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Purification of a Kunitz-type inhibitor from Acacia polyphylla DC seeds: Characterization and insecticidal properties against Anagasta kuehniella Zeller (Lepidoptera: Pyralidae) Suzy Wider Machado, Cezar da Silva Bezerra, Caio Oliveira, Olga Lima Tavares Machado, Sergio Marangoni, and Maria Ligia Rodrigues Macedo J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/jf3049565 • Publication Date (Web): 18 Feb 2013 Downloaded from http://pubs.acs.org on February 26, 2013
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Purification of a Kunitz-type inhibitor from Acacia polyphylla DC seeds: Characterization
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and insecticidal properties against Anagasta kuehniella Zeller (Lepidoptera: Pyralidae)
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Suzy Wider Machado§, Caio Fernando Ramalho de Oliveira†, Cezar da Silva Bezerra§, †, Olga
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LimaTavares Machadoψ, Sergio Marangoni†, Maria Ligia Rodrigues Macedo*§,
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§
Laboratório de Purificação de Proteínas e suas Funções Biológicas, Centro de Ciências Biológicas
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e Saúde, Universidade Federal do Mato Grosso do Sul, Campo Grande 79070-900, MS, Brazil
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ψ
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RJ, Brazil
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†
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Universidade Estadual do Norte Fluminense Darcy Ribeiro, Campo dos Goyatacazes 28013-600,
Departamento de Bioquimica, Instituto de Biologia, UNICAMP, Campinas 13083-970, SP, Brazil Departamento de Genética, ESALq/USP, Piracicaba 13418-900, SP, Brazil
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* Corresponding author: Maria Lígia R. Macedo. Phone: +55 67 33457612, Fax: +55 67 33457400,
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E-mail address:
[email protected] 25 26
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Abstract
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Anagasta kuehniella is a polyphagous pest that causes economic losses worldwide. This species
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produces serine proteases as its major enzymes for protein digestion. In this study, a new serine-
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protease inhibitor was isolated from Acacia polyphylla seeds (AcKI). Further analysis revealed that
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AcKI is formed by two polypeptide chains with a relative molecular mass of ~20kDa. The effects of
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AcKI on the development, survival, and enzymatic activity of Anagasta kuehniella larvae were
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evaluated, by incorporating AcKI in an artificial diet. Bioassays revealed a reduction in larval
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weight of ~50% with the lower concentration of AcKI used in the study (0.5%). Although
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additional assays showed an increase in endogenous trypsin and chymotrypsin activities, with a
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degree of AcKI-insensivity, AcKI produces an anti nutritional effect on A. kuehniella, indicating
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AcKI as a promising bioinsecticide protein for engineering plants that are resistant to insect pests.
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Keywords: Acacia polyphylla; Kunitz-type inhibitor; Anagasta kuehniella; pest insect.
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Introduction
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The incidence of pest insect attacks in crops of economic importance is a serious problem for food
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production, since losses due these attacks can reach 20% in large cultivars 1. It is known that some
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plants can decrease the damage caused by insects and pathogens through the synthesis of Protease
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Inhibitors (PIs), a group of plant proteins that act as a natural defense mechanism, triggered by
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herbivory 2. This group of proteins is widely found in the Fabaceae subfamilies and these enzymes
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are present in various plant tissues, including seeds, where they constitute up to 10% of the total
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protein 3.
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The most representative families of PIs are the Bowman-Birk and Kunitz inhibitors, where the latter
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are characterized by a molecular mass of between 18-22 kDa, with four cysteine residues that form
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two disulfide bridges and typically inhibit trypsin and chymotrypsin enzymes 4,5. As such, PIs have
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shown insecticide effects on different pests and transgenic plants expressing PIs have been assessed
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as an alternative strategy for protection against pest insects 6. Thus, there is an increasing demand
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for the discovery of novel PIs with different biological activities. However, the choice of study
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model needs to take into account its digestive physiology, as part of the PIs’ insecticide effects
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result from the direct inhibition of insect midgut enzymes. Since insects from the Lepidoptera order
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possess serine proteases, such as trypsin and chymotrypsin, as the most representative in their
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digestive physiology 7, these insects constitute good candidates for bioassays.
