Proline transport into brush border membrane vesicles from the midgut of Manduca sexta larvae

Leucine transport in brush border membrane vesicles from freshwater insect larvae

Leucine transport across brush border membrane vesicles prepared from four insect species common to European freshwater streams has been characterized. The species studied were: Ephemera danica (Ephemeroptera: Ephemeridae), Isoperla grammatica (Plecoptera: Perlodidae), Hydropsyche pellucidula (Trichoptera: Hydropsychidae), and Hybomitra bimaculata (Diptera: Tabanidae). The transport differed among the studied taxa for several features, including pH and sodium dependence, substrate affinity and specificity, and efficiency. In H. pellucidula and E. danica, leucine uptake was higher at pH 7.4 than at more alkaline or acidic pH values, whereas in I. grammatica and H. bimaculata, the uptake was rather constant when pH varied from 5.0 to 7.4, then strongly decreased at pH 8.8. All but E. danica displayed a transient intravescicular leucine accumulation in the presence of sodium, suggesting the existence of a cation-leucine symport mechanism. The sodium dependence ranged according to the following order: H. pellucidula > I. grammatica > H. bimaculata > E. danica. Moreover, in H. pellucidula and I. grammatica, the sodium-dependence was stronger at pH 8.8 than at pH 7.4. In E. danica, leucine uptake was sodium-independent at all pH values. The highest value of V(max) (45.3 pmol.s(-1).mg proteins(-1)) was in E. danica, which, however, displayed the lowest affinity (K(m) 137 muM) when compared to the kinetic parameters of other taxa. The V(max) and K(m) values were: 40 and 52.5, 32.1 and 12.5, and 4.5 and 230 for H. bimaculata, H. pellucidula, and I. grammatica, respectively. The obtained results are discussed within our current knowledge of amino acid transport systems in insects.

Leucine transport in membrane vesicles from Chironomus riparius larvae displays a mélange of crown-group features

Leucine uptake into membrane vesicles from larvae of the midge Chironomus riparius was studied. The membrane preparation was highly enriched in typical brush border membrane enzymes and depleted of other membrane contaminants. In the absence of cations, there was a stereospecific uptake of l-leucine, which exhibited saturation kinetics. Parameters were determined both at neutral (Km 33 +/- 5 microM and Vmax 22.6 +/- 6.8 pmol/7s/mg protein) and alkaline (Km 46 +/- 5 microM and Vmax 15.5 +/- 2.5 pmol/7s/mg protein) pH values. At alkaline pH, external sodium increased the affinity for leucine (Km 17 +/- 1 microM) and the maximal uptake rate (Vmax 74.0 +/- 12.5 pmol/7s/mg protein). Stimulation of leucine uptake by external alkaline pH agreed with lumen pH measurements in vivo. Competition experiments indicated that at alkaline pH, the transport system readily accepts most L-amino acids, including branched, unbranched, and alpha-methylated amino acids, histidine and lysine, but has a low affinity for phenylalanine, beta-amino acids, and N-methylated amino acids. At neutral pH, the transport has a decreased affinity for lysine, glycine, and alpha-methylleucine. Taken together, these data are consistent with the presence in midges of two distinct leucine transport systems, which combine characters of the lepidopteran amino acid transport system and of the sodium-dependent system from lower neopterans.

Substrate specificity of the brush border K+-leucine symport of Bombyx mori larval midgut

L-leucine uptake into membrane vesicles from Bombyx mori larval midgut was tested for inhibition by 55 compounds, which included sugars, N-methylated, alpha-, beta-, gamma-, delta-, epsilon-amino acids, primary amines, alpha-amino alcohols, monocarboxylic organic acids and alpha-ketoacids. Based on cis-inhibition experiments performed at the high pH (10.8) characteristic of the midgut luminal content in vivo, we find that the carrier binding site interacts with molecules which possess a well-defined set of structural features. Amino acids are preferentially accepted as anions and the ideal inhibitor must have an hydrophobic region and a polar head constituted by a chiral carbon atom bearing two hydrophilic groups, a deprotonated amino-group and a dissociated carboxylic group. Binding is reduced if one of the two hydrophilic groups is removed. Lowering the pH to less alkaline value (8.8) only affects the affinity of delta- and epsilon-amino acids, which are excluded from binding because of their positively charged side-chain. Modifications of the potassium electrochemical gradient increased the affinity constant values of the molecules, but have little effect on the rank of specificity. Physiological implications of the data reported are discussed.

Feeding rate and the structure of protein digestion and absorption in lepidopteran midguts

The transformation of leaf protein into insect tissue can be distilled grossly into four serial steps: consumption of protein, digestion of protein into small peptides and free amino acids, absorption of these fragments across the gut epithelium, and construction of tissue from the absorbed fragments. We examine these steps in caterpillars, with emphasis on determining which, if any, process is limiting and how digestive events influence-and are influenced by-the rate of consumption. Five competing hypotheses are (1) the steps are matched, (2) consumption is limiting, (3) digestion is limiting, (4) absorption is limiting, and (5) post-absorptive processes are limiting. For conditions appropriate to each of the five hypotheses, we use a chemical reactor model to predict the concentration profiles of protein and protein breakdown products along the midgut. We test the predictions in two ways. First, based on midgut morphology and rates of digesta flow and protein breakdown estimated from in vitro experiments, we calculate the axial concentration profile of protein in the midgut of fifth instar Manduca sexta. This calculation suggests that protein breakdown occurs primarily in the anterior midgut, thus excluding the hypotheses that feeding and digestive processes are matched or that digestion is limiting. In addition, caterpillar feeding responses to reduced concentrations of protein are not consistent with the hypothesis that consumption is limiting. Second, we review published studies that show midgut concentration profiles of protein and breakdown products, one supporting the hypothesis that absorption across the midgut epithelium is limiting and the other suggesting that post-absorptive processes are limiting. Consequently, we interpret compensatory feeding by larval M. sexta as a mechanism for maintaining saturated rates of protein absorption or of tissue construction. Arch. Copyright 1999 Wiley-Liss, Inc.

Effects of dietary protein concentration on L-proline transport by Manduca sexta midgut

Foliar protein concentrations vary widely. Phytophagous insects may respond to this variation by regulating the rate of absorption of protein breakdown products. Using fifth-instar Manduca sexta reared on artificial diets, we examined whether the rate of L-proline transport across the posterior midgut was affected by dietary protein concentration. Caterpillars were reared to the fifth instar on a low- or high-protein diet and then transferred to the same or opposite diet for 24 h prior to the flux measurements. Posterior midguts were mounted in Ussing chambers, and both potential difference and (14)C-proline flux across the tissue were measured during two-hour experiments. Midguts transported proline from lumen to hemolymph at about 6.5 &mgr;mol/cm(2)/h and from hemolymph to lumen at less than 0.43 &mgr;mol/cm(2)/h. Only a small fraction was oxidized (0.58 &mgr;mol/cm(2)/h), and proline was not converted to another compound while crossing the midgut epithelium. Neither proline transport rates nor potential differences across the epithelium differed significantly among treatment groups, suggesting that larval M. sexta do not regulate proline transport in response to dietary protein variation.