Enzymology of Plasma Membranes of Insect Intestinal Cells

The enzymology of insect intestinal cell plasma membranes isa field of scientific research that is in the earliest stagesof development. In this paper the few published studies specificallydesigned to isolate plasma membranes from insect intestinalcells and determine the enzymes associated with them are reviewedin light of both older studies that approached these problemsless directly and recent results from our laboratory. In the past few years reliable methods have been developed forthe isolation of specific portions of plasma membranes fromthe epithelial cells of the midguts of a few insect larvae.These membrane preparations have been assayed for a varietyof enzyme activities. Alkaline phosphatase, leucine aminopeptidaseand -glutamyl transpeptidase have shown promise as potentialmarkers for the plasma membranes of insect larval midgut cells.However, only the latter enzyme currently stands unchallengedas a marker for the apical portion of the plasma membrane ofinsect midgut columnar epithelial cells. No enzymes can yetbe considered to be even tentatively established as markersfor the basal or lateral portions of insect intestinal cells.