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Modern agriculture is defined by the incorporation of technological innovations to meet the
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growing demands of the world market in a sustainable manner; however, it is through basic research
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that novel tools are studied and finally applied in the field. Anagasta kuehniella (Zeller)
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(Lepidoptera, Pyralidae, Phycitinae), the Mediterranean flour moth, is a polyphagous pest that feeds
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on stored grains, fruits, nuts, jellies, cakes, and candies, causing economic losses worldwide 8.
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Depending on the temperature and humidity, a single female may lay nearly 600 eggs 9. It is,
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however, important to carry out a thorough study of physiological and biochemical aspects of
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potential insecticidal proteins, before suggesting alternative ways to control a specific pest, since
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adaptive responses may be triggered in pest insects, bypassing the insecticide effect expected, as
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reported for some pest insects
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biochemical properties of a new inhibitor purified from Acacia polyphylla seeds (AcKI). In
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addition, a complete study evaluating the insecticidal effects of AcKI on the larval development of
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A. kuehniella was performed, utilizing bioassays, measurements of physiological and nutritional
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parameters and finally in vitro assays, to show the potential of AcKI for the control of this pest.
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Materials and Methods
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Materials. A. polyphylla seeds were collected locally (Cerrado-Pantanal, Mato Grosso do Sul, MS,
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Brazil), packed in plastic bags and transported to the laboratory, where were washed and stored at -
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20 ºC until use. Bovine serum albumin (BSA), bovine pancreatic trypsin, bovine pancreatic α-
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chymotrypsin,
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nitroanilide (BTPNA), Succinyl-alanyl-alanyl-propyl-phenylalanyl-p-nitroanilide (SAAPPNA), N-
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p-tosyl-lysine chloromethyl ketone (TLCK), tosyl-L-phenylalanine chloromethyl ketone (TPCK)
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and electrophoresis reagents were purchased from Sigma (St. Louis, MO, USA). Chromatography
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supports were purchased from GE Healthcare. All other chemicals and reagents used were of
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analytical grade.
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Insects. The flour mouth (A. kuehniella) insect was originally supplied by Dr. J.R.P. Parra
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(Laboratório de Biologia dos Insetos, Escola Superior de Agronomia ‘‘Luiz de Queiroz’’,
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Universidade São Paulo, Piracicaba, SP, Brazil) and were housed at standard conditions (28±1 °C,
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RH of 70±5% and 16 h photo phase) in incubator B.O.D. (SPLABOR, SP-500) and maintained on a
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diet constituted of wheat germ and whole meal flour in a proportion of 3:2 wheat germ: whole meal.
10 ,11
. As such, this study aimed to describe the purification and
N-benzoyl-DL-arginine-p-nitroanilide
(BAPNA),
N-benzoyl-
L-tyrosyl-p-
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The determination of larval instars were made according Mbata and Osuji 12.
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Purification of AcKI. A. polyphylla seeds that were free of tegument were ground in a coffee mill
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and subsequently defatted with hexane. Crude extract from seeds was obtained by extraction of this
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meal with 0.1 M phosphate buffer, pH 7.6 (1:10, w/v), for 2 h followed by centrifugation at 10 000
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g for 30 min. The supernatant was dialyzed against distilled water for three days at 4°C and
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lyophilized. For purification of AcKI, the crude extract was dissolved in 0.1 M phosphate buffer,
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pH 7.6, containing 0.1 M NaCl and applied to a Sephadex G-50 column (2 x 50 cm) that was
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equilibrated with the same buffer. Fractions of 3 mL were collected with a flow of 36 mL/h. The
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fractions with inhibitory activity were pooled and dialyzed during 48 h at 4°C and lyophilized. The
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peak that showed inhibitory activity was applied to a trypsin-Sepharose column (12 cm × 3 cm)
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equilibrated with 0.1M sodium phosphate buffer, pH 7.6, containing 0.1M NaCl at a flow rate of 30
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mL/h. The protein bound to the column was eluted with 50mM HCl, dialyzed and lyophilized.
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Proteins were detected by monitoring the absorbance at 280 nm.
