Additional Studies on the Regulation of Carbohydrate-Dependent Enzymes in the Jejunum: Changes in Amino Acid Pools, Protein Synthesis, and the Effect of Actinomycin-D.

Abstract

Adaptive changes in rat jejunal disaccharidase levels during carbohydrate starvation and carbohydrate feeding were studied in relation to protein synthesis, amino acid pools, and cell migration. When animals were fed a sucrose-rich, normal protein diet for 24 hr after being on a casein-rich, carbohydrate-free diet for 7 days, there was 50% reduction in the size of the total amino acid pool without a significant change in the proportions of individual amino acids in the pool. The specific radioactivity of the leucine pool increased whenever carbohydrate was added to the diet and protein intake reduced. The incorporation of radioactive precursors into homogenate and brush border proteins reflected the characteristics of the pools, and was thus higher during carbohydrate feeding when the pool size was smaller. The rate of cell migration was unaffected by diet. Disaccharidase activities fell during prolonged carbohydrate starvation. When animals were treated with actinomycin-D, ribonucleic acid synthesis measured 18 hr later was reduced 50%, and cell migration was arrested. The drug did not inhibit the sucrose-stimulated rise in homogenate sucrase activity, although it did abolish the expected increase in fructokinase activity. With actinomycin-D treatment, brush border sucrase activity did not parallel the increases seen in the homogenate, but rather fell markedly. The data demonstrate that dietary carbohydrate increases jejunal disaccharidase activity without elevations in the rate of protein synthesis and that the effect is also independent of ribonucleic acid synthesis and cell migration. By contrast, jejunal fructokinase activity is probably regulated at the level of transcription.