The relation of size to the relative rates of degradation of intestinal brush border proteins.

Proteins associated with intestinal brush borders and their various fractions were solubilized with sodium dodecyl sulfate and beta-mercaptoethanol, and separated by electrophoresis on acrylamide gels containing sodium dodecyl sulfate. Brush borders contain at least 15 proteins or subunits, ranging in molecular weight from 19,000 to 270,000. The largest proteins (170-270,000 mol wt), including the disaccharidases, are removed from the brush borders by papain. Proteins belonging to the remaining membrane, including alkaline phosphatase, have an intermediate size (53-140,000 mol wt). The proteins corresponding to the filamentous "core" of the microvilli are the smallest (19-45,000). The relative rates of degradation of these proteins were studied by following the rate of decline of (14)C-labeled leucine activity in specific proteins, and by the double isotope technique of Schimke in which leucine-(14)C was given to intact rats intraluminally 10 hr before an intraluminal dose of leucine-(3)H. Heterogeneity of (3)H/(14)C ratios and thus of rates of turnover of brush border proteins was noted. In general, the largest proteins (including the disaccharidases), were turning over the fastest. Other membrane proteins (i.e. alkaline phosphatase) had an intermediate rate of degradation, and "core" proteins turned over slowly. Thus, there was a general correlation between relative degradation rate and size.