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HPLC Analysis of Purified AcKI. The peak eluted from the trypsin-Sepharose column was then
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submitted to Reverse Phase High Performance Liquid Chromatography (RP-HPLC). The sample
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was applied to a C5 column (Symmetric, Waters System) that had been previously equilibrated with
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0.1% (v/v) TFA (solvent A), followed by a 60-min linear gradient from 0 to 100% (v/v) of 66%
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(v/v) acetonitrile in 0.1% (v/v) trifluroacetic acid (solvent B). Proteins were detected by monitoring
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absorbance at 280 nm.
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Protein Quantification. Protein contents were determined by Coomassie blue staining, dye-
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binding method
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wells, where 5µL of standard concentrations of BSA (from 0.003125 to 0.5mg/mL) were used for
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construct the standard curve of known concentration. Subsequently, 5µL of samples of unknown
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concentration were added to microplate and then 250µL Bradford was added. The microplate was
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measured at 595nm and based in standard curve the concentration of protein in known samples
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were determined.
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Inhibitory Activity Assays. Inhibitory activity assays were determined by measuring the residual
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hydrolytic activity of bovine trypsin and chymotrypsin towards the substrates, BAPNA and
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BTPNA, respectively. Both of the enzymes were incubated with AcKI (1 mg/mL) for 10 min at 37
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°C, in Tris-HCl buffer (50 mM), pH 8.0, prior to the reaction. To start the reaction, 1mM BAPNA
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or 20mM BTPNA were added to the assay. After 15 min, the reaction was stopped by the addition
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, using BSA (1mg/mL) as standard. The assays were developed in microplate
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of 30% (v/v) acetic acid. The release of chromophore p-nitroanilide was measured at 405 nm for
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both substrates. The linearity of the relationship between the changes in absorbance with time was
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checked to ensure that the substrate concentrations were not limiting. Substrate and enzyme controls
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were run to ensure the validity of sample absorbance readings.
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Polyacrylamide Gel Electrophoresis and Isoelectric Focusing. Sodium dodecyl sulfate-
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polyacrylamide gel electrophoresis (SDS–PAGE, 12.5%) in the absence and presence of
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Dithiothreitol (DTT, 100 mM) was carried out as described by Laemmli
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detected by staining with 0.1% Coomassie brilliant blue R-250. For Native PAGE (15%), 2 µg of
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purified AcKI was used and the gel was submitted to APNE staining. The gel was fixed in a 12.5%
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TCA solution, incubated for 30 min in 0.1 mol/L sodium phosphate buffer, pH 7.4 containing 0.1
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mg/mL trypsin and then transferred to a solution containing APNE (N-acetyl-dl-phenylalanine-β-
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naphthyl ester) (2.5 mg/mL in dimethylformamide) and tetrazotized o-dianisidine (0.55 mg/mL in
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0.1 mol/L sodium phosphate buffer, pH 7.4) until the appearance of transparent bands against a pink
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background.
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Isoelectric focusing was carried out on a flat bed apparatus (LKB). Ampholine solutions (40%, v/v)
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in the pH range of 3.5–9.5 were used with subsequent Coomassie brilliant blue G-250 staining,
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according to Westermeier 15. The inhibitors were detected using the negative staining technique of
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Uriel and Berges 16.
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Temperature and pH Stability. An AcKI sample (1 mg/ml in 50 mM Tris–HCl buffer, pH 8.0)
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was heated for 30 min in a water bath at various temperatures (37°–100 °C), and then cooled to 0
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°C before testing for residual inhibitory activity. To measure pH stability, a solution of AcKI (1
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mg/ml) was diluted with an equal volume of various buffers (100 mM): sodium citrate (pH 2–4),
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sodium acetate (pH 5), sodium phosphate (pH 6–7), Tris–HCl (pH 8) and sodium bicarbonate (pH
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9–10). After incubation in each buffer for 1h at 37 °C, the pH was adjusted to pH 8.0 and the
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inhibitory activity on trypsin was assayed as described above. All experiments were carried out in
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triplicate and the results reported are the mean of three assays.
