The digestion of cellulose and other dietary components, and pH of the gut in the amphipod Gammarus pulex (L.)

SUMMARY. Gut extracts from Gammarus pulex hydrolysed native and other cellulose substrates in vitro. Digestive fluid cellulase is probably endogenous as cell-free fluid mediated cellulose hydrolysis, but no bacteria were isolated from the fluid which produced a detectable extra-cellular cellulase. There was no apparent digestion of plant cell walls during their passage along the digestive tract, which took about 5–7 h at 10°C. The pH sensitivities of the digestive enzymes and the pH of the various regions of the gut suggest that carbohydrate digestion occurs in the proventriculus, midgut glands and anterior midgut, but protein digestion may be largely limited to the posterior midgut. The pH of the digestive fluid was altered slightly, but significantly, by the consumption of different natural and artificial test diets and by starvation. The most probable reason for the non-digestion of plant cell-walls is the lack of necessary enzymes other than cellulase. The role of cellulase may be confined to digesting the many small, non-cellular particles which are present in the gut.     

The functional morphology of the alimentary tract of barnacles (Cirripedia: Thoracica)

The alimentary tract of barnacles is made up of cuticle-lined foregut and hindgut with an intervening U-shaped midgut associated anteriorly with a pair of pancreatic glands and perhaps midgut caeca. Epithelial salivary glands secrete acid mucopolysaccharide, glycoprotein or both. Cells of all the midgut regions are capable of absorption which is carried out mainly by the anterior midgut and caeca. Midgut cells of Balanus balanoides (L.) show a seasonal variation in the distribution of intracellular lipid droplets. Midgut cells rest on an elastic basal lamina and secrete a peritrophic membrane which contains mucopolysaccharide and protein. Cells of the stratum perintestinale connect with the midgut epithelial cells via cell processes which probably translocate absorbed materials. Glycoprotein globules and lipid droplets accumulate in the body parenchyma of B. balanoides and are transported to the ovaries to form yolk (glycolipovitellin). The pancreatic gland cells of all barnacles are active secretory cells secreting proteinaceous material (probably digestive enzymes).     

Ultrastructure of the hepatopancreas and associated tissues of the chelicerate arthropod,Limulus polyphemus

Summary The ultrastructure of the hepatopancreas and associated tissues of Limulus is described. All hepatopancreatic tubules contain L- and D-cells. L-cells exhibit apical pinocytosis and contain many different types of inclusions, including lipid containing calcium spherules of unique structure. They also produce a complex secretory product released by apocrine secretion. D-cells contain much rough endoplasmic reticulum, and show other evidence of active protein synthesis. They produce a single type of secretory granule released by merocrine activity. The appearance of L- and D-cells in tubules of various sizes and in fed and starved animals is described. Both cell types may produce digestive enzymes and absorb and transport nutrients to the hemolymph, but neither serves as a major storage site. Storage is an important function of the intertubular R-cells. The appearance of R-cells in fed and starved animals is described. A previously undescribed hemocyte is reported, as are apparent gap junctions between L- and R-cells. A structural and functional comparison of these glands with those of other arthropods is presented.Supported by NSF Grant No. GB-16607 and by the University of Minnesota Graduate School.     

CARBOHYDRASES IN THE DIGESTIVE SYSTEM OF THE SPINED SOLDIER BUG,Podisus maculiventris (SAY) (HEMIPTERA: PENTATOMIDAE)

The spined soldier bug, Podisus maculiventris, is a generalist predator of insects and has been used in biological control. However, information on the digestion of food in this insect is lacking. Therefore, we have studied the digestive system in P. maculiventris, and further characterized carbohydrases in the digestive tract. The midgut of all developmental stages was composed of anterior, median, and posterior regions. The volumes of the anterior midgut decreased and the median midgut increased in older instars and adults, suggesting a more important role of the median midgut in food digestion. However, carbohydrase activities were predominant in the anterior midgut. In comparing the specific activity of carbohydrases, α‐amylase activity was more in the salivary glands (with two distinct activity bands in zymograms), and glucosidase and galactosidase activities were more in the midgut. Salivary α‐amylases were detected in the prey hemolymph, demonstrating the role of these enzymes in extra‐oral digestion. However, the catalytic efficiency of midgut α‐amylase activity was approximately twofold more than that of the salivary gland enzymes, and was more efficient in digesting soluble starch than glycogen. Midgut α‐amylases were developmentally regulated, as one isoform was found in first instar compared to three isoforms in fifth instar nymphs. Starvation significantly affected carbohydrase activities in the midgut, and acarbose inhibited α‐amylases from both the salivary glands and midgut in vitro and in vivo. The structural diversity and developmental regulation of carbohydrases in the digestive system of P. maculiventris demonstrate the importance of these enzymes in extra‐oral and intra‐tract digestion, and may explain the capability of the hemipteran to utilize diverse food sources.     

