Digestion Of Lipids And Phosphates

Oils and fats are triacylglycerols and are hydrolyzed by a triacylglycerol lipase that preferentially removes the outer ester links of the substrate (Fig. 1D) and acts only on the water-lipid interface. This interface is increased by surfactants that, in contrast to the bile salts of vertebrates, are mainly lysophosphatides. The resulting 2-monoacylglycerol may be absorbed or further hydrolyzed before absorption.

Membrane lipids include glycolipids, such as galactosyl-diacylglycerol and phosphatides. After the removal of galactose residues from mono- and digalactosyldiacylglycerol, which leaves diacylglycerol, it is hydrolyzed as described for triacylglycerols. Phospholipase A removes one fatty acid from the phosphatide, resulting in a lysophosphatide (Fig. 1D) that forms micellar aggregates, causing the solubilization of cell membranes. Lysophosphatide seems to be absorbed intact by insects.

Nonspecific phosphatases remove phosphate moieties from phosphorylated compounds to make their absorption easier. Phosphatases are active in an alkaline or acid medium.

See Also the Following Articles

Metabolism • Nutrition • Phytophagous Insects • Salivary Glands

Further Reading

Cristofoletti, P. T., and Terra, W. R. (1999). Specificity, anchoring and subsites in the active center of a microvillar aminopeptidase purified from Tenebrio molitor (Coleoptera) midgut cells. Insect Biochem. Mol. Biol. 29, 807-819.

Kerkut, G. A., and Gilbert, L. I. (eds.) (1985). "Comprehensive Insect Physiology, Biochemistry and Pharmacology." Pergamon Press, Oxford. [See especially Vol. 4, Chaps. 5 and 7] Lehane, M. J., and Billingsley, P. F. (eds.) (1996). "Biology of the Insect Midgut."

Chapman & Hall, London. [See especially Chaps. 3, 6, 7, 11, and 14] Silva, C. P., Terra, W. R., de Sa, M. F. G., Samuels, R. I., Isejima, E. M., Bifano, T. D., and Almeida, J. S. (2001). Induction of digestive alpha-amylases in larvae of Zabrotes subfasciatus (Coleoptera: Bruchidae) in response to ingestion of common bean alpha-amylase inhibitor 1. J. Insect Physiol. 47, 1283-1290. Terra, W. R., and Ferreira, C. (1994). Insect digestive enzymes: Properties, compartmentalization and function. Comp. Biochem. Physiol. 109B, 1-62. Vonk, H. J., and Western, J. R. H. (1984) "Comparative Biochemistry and

Physiology of Enzymatic Digestion." Academic Press, London. Watanabe, H., Noda, H., Tokuda, G., and Lo, N. (1998). A cellulase gene of termite origin. Nature 394, 330-331.

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