MIDGUT DIPEPTIDASES FROM RHYNCHOSCIARA-AMERICANA (DIPTERA) LARVAE - PROPERTIES OF SOLUBLE AND MEMBRANE-BOUND FORMS

Dipeptidase activity in Rhynchosciara americana (Diptera: Sciaridae) i s found mainly in midgut caeca cells, The caecal dipeptidase activity is partly soluble and partly membrane- bound, Differential centrifugat ion of midgut caeca homogenates, followed by assays of enzyme markers and dipeptidase, suggest that soluble dipeptidase is cytosolic or weak ly associated with the cell glycocalyx, Membrane-bound dipeptidase is likely a microvillar enzyme, Soluble dipeptidase activity is resolved by gel filtration and ion exchange chromatography into two enzymes (M, 63,000 and 73,000), which hydrolyze both Gly-Leu and Pro-Gly, althoug h with different efficiency, The two enzymes also differ in their stab ility in the presence of EDTA and degree of inhibition by phenanthroli ne and aminoacyl hydroxamates. Dipeptidase inhibition by phenanthrolin e is reversed by dialysis, Membrane-bound dipeptidase activity was sol ubilized by Triton X-100 and papain, Density-gradient ultracentrifugat ion, gel filtration, and ion-exchange chromatography suggest that ther e is only one detergent (M, 86,000) form of this enzyme, which is acti ve on Gly-Leu and Pro-Gly. No activity upon Gly-Pro was found in R, am ericana midguts, whereas the weak activity observed upon carnosine is independent of the enzymes hydrolyzing Gly-Leu and Pro-Gly, Thus, R, a mericana midguts seem to have major soluble and membrane-bound dipepti de hydrolases (EC 3.4.13.11), which in contrast to the mammalian enzym e, are very active upon Pro-Gly, R, americana also has a minor carnosi nase (EC 3.4.13.3).