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. The proteins were
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Amino-Terminal Sequencing. The N-terminal amino acid sequence of AcKI was determined by
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direct sequencing using automated Edman degradation and a PROCISE amino acid sequencer
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(Applied Biosystems). The phenylthiohydantoine (PTH) amino acids were identified in a model
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140C microgradient PTH amino acid analyzer, based on their retention times. The sequence was
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submitted to automatic alignment, which was performed using the NCBI-Blast search system.
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Midgut Preparation. Proteases were obtained from the midgut larvae, according to a protocol by
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Macedo et al.17. The larvae were cold-immobilized and dissected, and the midguts were surgically
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removed and placed in iso-osmotic saline solution; aliquots of 10 midguts were prepared. The A.
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kuehniella midguts were subsequently homogenized in 150 mM NaCl and centrifuged at 6 000 x g
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for 20 min at 4ºC, and supernatants were stored at -20 °C until use in enzymatic assays.
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Activity Inhibitory of AcKI, TLCK and TPCK against different enzymes. Bovine trypsin and
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chymotrypsin were incubated with synthetic trypsin (TLCK) and chymotrypsin inhibitors (TPCK),
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respectively, as well as AcKI (2µg) at 37°C, in 50 mM Tris-HCl buffer, pH 8.0. The substrates,
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BAPNA and BTPNA, were used as described above. The same assay was performed for the midgut
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extract of fourth-instar A. kuehniella larvae (5µg), using the substrates BAPNA and SAAPPNA, the
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latter being specific for chymotrypsin. All assays were performed in triplicate.
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Effects of AcKI on A. kuehniella Development. To examine the effects of AcKI on A. kuehniella
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development, larvae were fed on a diet containing AcKI at concentrations of 0.5%, 1.0% and 2.0%
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(w/w) until the fourth-instar. For obtainment of eggs, adults were aspirated from colony into a 300
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mL jar covered with a screen of mesh size of 0.8 mm and then the jar was inverted on to a Petri
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dish. The eggs were collected and the neonate larvae were transferred to a clean bottle containing
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artificial diet with a soft brush. The control diet was prepared without inhibitor addition and for
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each AcKI concentration evaluated, 4 neonate larvae were placed in a clean bottle and this process
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was repeated ten times (n = 40). Following incubation for 22 days under standard conditions, the
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weight and number of larvae were determined. Linear regression analysis was used to evaluate the
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response of A. kuehniella to the concentrations of AcKI. Effective doses for the 50% response were
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defined as the concentration of AcKI that decreased the larval weight to 50% in relation to the
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control group. After the choice of the ideal concentration of AcKI (weight reduction of close to
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50%), further bioassays were performed, in order to evaluate the weight and survival in third (16
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days), fourth (22 days) and fifty-instar larvae (28 days), under the same conditions established in
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the first experiment. Diet intake and fecal output were measured and used for the elaboration of
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Nutritional Parameters. Biochemical assays with the major enzymes present in the A. kuehniella
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midgut were also performed.
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Nutritional Parameters. The nutritional parameters were compared among third, fourth and fifth-
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instar larvae exposed to either the AcKI-fed diet or the control diet. The larvae, feces, and uneaten
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food were separated using a microscope stereoscope, dried and weighed. Nutritional parameters of
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consumption, digestion and utilization of food were calculated, as described by Waldbauer
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Farrar et al. 19. The nutritional index, denominated efficiency of conversion of ingested food (ECI),
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efficiency of conversion of digested food (ECD) and approximate digestibility (AD) were
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calculated as follows:
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ECI (∆B/I) x 100; ECD [∆B/I - F)] x 100; and AD [(I - F)/I] x 100, where I = weight of food
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consumed, ∆B is change in body weight, and F = weight of feces produced during the feeding
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period. Metabolic cost (CM) was calculated as: 100 - ECD.
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Protease Activity of Midgut Extracts in Zymography Gel Electrophoresis. Proteins extracted
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from the midgut extracts of A. kuehniella larvae were run on SDS–PAGE (12.5%) containing 0.1%
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gelatin 20. Following electrophoresis at 4 °C, the gels were washed with 2.5% Triton X-100 solution
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for 2 h with shaking to remove the SDS, after which the gels were incubated with 0.1M Tris–HCl,
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pH 8.0, for 2–3 h. The gels were subsequently stained with Coomassie brilliant blue R-250. Bands
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of proteolytic activity appeared as clear zones against a blue background.
