Low sucrase activity in the small intestine of a senescence-accelerated strain of mouse, SAMP1.
The small intestinal sucrase activity in a senescence-accelerated strain of mouse, SAMP1, was significantly lower than that in other strains, including its control strain, SAMR1. In contrast, the activity of isomaltase, which usually associates with sucrase to form a complex enzyme (SI complex), in SAMP1 was comparable to that in other strains. Thus, the ratio of the sucrase to isomaltase activities (S/I ratio) in SAMP1 was very low (about 0.15), compared with that in other strains (around 0.7). The S/I ratio in SAMP1 was abnormally low, even at a young age, indicating that senescence did not result in the low sucrase activity. Western blot analysis suggests that a large part of the isomaltase subunit occurred alone without the association of the sucrase subunit in this strain. In contrast, Northern blot analysis shows that the level of mRNA for the SI complex in SAMP1 was comparable to that in SAMR1. When the pancreatico-biliary ducts were ligated in SAMP1 to reduce the level of pancreatic proteases, a remarkable increase was observed in the sucrase activity, whereas the isomaltase activity was increased to a much smaller extent. This marked increase in sucrase activity resulted in the S/I ratio increasing to 0.84 18 h after the ligation. These results suggest the sucrase subunit of the SI complex to be abnormally unstable against pancreatic proteases in SAMP1.