LITERATURE REVIEW 

 Polyphenol Oxidase 



Polyphenol oxidase (PPO) (E.C. 1.14.18.1.). also known as 

 tyrosinase, polyphenol ase, phenol ase, catechol oxidase, cresolase, and 

 catecholase, is widely distributed in nature (Schwimmer, 1981). In 

 addition to its general occurrence in plants, it can also be found in 

 microorganisms, especially fungi, and in some animal tissue (Brown, 

 1967). PPO plays an important role in the resistance of plants to 

 microbial or viral infections and probably to adverse climate (Vamos- 

 Vigyazo, 1981). The vast literature dealing with the role of the PPO- 

 polyphenol system in plant pathology has been extensively reviewed 

 (Bonner, 1957; Parkas and Kiraly, 1962). A simplified explanation of this 

 role in the resistance of plants to infections is that the quinones formed 

 upon the action of the enzyme undergo secondary polymerization reactions 

 yielding dark, insoluble polymers; the tissues impregnated with these 

 polymers act as barriers to prevent further spreading of the infection. 

 This is considered by some authors to be the main function of the enzyme 

 (Macrae and Duggleby, 1968). However, plants resistant to adverse 

 climatic conditions have, in general, higher PPO activities than 

 susceptible varieties (Khrushcheva and Krehin, 1965). 



In addition to the involvement in phenolic compounds biosynthesis, 

 the PPO enzymes also indirectly participate in auxin biosynthesis. The 



