Blood digestion in the mosquito, Anopheles stephensi Liston (Diptera: Culicidae): partial characterization and post-feeding activity of midgut aminopeptidases

Aminopeptidase activity was partially characterized from midguts of Anopheles stephensi Liston which had been dissected 30 h after blood feeding. In crude midgut homogenate supernatants the aminopeptidases showed optimum activity at pH 8.0 and preferentially hydrolyzed alanine- and leucine-terminal amino acid substrates. Methionine, proline, lysine, and arginine terminal substrates were hydrolysed, but not glutamic acid. Activity was stimulated by Mg2+, EDTA, and low Ca2+ concentrations, while Mn2+, Tris, 1,10 phenanthroline, and higher Ca2+ concentrations were inhibitory. Supernatants from midguts homogenized in 1% Triton X-100 showed a two-fold increase in activity. Differential centrifugation of midgut homogenates demonstrated 45% of the total activity in a putative microvillar pellet and 32% in a soluble fraction. More than 92% of the total activity was solubilized after homogenization in Triton X-100. Activity in homogenate supernatants was restricted to one major peak (Mr = 552,000) with a higher molecular weight shoulder. Three distinct peaks of aminopeptidase activity were observed following Triton X-100 treatment: a minor high molecular weight peak (Mr = 552,000), and two major peaks at Mr = 123,000 and Mr = 32,000 respectively. The activity of aminopeptidase increased after a blood meal, in parallel to the post-feeding changes in trypsin activity, indicating its important role in secondary digestion of blood meal proteins.