Compartmentalization of the digestive process in Abracris flavolineata (Orthoptera: Acrididae) adults

Carbohydrases predominate in the crop and their pH optima agree with pH prevailing in crop contents. Major amounts are also found in caecal contents. Aminopeptidase and trypsin are active mainly in the caeca, where they predominate in cells and contents, respectively. Aminopeptidase is partly membrane-bound. Except for trehalase, salivary glands display negligible amounts of digestive enzymes. The specific activity of digestive enzymes is high in all midgut cells and the enzyme molecules do not differ among gut compartments, as judged by polyacrylamide gel electrophoresis. Thus, it is probable that digestive enzymes are synthesized and secreted by all midgut cells (mainly in caeca) and then passed forward into the crop. Digestive enzymes are found in hindgut in concentrations similar to those in ventricular contents and, since they are stable in gut contents, they are likely excreted at a rate similar to undigested food. The data support the hypothesis that carbohydrate and protein are digested mainly in crop and caecal lumina, respectively, with part of the final digestion of proteins occurring at the surface of caecal cells. The peculiar features of the digestion of A. flavolineata grasshoppers, including the lack of midgut countercurrent fluxes, are thought to be derived from putative Polyneoptera ancestors.     

The ultrastructural investigation of the midgut in the quill mite Syringophilopsis fringilla (Acari,Trombidiformes: Syringophilidae)

The midgut of the females of Syringophilopsis fringilla (Fritsch) composed of anterior midgut and excretory organ (=posterior midgut) was investigated by means of light and transmission electron microscopy. The anterior midgut includes the ventriculus and two pairs of midgut caeca. These organs are lined by a similar epithelium except for the region adjacent to the coxal glands. Four cell subtypes were distinguished in the epithelium of the anterior midgut. All of them evidently represent physiological states of a single cell type. The digestive cells are most abundant. These cells are rich in rough endoplasmic reticulum and participate both in secretion and intracellular digestion. They form macropinocytotic vesicles in the apical region and a lot of secondary lysosomes in the central cytoplasm. After accumulating various residual bodies and spherites, the digestive cells transform into the excretory cells. The latter can be either extruded into the gut lumen or bud off their apical region and enter a new digestive cycle. The secretory cells were not found in all specimens examined. They are characterized by the presence of dense membrane-bounded granules, 2–4 μm in diameter, as well as by an extensive rough endoplasmic reticulum and Golgi bodies. The ventricular wall adjacent to the coxal glands demonstrates features of transporting epithelia. The cells are characterized by irregularly branched apical processes and a high concentration of mitochondria. The main function of the excretory organ (posterior midgut) is the elimination of nitrogenous waste. Formation of guanine-containing granules in the cytoplasm of the epithelial cells was shown to be associated with Golgi activity. The excretory granules are released into the gut lumen by means of eccrine or apocrine secretion. Evacuation of the fecal masses occurs periodically. Mitotic figures have been observed occasionally in the epithelial cells of the anterior midgut.     

Biochemical characterization of digestive α-amylase, α-glucosidase and β-glucosidase in pistachio green stink bug,Brachynema germari Kolenati (Hemiptera: Pentatomidae)

The pistachio green stink bug, Brachynema germari, has 3–5 generations per year and causes severe damages to pistachio crops in Iran. Physiological digestive processes, such as digestive carbohydrases, can be used to design new strategies in IPM programs for controlling this pest. The enzyme α-amylase digests starch during the initial stage of digestion. Complete breakdown of carbohydrates takes place in the midgut where α- and β-glucosidic activities are highest. Alpha-amylase and α- and β-glucosidase activities were found in the midgut and salivary glands of pistachio green stink bug adults. Overall enzyme activities were significantly higher in the midgut than in salivary glands. The highest α-amylase and α- and β-glucosidase activities were in section v3, whereas the lowest activities were in section v4. V_max was higher and K_m was lower in the midgut than in the salivary glands for these enzymes. In the pistachio green stink bug, the optimal pH was pH 5–6.5 and the optimal temperature was 30 °C to 35 °C for these enzymes. Alpha-amylase activity in the midgut and salivary glands decreased as the concentrations of MgCl₂, EDTA and SDS increased. Enzyme activities in both midgut and salivary glands increased in the presence of NaCl, CaCl₂, and KCl. NaCl had a negative effect on alpha-amylase extracted from salivary glands.     

