH -i- H t- \ 



/ Systems of \ 

 y oxidation J 



DPNH -'- H+ 



\^ Light 



\ 



^ Lumi-rhodopsin 



I -20='C. 



Meta-rhodopsin 



Retincne -p Opsin 



Fig. 92 (G. Wald). — The rhodopsin system. 



A frequent form of evolution of enzyme systems and their associated 

 systems (substrates, enzymes, coenzymes, hormonal regulators, etc.) is 

 by the introduction of one or several of their components into a more 

 specialized system. We have already noted (p. 290) the more specialized 

 nature of rhodopsin, present in the retina of salt-water fish, reptiles, birds 

 and mammals, compared to the more primitive porphyropsin of fresh- water 



