Gastrointestinal Transit of Soybean Trypsin Inhibitor and Fluctuations in Intraluminal Trypsin

Soybean trypsin inhibitor of Kunitz type (STI) was modified by reduced alkylation with NaBH₄ and HCHO, and examined for resistance to in vitro digestion from the viewpoint of inhibitory activity and immunoreactivity. Methylated STI was exactly alike in that respect. Hence, STI was in part replaced by its [¹⁴C]methyl-labeled specimen and their (17: 3) mixture was used as a proteinous and unabsorbable marker for digestibility experiments. When STI was immunochemically measured in intraluminal leavings in segments of the rat digestive tract, its recovery decreased progressively with the elapse of time. As a result of radioactivity measurement, however, the recovery of [¹⁴C]labeled STI proved almost quantitative all together over a period of 7 h postprandial. Concurrently, gastric emptying and intestinal transit of digesta were assessed in terms of intraluminal STI movement. After moving out from the stomach, STI rapidly passed through the upper small intestine with depression of the trypsin activity, and stayed in the lower small intestine for a few or several hours (all that while, the trypsin activity was not depressed so much). A similar pattern was observed for the intraluminal movement of a food additive ‘indigo carmine’ in another experiment. It was assumed from these observations that digesta would also have gone past the upper small bowel irrespective of ingesting either a powdered 20% casein diet or a dumpling-kneaded artificial bait.