Histochemical patterns of lipolytic enzymes of the hepatopancreas of Scylla serrata and their possible relation to eyestalk factor(s)

Of the lipolytic enzymes studied histochemically, lipase (Tween 80 specific enzyme) showed highest activity while non-specific esterases (Tween 60, α-naphthyl acetate, β-naphthyl acetate and 5-bromo-indoxyl acetate esterases) and Tween 20 and Tween 40 esterases exhibited medium and lowest activities respectively. The pattern of distribution of these enzymes was found to be variable among the various elements of the hepatopancreas. Lipase. Tween 20 and Tween 40 esterases were localized at the apical parts of the R-cell, F-cell cytoplasm, the B-cell vacuolar contents and the lumina of the hepatopancreatic acini. Non-specific esterases were primarily present in the cytoplasm of the R-cells, brush border of the tubules and connective tissue. A striking reduction in the lipase activity of the R- and B-cells was apparent 4 hours after the bilateral extirpation of eyestalks. Restoration of activity was observed 48 hours after the operation. On the other hand, a rise in the level of non-specific esterases was conspicuous 4 hours after the eyestalk amputation. This increased enzyme activity was restored to normal 24 hours after the operation. Administration of eyestalk extract into normal and de-stalked animals caused an increase in the lipase activity of the R- and B-cells. Surprisingly, the activity of non-specific esterases also enhanced considerably in the R-cell cytoplasm and connective tissue. The enhanced activity of lipase and non-specific was noted 4 hours after the treatment. From the present findings it appears that the eyestalk hormone(s) are directly involved in regulating the activity of the lipolytic enzymes. The physiological role of the F- and B-cells seems to be secretion and extrusion of lipase respectively. The R-cells and connective tissue appear to be associated with storage and mobilization of lipids.     

Enzymatic profile of Toxoplasma gondii

Zymogram patterns of nine strains of Toxoplasma gondii were studied using the API enzyme research kit. This system uses chromogenic substrates to detect the presence or absence of 84 enzymes. Enzyme classes assayed for included amino-peptidases, glycosidases, esterases, lipases, phosphoamidase and phosphatases.
All strains were positive for 24 enzymes: 15 arylamidases, seven esterases, alkaline phosphatase and acid phosphatase. In contrast, 27 enzyme tests were negative in all strains. Thirty-three enzymatic reactions were different in one or more strains.     

Digestive enzymes of leaf-cutting ants, Acromyrmex subterraneus (Hymenoptera: Formicidae: Attini): distribution in the gut of adult workers and partial characterization

Enzyme activities associated with the labial glands, midgut and rectum of adult Acromyrmex subterraneus were investigated in order to understand their role in digestion of plant and fungal material. High chitinolytic activity was detected in the labial glands, indicating a possible role in the degradation of fungus ingested by the ants. Chitinolytic activity seen in other compartments of the alimentary canal probably originated in the labial glands. The highest activity detected in the midgut was for alpha-glucosidase, which was considered to be of insect origin due to its association with midgut epithelium and it is probably involved in glucose assimilation from nutrient sources such as maltose and sucrose present in plant material. A large range of enzyme activities were detected in the rectal lumen contents, and as in the midgut the highest values were for alpha-glucosidase activity. The absence of activity associated with the epithelium, in the particulate fraction, indicates that the rectal epithelium does not have a secretory function. The detection of enzymes in the rectal lumen contents, which were not detected in the midgut lumen contents, indicates that the rectum acts as a reservoir, accumulating enzymes. The major digestive enzymes were partially characterized using hydrophobic interaction chromatography, gel filtration and SDS-PAGE. The pH of the adult intestinal tract and flow rate of dye through the tract was investigated. A gradual acidification of the intestinal tract was noted commencing with the crop (pH 6-8.2) and terminating with the rectum (pH 3-5). The flow of dye through the different compartments of the tract showed a rapid fill time for all the gut compartments and a short residence time in the crop. In all other compartments, the dye remained detectable for 10 days or longer.     

