alpha-galactosidases from the larval midgut of Tenebrio molitor (Coleoptera) and Spodoptera frugiperda (Lepidoptera)

There are three midgut alpha -galactosidases (TG1, TG2, TG3) from Tenebrio molitor larvae that are partially resolved by ion-exchange chromatography. The enzymes have approximately the same pH optimum (5.0), pI value (4.6) an d Mr value (46 000-49 000) as determined by gel filtration or native electr ophoresis run in polyacrylamide gels with different concentrations. Substra te specificities and functions were proposed for the major T. molitor midgu t alpha -galactosidases (TG2 and TG3) based on chromatographic, carbodiimid e inactivation, Tris inhibition, and on substrate competition data. Thus, T G2 would hydrolyse alpha -1,6-galactosaccharides, exemplified by raffinose, whereas TG3 would act on melibiose and apparently also on digalactosyldigl yceride, the most important compound in the thylacoid membranes of chloropl asts. Most galactoside digestion should occur in the lumen of the first two thirds of T. molitor larval midguts, since alpha -galactosidase activity p redominates there. Spodoptera frugiperda larvae have three midgut alpha -ga lactosidases (SGI, SG2, SG3) partially resolved by ion-exchange chromatogra phy. The enzymes have similar pH optimum (5.8), pI value (7.2) and Mr value (46000-52000), and at least the major alpha -galactosidase must have an ac tive carboxyl group in the active site. Based on data similar to those desc ribed for T. molitor, SG1 and SG3 should hydrolyse melibiose and SG3 should digest raffinose and, perhaps, also digalactosyldiglyceride. The midgut di stribution of alpha -galactosidase activity supports the proposal that alph a -galactosidase digestion occurs at the surface of anterior midgut cells i n Spodoptera frugiperda larvae. (C) 2001 Elsevier Science Inc. All rights r eserved.