ELECTRON-MICROSCOPIC OBSERVATION OF COTTON CELLULOSE DEGRADATION BY EXO-TYPE AND ENDO-TYPE CELLULASES FROM IRPEX-LACTEUS

The interaction of two highly purified cellulases, exo- and endo-type cellulases from Irpex lacteus, with pure cotton and amorphous cellulos e was investigated by electron microscopy. The morphological observati ons indicated that exo- and endo-type cellulases are both strongly ads orbed on the internal microfibril of cotton fiber before enzymatic hyd rolysis, and then initiate their action toward the internal cellulose microfibrils with retention of the original shape. The two cellulases, however, caused considerably different morphological changes in cotto n cellulose, and each cellulase seems to degrade native cellulose with a distinct mode of action. In the hydrolysis of cotton with exo-type cellulase, deep transverse cracks were produced and they extended from the fiber surface to the lumen structure located inside the fibers. I n contrast, it was found that there were no deep cracks on fibers trea ted with endo-type cellulase, but severe internal erosion and cavitati on occurred along fibril or microfibril layers inside the fibers. Thus , the degradation of cotton by exo- and endo-type cellulases yielded q uite different morphological patterns, while little difference was fou nd for regenerated celluloses. The mode of enzymatic hydrolysis of cel lulose shown by cellulases with different degrees of randomness (exo a nd endo types) appears to be markedly affected by the fine structure o f cellulose fibers.