The present study was undertaken to provide further evidence for the mechanisms proposed for the toxicity of ingested bean lectins in animals: to show the stability of concanavalin A (Con A) in the gastrointestinal tract so that it has enough time to interact with some enzymes localized in the intestinal membrane and to find its effect on the activities of those enzymes that have been adopted as criteria for adaptive changes in response to altered diets, namely intestinal sucrase, alkaline phosphatase and leucine aminopeptidase. Significant amounts of ingested Con A were recovered unaltered (as seen from affinity chromatography and electrophoresis) from the cecal content of rats 4 h after its oral administration and from feces (90% recovery) 4 d later. This indicated that Con A is quite stable during its passage through the gastrointestinal tract. Con A, given at a level of 0.3 or 0.5% in the diet, completely prevented adaptive changes in the activities of those enzymes. These results substantiate the mechanisms proposed earlier by other investigators that the toxicity of ingested bean lectins involves their binding to the luminal surface of the small intestine, where they disturb the function of the brush border membrane.
In the previous studies,1>2) in which the mechanisms involved in the toxicity of ingested concanavalin A (Con A) were investigated, we observed the following: 1. The ingested Con A remained unaltered during its passage through the gastrointestinal tract and was rapidly excreted in the faces. 2. It prevented the adaptive responses of the intestinal sucrase, alkaline phosphatase and leucine aminopeptidase activities, which are localized in the brush border membrane, to dietary nutrients. 3. The excreted Con A in the feces was bound to water-insoluble components in the feces, which was suggested by it being released from the feces with an a-methyl-D-mannoside solution but not with water. 4. The water-insoluble components in the feces were confirmed to have an affinity to Con A in vitro. These findings strongly suggested the presence of a Con A-binding glycoprotein in the small intestine and feces of rats.
The present study was undertaken to define more precisely the mechanismsinvolved in the toxicity of ingested concanavalin A (Con A) in rats, by observation of its behavior in the gastrointestinal tract. Con A was observed to have a high affinity for water-insoluble components offeces. The addition of a massive amountof the water-insoluble componentsto a high sucrose diet (HSD) containing Con A did not ameliorate the adverse effect of ingested Con A on the adaptive response of the intestinal, sucrase activity to dietary sucrose. The addition of Con A bound feces to HSDalso prevented the adaptive response of the intestinal sucrase activity to dietary sucrose. The luminal pH of the stomach 5 hr after HSDadministration was 3 to 4, which extremely reduced the affinity of Con A for carbohydrate moieties of the cell membrane,whereas those ofthejejunum and cecum ranged from 6 to 7. These results substantiate the proposed mechanism, that is, that the toxicity of ingested Con A involves its binding to the luminal surface of the small intestine, and it then in turn disturbs the functional formation of the brush border membranein response to dietary alterations.
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