Properties of the digestive enzymes and the permeability of the peritrophic membrane of Spodoptera frugiperda (Lepidoptera) larvae

Abstract

The physical and kinetic properties of several soluble and detergent-solubilized membrane-bound midgut hydrolases of Spodoptera frugiperda (Lepidoptera) were investigated. The following techniques were employed: isoelectric focusing and electrophoresis in polyacrylamide gels, density-gradient ultracentrifugation and gel filtration in Superose columns. In vivo molecular weights of the hydrolases were considered to correspond with those determined by centrifugation, which is the more gentle of the procedures employed. Using this criterion, the soluble hydrolases display the following number of sub-units: acetylglucosaminidase [pHo5 (pH optimum), Mr 123,000], 2; aminopeptidase (pHo 7.4, Mr 107,000), 2; amylase (pHo9.6, Km for starch 0.38%, Mr 87,000), 1; carboxypeptidase A (pHo 8.0, Mr 45,000), 2; cellobiase (pHo 7.0, Mr 124,000), 2; dipeptidase (pHo 8.0, Mr 95,000), 2; maltase (pHo 5.0, Mr 150,000), 2; trypsin (pHo 7.9, Mr 51,000), 2. Amylase is not activated by chloride. A comparison between the diameters of the enzymes which pass through the peritrophic membrane (amylase, carboxypeptidase and trypsin) with that which is secreted into the lumen but do not pass through the peritrophic membrane (acetylglucosaminase) suggests that the pores of S. frugiperda larval peritrophic membrane have diameters of 7.5–8 nm.