60 THE PLANT CELL WALL 



Our broad view of wall substances calls for inclusion of col- 

 lagenase, an enzyme associated with the resorption of bone by 

 osteoclastic cells, and the proteolytic enzyme(s) which degrade 

 arthropodin in the insect cuticle. 



Lignin is one of the most resistant cell wall substances, but 

 it is slowly degraded by some wood-destroying fungi. As an 

 aromatic polymer, its degradation must proceed through oxidative 

 rather than hydrolytic cleavage. 



III. Surface Processes in Lignin Polymer Formations 



The synthesis of lignins in tissues has been studied in various 

 ways including its overall biogenesis from carbohydrate-level 

 constituents; the incorporation of radioactivity in the naturally 

 formed substance following the feeding of carbon- 14-labeled 

 precursors such as tyrosine phenylalanine, etc., or by the 

 "forced" production of lignin-like polymers from suitable pre 

 cursors. 



Each technique has contributed significantly, and uniquely, 

 to our concepts of the physiology and biochemistry and chemistry 

 of lignification, and each possesses limitations as well. 



A recent method of particular interest has centered about 

 forced, rapid synthesis of lignins from "Baustein" molecules in 

 the presence of high concentrations of hydrogen peroxide. This 

 procedure permits phenolic substances which may not be identical 

 with the native precursors to undergo transformation into lignins. 

 Nevertheless, a great deal has been learned by the application of 

 such molecules in model systems which could not have been made 

 evident by alternative methods of study. As we have already learned, 

 the rapid synthesis of lignin from, say, eugenol, is effected by the 

 cell wall itself rather than soluble or particulate components of 

 the protoplast. How shall we interpret such a finding? One explana- 

 tion which might be put forth is that the cell wall contains a neces- 

 sary biocatalyst bound, perhaps, to its very framework. Although 

 appreciable peroxidase resides in the washed cell wall, most of 

 this enzyme is located in the soluble cytoplasmic fraction. We must 

 conclude that this enzyme, which may be essential to polymer 