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Effect of Ingested AcKI on A. kuehniella Midgut Proteases. Trypsin and chymotrypsin enzymes
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of midgut extracts from A. kuehniella larvae (both AcKI-fed and control-fed larvae) were assayed
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using the substrate BAPNA and SAAPPNA. A sample of midgut extract (5µg) was incubated at
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37°C for 10 min before adding the substrate to start the reaction, which was allowed to proceed for
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15 min for BAPNA and 5 min for SAAPPNA. The reaction was then stopped by adding 30% (v/v)
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acetic acid.
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To analyze the ability of AcKI to inhibit the enzymatic activity of A. kuehniella midgut enzymes,
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midgut samples were incubated with increasing concentrations of AcKI (0-2 µg). Inhibition assays
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for trypsin and chymotrypsin were then performed. All assays were performed in triplicate.
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AcKI Digestion. To verify the possibility that AcKI may be digested by digestive enzymes of A.
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kuehniella, AcKI was incubated with 50 mM Tris buffer pH 8.0 and digestive enzymes of the
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midgut extract from fourth-instar larvae, dissected and processed as described previously, for 72
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hours at 30°C (same protein concentration for the midgut extract and AcKI). The digestion was
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stopped by immersing the tubes in an ice bath. The remaining inhibitory activity of AcKI was
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analyzed by inhibition of trypsin activity, using BAPNA as substrate.
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Statistical Analysis. The effects of AcKI on thermal and pH stabilities, enzymatic activities, weight
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gain, survival and nutritional parameters were examined using one-way analysis of variance
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(ANOVA) with post-test using Tukey as multiple comparison in order to identify the means that
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differed if the ANOVA test indicated significance. The p value < 0.05 was considered to be
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significant.
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Results and Discussion
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AcKI Purification. Different chromatography techniques were used for the purification of AcKI. In
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the first step, the gel filtration separated the crude extract into two major groups, where only the
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second peak showed strong inhibitory activity for trypsin (Fig. 1A). In the second step, the second
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peak from the gel filtration was submitted to bio affinity chromatography and the peak eluted from
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the column exhibited trypsin activity inhibition (Fig. 1B). The inhibitor purified by the affinity
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column was subjected to RP-HPLC. Using a linear gradient of buffer B, the presence of 4 major
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peaks were observed, with a similar retention time (Fig. 1C), suggesting the presence of
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isoinhibitors, as reported for other Kunitz inhibitors 5.
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SDS-PAGE (12.5%) analysis revealed that a single protein had been purified, showing a relative
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mass of approximately 20 kDa under non-reducing conditions. Thus, this protein was named AcKI
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and used for further characterization. Even though AcKI presented a single band under non-
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reducing conditions, the treatment of AcKI with DTT demonstrated the presence of two bands of
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distinct molecular masses, indicating that AcKI is composed of two polypeptide chains (Fig. 1A,
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inset). Other PIs isolated from the Acacia genus have a range of masses of 18-21 kDa: A. elata ~20
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kDa 21, A. siberiana ~19 kDa 22, A. confusa ~21 kDa 23, A. victoriae ~18.3 kDa 4, A. plumosa ~20
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kDa
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another characteristic shared among inhibitors from the Acacia genus, as for a number of other
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inhibitors from the Mimosoideae subfamily.
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Isoelectric focusing analysis of AcKI showed the presence of four bands with pIs ranging from 6.5
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to 5.85 (Fig. 1D), indicating the occurrence of isoinhibitors. Protease inhibitors may present various
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molecular forms and differ in their pI values 3, however most kunitz-type inhibitors are acidic 14, 26.
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The existence of isoforms in nature is not completely elucidated, although the hypothesis that these
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isoforms and the synergism among them ensure the plant’s survival 5 is recognized.
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Thermal and pH Stability. Kunitz inhibitors often demonstrate characteristics of tolerance at high
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temperatures and over a wide variation of pH
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inhibitory activity affected by increase in temperature. A significant reduction of AcKI biological
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activity is noticed from 60ºC up to higher temperatures analyzed (P
