MIDGUT BETA-D-GLUCOSIDASES FROM ABRACRIS-FLAVOLINEATA (ORTHOPTERA, ACRIDIDAE) - PHYSICAL-PROPERTIES, SUBSTRATE SPECIFICITIES AND FUNCTION

A combination of gel filtration, ion-exchange chromatography, polyacry lamide gel electrophoresis, and heat inactivation data revealed the ex istence of three beta-glucosidases with M(r) 82,000 in Abracris flavol ineata midgut contents: 1, a major heat-stable activity against cellob iose (cellobiase-argl beta-glucosidase); 2, a minor heat-unstable acti vity against p-nitrophenyl beta-D-glucoside (NP beta Glu) (aryl beta-g lucosidase); 3, an activity against octyl-beta-glucoside (alkyl beta-g lucosidase). The cellobiase-aryl beta glucosidase has a pH optimum of 5.5 and is more active on cellobiose and laminaribiose than on synthet ic or natural aryl beta-glucosides. Experiments involving competition between substrates and the use of inhibitors suggested that cellobiase -aryl beta-glucosidase hydrolyzes cellobiose and aryl beta-glucosides at different active sites. Alkyl beta-glucosidase (pH optimum 4.8) has a sigmoidal activity-octyl beta-glucoside-concentration profile, whic h changes to a hyperbolic profile in the presence of excess Triton X-1 00, NP beta Glu, which is hydrolyzed at the same site as octyl beta-gl ucoside, has a hyperbolic activity-NP beta Glu-concentration profile t hat increases in the presence of Triton X-100. It seems that amphipath ic molecules activate the alkyl beta-glucosidase, which is inactive on methyl beta-D-glucoside and is most active on C-7-C-10 alkyl-beta glu cosides. The aryl beta-glucosidase activity of the cellobiase-aryl bet a-glucosidase and the alkyl beta-glucosidase are probably responsible for in vivo digestion of beta 1,3-glucans and glucosylceramides, respe ctively. Activation by detergent-like molecules is supposed to maintai n high alkyl beta-glucosidase activity only during plant cell membrane digestion, This avoids extensive hydrolysis of toxic plant beta-gluco sides which may be ingested by the insects.