Nutritional utilization by monogastric animals of Glycoprotein II (Phaseolin), the major 7S protein from kidney beans (Phaseolus vulgaris) : in vivo and in vitro degradation of Glycoprotein II by rat intestinal proteases
Native Glycoprotein II (Phaseolin, G-II), the major 7S storage protein from Phaseolus vulgaris seeds, var. 'Processor' is known to be resistant to in vitro proteolysis by most endopeptidases. On sequential treatments with pepsin and a mixture of trypsin and chymotrypsin, the sub-unit polypeptides of G-II were split midchain. The fragments produced however, retained reactivity with the antibody raised against native G-II quantitatively. When measured by rocket immunoelectrophoresis, the extent of in vitro degradation of G-II by these endopeptidases was negligible. This procedure was used for monitoring the in vivo or in vitro degradation of G-II by gut enzymes other than trypsin or chymotrypsin. Diets containing 10% of a highly purified G-II preparation, did not support growth of rats adequately. Faecal N outputs were elevated and the true N digestibility based on Kjeldhal estimation was only 37%. In contrast, the true GII-N digestibility, based on immunological estimations, was high. It is suggested that G-II and/or its limited breakdown fragments (by trypsin or chymotrypsin) are stimulants of endogenous N secretion in the small intestine. The higher extent of the degradation of G-II in the small intestine of rats in vivo than that obtained by pure endopeptidases in vitro suggested the presence in this tissue of other enzymes capable to act upon and modify the structure of G-II, prior to the action of trypsin and chymotrypsin. These other modifying proteolytic enzymes render the G-II molecule more negatively charged and more susceptible to the subsequent action of trypsin and chymotrypsin. It is suggested that protease content and the ratio of the concentration of the GII-modifying protease(s) to that of trypsin and chymotrypsin may vary appreciably along the small intestine. Accordingly, the dependence of the degradation of G-II in vivo on the competition between all the enzymes capable of attacking it during its passage through the gut may explain the variability of GII breakdown in vivo.