30 Applied Biophysics 



contains an enzyme system which oxidizes catechol very rapidly 

 in the presence of hydrogen peroxide. This enzyme system is 

 involved in the natural hardening of insect cuticle. Both the 

 enzyme and natural polyphenol substrate are secreted into the 

 cuticle by specialized epidermal cells. The rate of natural tan- 

 ning is increased by abrading the outer layer of the cuticle and 

 impregnating the abraded layer with a concentrated aqueous 

 horseradish-peroxidase extract. Alternatively, the penetration 

 of catechol into the cuticle is increased by treating the cuticle 

 with a fat solvent and then immersing the insect in an aqueous 

 catechol substrate containing hydrogen peroxide. The catechol 

 is oxidized to the o-quinone inside the cuticle framework, but 

 the diffusion of the quinone within the membrane framework 

 does not take place uniformly owing to the mosaic structure, 

 and in this respect the insect cuticle differs from the simpler 

 homogeneous gelatin membrane. 



However, when we examine the pattern of enzymic tanning 

 which has been induced in the cuticle (figures 3d, 3e, 3/), we see 

 that it is similar to that induced by the nonenzymic tanning 

 with /'-benzoquinone (figures 3a, 3b, 3c). We note further, 

 that there is a general parallelism between the degree of induced 

 enzymic cuticle tanning produced by sensitizing the cuticle with 

 fat solvents such as hexane, heptane, benzene, ether, or chloro- 

 form, and the degree of nonenzymic tanning by /'-benzoquinone 

 induced by the action of these fat solvents on the protective 

 lipids in the cuticle framework. We conclude that access of 

 catechol to the cuticle enzyme receptors is similarly influenced 

 by a permeability factor or by competitive action of protective 

 . lipid on the structural protein-enzyme complex. 



Analogy between Insect Cuticle and Cell Membrane 



It may well be argued that the insect cuticle is a highly 

 specialized membrane which has little in common with the 

 more complex and submicroscopic cell membrane. But when 

 intact isolated insect tissues are treated with a fat solvent such 

 as chloroform, there is a large increase in tissue-peroxidase 

 activity, suggesting a similar sensitization of the bounding mem- 



