Properties of digestive glycosidases and peptidases and the permeability of the peritrophic membranes of Abracris flavolineata (Orthoptera: Acrididae)

Abstract

In Abracris flavolineata midguts, cellulose is hydrolyzed by at least three enzymes, whereas the most active hemicellulases are a laminarinase (pH optimum 5.7, M r 146 k) and three lichenases (pH optima in the range 5–7.3; M r values: 22, 71, and 97 k). Digestion of hemicellulose is completed by β-glucosidases described elsewhere and by an α-mannosidase (pH optimum 4.7, K m 1.7 mM). There are a major and two minor amylases (pH optimum 6.5; M r values: 42, 45, and 108 k) activated by chloride. Two of the α-glucosidases ( M r 74 and 94 k) are active on maltose and hence should finish the digestion of starch. It is not clear what is the natural substrate of the remaining α-glucosidase ( M r 93 k). The major α-galactosidase ( M r 112 k) is active on melibiose and raffinose, whereas the minor ( M r 70k) may be active on digalactosyl diglycerides. The effect of pH on azocasein hydrolysis and electrophoresis data suggest that trypsin is a major and chymotrypsin and cysteine proteinase are minor enzymes. The cysteine proteinase may derive from the leaves ingested by the grasshopper, taking into account its activity in leaves. Protein digestion is finished by two soluble aminopeptidases ( M r 92 and 105 k), a major membrane-bound aminopeptidase ( M r 97 k) and two membrane-bound dipeptidases ( M r 87 k). The sizes of the digestive enzymes recovered in A. flavolineata crops suggest that the pores of the semi-fluid caecal peritrophic membrane have diameters larger than 8.6 nm.