The anatomy and histology of the midgut and hindgut of Charybdis (Goniohellenus) truncata (Fabricius, 1798) (Decapoda: Brachyura)

The midgut of C. (G.) truncata accounts for half of the postgastric intestinal tract. The paired anterior midgut caeca arise just behind the pyloric stomach, on either side of the midgut. The unpaired posterior midgut caecum arises dorsally at the rear end of the midgut, where this joins the hindgut. The midgut and its caeca help in the digestive absorption of food. The hindgut is of ectodermal origin and is lined with chitin of a collagenous nature. The connective tissue of the anterior part of the hindgut is packed with tegumental glands whose secretion contains both sulphated and weakly acidic mucosubstances, which facilitate the passage of faecal matter and help to bind food particles. The digestive gland - the hepatopancreas - opens into the anterior part of the midgut, below the anterior midgut caeca. Histologically, its tubules contain three different types of cells - "F", "R" and "B" cells.     

A review on digestive TAG-lipases of insects

Digestion in insects is a multi-step process to afford nutritional requirements of biological activities. The process starts with nervous stimuli and continues with biochemical activities of digestive enzymes as well as several pumps to digest and absorb the obtained molecules. Carbohydrases, lipases and proteases are the three main digestive enzymes involved in digestion process. Lipases seem to be very important not only for digestive role but also for esteratic activity so that some experts consider lipases as the Class 3 of general esterases. Digestive lipases divided into different groups based on their biological roles namely triacylglycerol lipases, phospholipases and two types of phosphatases. Briefly, triacylglycerol lipases (TAG-lipases) are the hydrolysing enzymes that affect the outer esteric links of triacylglycerols in ingested food. Phospholipases including PLA2 and PLA1 remove phosphatide fatty acids attached to the Position 2 and Position 1. Finally, Alkaline and acid phosphatases are the enzymes that hydrolyse phosphomonoesters under alkaline or acid conditions, respectively. In this review, presence and physiological role of digestive TAG-lipases are explained and their possible importance will be discussed in insect.     

An hypothesis of the mechanism controlling proteolytic digestive enzyme production levels in Stomoxys calcitrans

Flies fed a human blood meal and sacrificed 9 h later were assayed to give information on unfed fly weight, meal weight, total midgut protein, total midgut proteolytic activity, anterior midgut protein, anterior midgut proteolytic activity, posterior midgut protein, and posterior midgut proteolytic activity; correlation coefficients were calculated for all pairings of these parameters. Posterior midgut protein showed a positive correlation with posterior midgut proteolytic activity and on this evidence it is concluded that proteolytic digestive enzyme secretion in the midgut of Stomoxys calcitrans is controlled by a secretogogue mechanism.It is proposed that the only direct stimulus the food supplies in the control of digestive enzyme production is that for digestive enzyme release from the production cells. It is also proposed that the basis of the secretogogue mechanism is that digestive enzymes are produced in direct proportion to the quantities of amino-acids available for their synthesis and that this is a consequence of the quantities of amino acids released from the food during digestion.     

Fine structure of the midgut gland of Phalangium opilio (Chelicerata,Phalangida)

Summary The fine structure of the midgut gland and the changes in composition associated with the digestive activity were examined in Phalangium opilio. In the epithelium four different types of cells are present: ferment cells, resorption cells, and digestion cells which probably turn into excretion cells, as can be seen by many intermediate stages. Ferment cells are found only in the midgut gland and in no other epithelia; therefore they should be regarded as a cell type. The relationship between digestion and resorption cells is not yet clear. No regeneration zone or single regeneration cells could be identified.The ultrastructural changes in these different cells during digestion are described, and their functional aspects are discussed. A hypothetical digestive cycle is constructed from these data. The results are compared with those on other chelicerate midgut glands.     

Influence of starvation and feeding on the hepatopancreas of larvalHyas araneus (Decapoda,Majidae)

Zoea-1 larvae ofHyas araneus were kept under different nutritional conditions. Their midgut glands were investigated with a transmission electron microscope. The glandular epithelium consists of the cell types known from adult decapods. It is mainly the R-cell type that undergoes ultrastructural alterations which reflect nutritional conditions. R-cells of fed larvae are characterized by large lipid inclusions; after a certain period of food deprivation (point-of-no-return) the original ultrastructure cannot be reestablished. Refeeding results in large glycogen deposits in these cells.     

