344 UNITY AND DIVERSITY IN BIOCHEMISTRY 



Vertebrates. Rhodopsin is a derivative of vitamin Aj and porphyropsin 

 is a derivative of vitamin Ag. The visual function of the A vitamins 

 is the only one it has been possible to demonstrate in animals other than 

 the mammals and birds. In the two latter, it plays the additional role of a 

 vitamin essential for the normal function of epithelial tissue. This last fact 

 shows us the development of a new biochemical system in which has been 

 inserted a biochemical component already utilized in another system. The 

 photoreceptor system of the rods in the retina of animals having a differen- 

 tiated eye furnishes us with another example of this type of evolution in an 

 enzyme system. Figure 92 indicates the changes taking place during 

 photoreception in the rhodopsin of the eye. Retinene is produced by the 

 reduction of vitamin A^. The enzyme catalysing the transformation was 

 first called retinene-reductase. We know today that it is alcohol- 

 dehydrogenase (p. 162) as has been shown by Bliss. This universally 

 distributed enzyme is found to have been inserted here into a new system 

 that is extremely specialized. 



The mechanisms of hormonal regulation provide us with many instances 

 of insertions into new systems. The secretion of milk, due to the bio- 

 chemical differentiation of one type of Mammalian cell (p. 306) is provoked 

 and controlled by prolactin, resulting from the biochemical specialization 

 of another type of cell, the adenohypophysis. But prolactin is secreted by 

 the adenohypophysis of fish, amphibians and reptiles. Its intervention 

 in the secretion of milk in mammals is thus an insertion into a new bio- 

 chemical system. 



Another example of the same type is the action of pitocin on the 

 mammalian uterus. This hormone is present in all Vertebrates and 

 acts in the control of water metabolism. Its action on mammalian uterus 

 demonstrates its insertion into a more specialized system. 



(e) Specialization of a Primitive Biochemical System by the Introduction 

 of a Constituent of another Primitive System 



One of the important aspects of the biochemical evolution of Vertebrates 

 has been the acquisition by the cells of the mesoderm of enzyme systems 

 for the biosynthesis of new types of steroid (heteromorphic evolution). 

 One of the physiological effects of this evolution is the ionic regulation 

 brought about by the action of the corticosteroid hormones at the urinary 

 tube. In the Amphibiae this action is established in conjunction with a 

 pre-existing system, that of the regulation controlled by the adeno- 

 hypophysis. 



The adaptation to terrestrial life in certain amphibians, such as the toad, 

 in fact depends on the ability to reabsorb water controlled by the active 

 principles of the hypophysis, and these substances are fundamental con- 

 stituents in these animals (see Jones, 1957). 



