The Intestinal Brush Border Membrane in Diabetes

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Abstract
Diabetes stimulates the functional activity of the intestinal brush border membrane with enhancement of both hydrolytic enzyme activity and membrane transport systems. To determine the mechanism of this effect, we studied the effects of streptozotocin diabetes on the metabolism of one membrane protein, sucrase-isomaltase, which increases its activity in diabetes. The protein was purified and an antiserum prepared. Sucrase-isomaltase from control and diabetic rats was immunologically identical as shown by Ouchterlony double-diffusion analysis of papain-solubilized mucosal proteins. The increase in sucrase enzyme activity in diabetic animals (31.0±1.4 U SEM 5 days after streptozotocin vs. 13.1±1.0 in controls) was the consequence of increased enzyme protein and not an alteration in catalytic efficiency as demonstrated by quantitative immunoprecipitin reactions. To account for increased sucrase-isomaltase protein in diabetes we studied papain-solubilized mucosal proteins labeled by injection of [14C]carbonate and [14C]leucine and analyzed incorporation into sucrase-isomaltase protein (anti-serum precipitable) and total protein (trichloroacetic acid precipitable). We found that diabetes did not affect the decay of labeled total protein, but prolonged the decay of labeled sucrase-isomaltase. t½ of sucrase-isomaltase was 4.4 h in control animals after [14C]carbonate injection and 8.8 and 10.2 h, respectively, 2 and 5 days after induction of streptozotocin diabetes. We obtained similar results in experiments with [14C]leucine with diabetes increasing t½ from 6 to 13.6 h. Diabetes did not appear to increase the rate of addition of sucrase-isomaltase to the brush border membrane, since it did not affect the 10- and 60-min incorporations of isotope into sucrase-isomaltase protein relative to incorporation into total protein and did not alter rate constants for synthesis calculated from the t½ and the change in enzyme mass over time. Thus, enhanced sucrase activity in the diabetic animal is the consequence of an increase in sucrase-isomaltase protein which develops because of a decrease in its rate of degradation.