Food Uptake And Utilization

example, firebrats and silverfish (Zygentoma), larvae of wood-boring Cerambycidae and 499

Anobiidae (Coleoptera), as well as both lower and higher termites, have endogenous cel-lulases, though in most insects production of this enzyme is restricted to microorganisms present in the hindgut. Curiously, lower termites produce cellulase in the saliva, whereas in higher termites the midgut is the source of this enzyme. Scolytinae (Coleoptera) produce a hemicellulase, chitinase is reported to occur in the intestinaljuice of Periplaneta, and some herbivorous Orthoptera produce lichenase.

As in other organisms, the protein-digesting enzymes produced by the midgut are divisible into two types: endopeptidases, which effect the initial splitting of proteins into polypeptides, and exopeptidases, which bring about degradation of polypeptides by the sequential splitting off of individual amino acids from each end of a molecule. Exopeptidases can be further categorized into carboxypeptidases, which remove amino acids from the car-boxylic end of a polypeptide, and aminopeptidases, which cause hydrolysis at the amino end of a molecule. A dipeptidase also is frequently present. In some species only endopeptidases occur in the midgut lumen (specifically within the endoperitrophic space), the exopeptidases being found outside the PM or even attached to the apical plasma membrane of the epithelial cells. Some insects produce specific enzymes for the digestion of particularly resistant structural proteins. Collagenase has been mentioned already. Keratin, the primary constituent of wool, hair, and feathers, is a fibrous protein whose polypeptide components lie side by side linked by highly stable disulfide bonds between adjacent sulfur-containing amino acids, such as cystine and methionine. A keratinase has been identified in clothes moth larvae (Tineola) and may also occur in other keratin-digesting species, such as der-mestid beetles and Mallophaga. The keratinase is active only under anaerobic (reducing) conditions and, in this context, it is interesting to note that the midgut of Tineola is poorly tracheated.

Dietary fats of either animal or plant origin are almost always triglycerides, that is, glycerol in combination with three fatty acid molecules. The latter may range from unsat-urated to fully saturated. Lipases, which hydrolyze fats to the constituent fatty acids and glycerol, have low specificity. Therefore, the presence of one such enzyme will normally satisfy an insect's needs. In a few species, however, at least two lipases have been identified, having different pH optima and acting on triglycerides of different sizes. Fat digestion is generally somewhat slow as insects lack anything comparable to the bile salts of vertebrates that would emulsify and stabilize lipid droplets.

4.2.2. Factors Affecting Enzyme Activity

According to House (1974), three factors markedly affect digestion in insects: pH, buffering capacity, and redox potential of the gut.

The pH determines not only the activity of digestive enzymes, but also the nature and extent of microorganisms in the gut and the solubility of certain materials in the gut lumen. The latter affects the osmotic pressure of the gut contents and, in turn, the rate of absorption of molecules across the gut wall. Analyses of the pH in various regions of the gut have been made for a wide range of species, and various authors have attempted to correlate these with the feeding habits or phylogenetic position of an insect. At best, these correlations are only broadly correct, and many exceptions are known. In most insects the gut is slightly acid or slightly alkaline throughout its length. Further, the pH generally increases from foregut to midgut, then decreases from midgut to hindgut. Though the latter is true for most

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