α-galactosidase activity in ingested seeds and in the midgut ofDysdercus peruvianus (Hemiptera: Pyrrhocoridae)

The midgut of Dysdercus peruvianus is divided into three main sections (V1-V3) and is linked through V4 to the hindgut. The distribution of α-galactosidase activity in the different gut segments of D. peruvianus females was studied. α-galactosidase hydrolyzes the trisaccharide raffinose, the major carbohydrate of cotton seeds, on which the insects live. In D. peruvianus midgut, α-galactosidase activity is mainly found in soluble fractions of V1 contents. However, a comparison between specific activities using different α-galactosidase substrates in cotton seed extracts, V1 tissue homogenate, and midgut contents suggested that the contribution of the enzymes from seeds may be very significant. Gel filtration on Sephacryl S-200 of samples from seed extracts, V1 tissue, and V1 contents revealed that in all samples raffinose hydrolysis is accomplished by α-galactosidases with similar Mr (30,000 ± 3,000) and does not involve the activity of a β-fructosidase. Thermal inactivation studies of extracts from the three sources suggested that there was only one molecular form of the insect α-galactosidase and that the activity found in V1 contents includes enzymes derived from the seed kernel. In insects fed with cotton seeds, the α-galactosidase activity increased in parallel with diet ingestion. In starved insects fed with tablets of sucrose plus raffinose, an increase in α-galactosidase activity was also observed, confirming that the insect is able to synthesize part of the gut enzyme. The results indicated that raffinose digestion starts in V1 utilizing α-galactosidases derived from the seed kernel and by an additional α-galactosidase synthesized by insect tissues. The action of α-galactosidases liberates galactose and sucrose, which are sequentially hydrolyzed by the major membrane-bound α-galactosidase releasing glucose and fructose in V1 and V2 lumina. Arch. Insect Biochem. Physiol. 34:443–460, 1997.