PubMed1980
Past investigation has revealed that the circadian rhythm of intestinal sucrase activity in rats is primarily cued by the time of feeding. We examined the mechanism of the circadian rhythm by methods involving quantitative immunoprecipitation of sucrase-isomaltase protein and study of decay of radioactively labeled protein. Rats were placed on a controlled feeding regimen (1000-1500 h) and then sacrificed at 3-h intervals over a 24-h period. Immunotitration experiments indicated that the circadian rhythm was the result of changes in the absolute amount of sucrase-isomaltase protein present and not of changes in the enzyme's catalytic efficiency. To study the mechanism of this circadian variation in sucrase-isomaltase mass, [(14)C]sodium carbonate was injected and, after maximum incorporation into brush border protein, the rats were sacrified at 3-h intervals. Sucrase-isomaltase protein was isolated by immunoprecipitation, and the decrease in total disintegrations per minute over time was used to study degradation of the protein. Enzyme degradation was not constant but exhibited a clear circadian rhythm. The period of increasing enzyme mass was characterized by virtual cessation of enzyme degradation (t((1/2)) of 38 h), and the period of declining enzyme mass by rapid degradation (t((1/2)) of 6 h or less). We found similar changes in enzyme degradation in fasted animals, demonstrating that the changes were not the result of decreased isotope reutilization during feeding. We found no evidence of a circadian rhythm in [(14)C]leucine incorporation into the protein, suggesting that enzyme synthesis was constant. These results indicate that the circadian rhythm of sucrase activity represents changes in the total amount of enzyme protein that are, at least in large part, secondary to changes in the enzyme's degradation rate.