Digestion of starch granules by α-amylases from the rice weevil, Sitophilus oryzae: effect of starch type, fat extraction, granule size, mechanical damage, and detergent treatment

Abstract

In vitro digestion of raw starch granules by α-amylases from the rice weevil, Sitophilus oryzae (L.), was studied. Digestion of starch granules isolated from ball-milled cereals, especially corn and wheat, was extremely slow. Relative to intact starch granules of rice, corn, and wheat, gelatinized samples were 550-, 2000-, and 3300-fold, respectively, more susceptible to enzyme action. Among commercial starch samples tested, granular potato starch was most resistant to α-amylases from S. oryzae. Relative to granular potato starch, activity against other tested starches ranged from 2.1-fold greater for tapioca starch to 15.9-fold greater for an unmodified corn starch. Size distribution of granules of an unmodified wheat starch was determined by sequential passage through a series of nylon screens. Larger granules (15–20 μ) were 2.4-fold more susceptible than smaller granules (5–10 μ) to α-amylase. Defatted granules were slightly more susceptible than nondefatted granules to enzymatic degradation. α-Amylase activity against intact starch granules purified from soft wheat kernels (“Florida 302”) was increased 119-fold when the granules were extensively homogenized in a ground glass homogenizer. Activity against granules damaged by grinding in a mortar and pestle or a mechanical grinder was also increased. Presence of a detergent (Triton X-100) in the grinding buffer gave only a slight increase (14%) in susceptibility. Studies comparing the activity of α-amylase and crude gut homogenates of S. oryzae indicated that α-amylase is the primary enzyme that attacks granular starch in the gut. In addition, we could find no evidence that α-amylases from S. oryzae bind for a relatively long duration to intact or mechanically damaged granules from Florida 302. Based on these results, in our view, the precise mechanisms and efficiencies of the enzymatic conversion of granular starch to utilizable nutrients in vivo cannot be adequately assessed from in vitro assays with gelatinized starch and thus remain to be determined. A hypothesis for the in vivo digestion of granular starch by phytophagous insects is presented.