The effect of adipokinetic hormone on midgut characteristics in Pyrrhocoris apterus L. (Heteroptera)

Digestive processes and the effect of adipokinetic hormone (Pyrap-AKH) on the amount of nutrients (lipids, proteins, and carbohydrates), and on the activity of digestive enzymes (lipases, peptidases, and carbohydrases) were studied in the midgut of the firebug, Pyrrhocoris apterus. The analyses were performed on samples of anterior (AM), middle (MM) and posterior (PM) midgut parts. The results revealed that the digestion of lipids, carbohydrates and proteins take place in the acidic milieu. The Pyrap-AKH treatment increased significantly the level of lipids and proteins in the midgut, and also the level of triacylglycerols (TGs) predominantly in the AM, and the level of diacylglycerols (DGs) in the MM. The increase was not uniform for all present TG and DG species - those containing the linoleic fatty acid were predominant. No hormonal effect on lipase activity was recorded, while peptidase and glucosidase activity was increased in the MM and PM. All these facts indicate that the Pyrap-AKH probably stimulates digestion by more intensive food ingestion or turnover, and perhaps by the stimulation of metabolite absorption; the activation of digestive enzymes seems to be secondary or controlled by other mechanisms.     

Nutrition in terrestrial isopods (Isopoda: Oniscidea): an evolutionary-ecological approach

The nutritional morphology, physiology and ecology of terrestrial isopods (Isopoda: Oniscidea) is significant in two respects. (1) Most oniscid isopods are truly terrestrial in terms of being totally independent of the aquatic environment. Thus, they have evolved adaptations to terrestrial food sources. (2) In many terrestrial ecosystems, isopods play an important role in decomposition processes through mechanical and chemical breakdown of plant litter and by enhancing microbial activity. While the latter aspect of nutrition is discussed only briefly in this review, I focus on the evolutionary ecology of feeding in terrestrial isopods. Due to their possessing chewing mouthparts, leaf litter is comminuted prior to being ingested, facilitating both enzymatic degradation during gut passage and microbial colonization of egested faeces. Digestion of food through endogenous enzymes produced in the caeca of the midgut glands (hepatopancreas) and through microbial enzymes, either ingested along with microbially colonized food or secreted by microbial endosymbionts, mainly takes place in the anterior part of the hindgut. Digestive processes include the activity of carbohydrases, proteases, dehydrogenases, esterases, lipases, arylamidases and oxidases, as well as the nutritional utilization of microbial cells. Absorption of nutrients is brought about by the hepatopancreas and/or the hindgut epithelium, the latter being also involved in osmoregulation and water balance. Minerals and metal cations are effectively extracted from the food, while overall assimilation efficiencies may be low. Heavy metals are stored in special organelles of the hepatopancreatic tissue. Nitrogenous waste products are excreted via ammonia in its gaseous form, with only little egested along with the faeces. Nonetheless, faeces are characterized by high nitrogen content and provide a favourable substrate for microbial colonization and growth. The presence of a dense microbial population on faecal material is one reason for the coprophagous behaviour of terrestrial isopods. For the same reason, terrestrial isopods prefer feeding on decaying rather than fresh leaf litter, the former also being more palatable and easier to digest. Acceptable food sources are detected through distance and contact chemoreceptors. The 'quality' of the food source determines individual growth, fecundity and mortality, and thus maintenance at the population level. Due to their physiological adaptations to feeding on and digesting leaf litter, terrestrial isopods contribute strongly to nutrient recycling during decomposition processes. Yet, many of these adaptations are still not well understood.     

Changes in the digestive gland of the loliginid squid Sepioteuthis lessoniana (Lesson 1830) associated with feeding

Changes associated with feeding in the histological and cytological structure of the digestive gland of the loliginid squid Sepioteuthis lessoniana were examined, along with the nature of both the intracellular and extracellular enzymes produced by the gland. The timing of the release of the extracellular enzymes during the digestive cycle was also determined using a quantitative experimental program. Like that of all coleoid cephalopods, the digestive gland was characterised by one type of cell with several functional stages. As is the case for other loliginid squids, however, the digestive cells did not contain the large enzyme-carrying boules that characterise the digestive glands of most cephalopods. Instead, smaller secretory granules were found in the digestive cells and these may be the enzyme carriers. The prominent rough endoplasmic reticulum, large mitochondria and active Golgi complexes present in the digestive cells are characteristic of cephalopods and indicate a high metabolic activity. Like that of other cephalopods, endocytotic absorption of nutrients and intracellular digestion occurs in the digestive gland of this squid. From quantitative and qualitative examinations of structural changes in the digestive gland of S. lessoniana after feeding, a schedule of its function during the course of digestion was proposed. This indicated that digestion was very rapid, being completed in as little as 4 h in S. lessoniana. Extracellular digestive enzymes were only released after the first hour following feeding, which implies that they are stored in the stomach between meals to increase digestive efficiency.