Presence and properties of cellulase and hemicellulase enzymes of the gecarcinid land crabs Gecarcoidea natalis and Discoplax hirtipes.

Digestive juice from the herbivorous gecarcinid land crabs Gecarcoidea natalis and Discoplax hirtipes exhibited total cellulase activity and activities of two cellulase enzymes; endo-beta-1,4-glucanase and beta-1,4-glucosidase. These enzymes hydrolysed native cellulose to glucose. The digestive juice of both species also contained laminarinase, licheninase and xylanase, which hydrolysed laminarin, lichenin and xylan, respectively, to component sugars. The pH optima of beta-1,4-glucosidase, endo-beta-1,4-glucanase and total cellulase from G. natalis were 4-5.5, 5.5 and 5.5-7, respectively. In the digestive juice from D. hirtipes, the corresponding values were 4-7, 5.5-7 and 4-9, respectively. The pH of the digestive juice was 6.69+/-0.03 for G. natalis and 6.03+/-0.04 for D. hirtipes and it is likely that the cellulases operate near maximally in vivo. In G. natalis, total cellulase activity and endo-beta-1,4-glucanase activity were higher than in D. hirtipes, and the former species can thus hydrolyse cellulose more rapidly. beta-1,4-glucosidase from G. natalis was inhibited less by glucono-d-lactone (K(i)=11.12 mmol l(-1)) than was the beta-1,4-glucosidase from D. hirtipes (K(i)=4.53 mmol l(-1)). The greater resistance to inhibition by the beta-1,4-glucosidase from G. natalis may contribute to the efficiency of the cellulase system in vivo by counteracting the effects of product inhibition and possibly dietary tannins. The activity of beta-1,4-glucosidase in the digestive juice of D. hirtipes was higher than that of G. natalis.