Histochemical observations on the localization of some enzymes associated with digestion in four species of Brazilian nemerteans.

Abstract

The processes of digestion have been investigated histochemically for several nemertean species (Jennings, 1962a ; Gibson and Jennings, 1969 ; Jennings and Gibson, 1969 ; Gibson, 1970) , and results so far obtained suggest that although the fundamental digestive sequence is essentially similar for all the species, differences in details can be related either to their systematic position or mode of life. Irrespective of the nature of the food utilized, digestion occurs in two distinct phases. Initial extracellular digestion is accomplished in the intestinal lumen at an acidic pH and involves mainly proteolytic enzymes secreted by the gastrodermis. Food particles are subsequently engulfed by lamellar outgrowths of the distal ciliated cell walls (Jennings, 1969) and food vacuoles passed back into the gastro dermis for the second, intracellular, stage in digestion. This involves exopeptidases (arylamidases), acid and alkaline phosphatases and, in some species at least, carbohydrases and lipases. The most uniform patterns of digestion are found in anoplan neinerteans (Jennings, 1962a ; Jennings and Gibson, 1969). Acidophilic gland cells in the foregut, rich in carbonic anhydrase, discharge their contents during ingestion which serve both to kill prey taken alive and provide the correct lumenar pH for extracellular digestion . Mucoid secretions, di scharged from other foregut glands, lubricate the food as it is passed into the intestine. Food entering the intestinal lumen stimulates the gastrodernial gland cells to discharge endopeptidases, which are responsible for early proteolysis and function optimally under acidic conditions. Intracellular digestion initially involves endopeptidases phagocytosed along with food particles and is marked by a sharp increase in acid phosphatase activity in and around food vacuoles. Acid phosphatases may be concerned in some way with the maintenance of the correct pH for intralumenar endopeptic activity (Jennings and Gibson, 1969) or with food vacuole formation (Rosenbaum and Rolon, 1960). Slinger and Gibson (1974) demonstrating biochemically that these enzymes func tion optimally in the range pH 4.1—5.0,depending upon the species. A few hours after the commencement of phagocytosis acid phosphatase and endopeptidase activity in the food vacuoles and surrounding cytoplasm declines, being replaced by alkaline phosphatases and exopeptidases. This final, alkaline, phase in digestion operates within the pH range 8.7—10.1 (Slinger and Gibson, 1974), and continues until digestion has been completed. Endopeptidase enzymes can be demonstrated in the gastrodermal gland cells at all times, irrespective of the nutritive state, but exopeptidase activity can only be visualized histochemically in gut cells at the appropriate stage in digestion. 352