Intracellular processing of disaccharidases: The effect of actinomycin D

Abstract

1. 1. The effects of actinomycin D on the specific activities and intracellular distribution of rat jejunal disaccharidases were studied in rats fed a standard lab chow diet and a carbohydrate-free diet. With the standard diwet, brush border sucrase and lactase activities decreased 50% within 18 h of actinomucin D injection (0.25 μg/g body wt.), and continued to fall with further drug treatment, Maltase activity in teh brush border remained at control levels for the first 23 h, but then declined progressively. By contrast, sucrase and lactase activities in both jejunal homogenates and microsomal fractions increased sifnificantly within 24 h of actinomycin D administration. However, with continued treatment sucrase activity in the homogenates fell rapridly while lactase and maltase activities declined more slowly. No significant change occurred in isolmaltase activity in either homogenates or brush borders. Actinomycin D cuased a 50% reduction in total jejunal RNA synthesis 18 h after injection. Incorporation of 14C-labelled amino acids into total jejunal and microsomal proteins was not affected, but the labelling of brush border proteins showed a profressive reductino during the first 24 h after injection of actinomycin D. In rats maintained on a carbohydrate-free diet, disaccharidase activities fell below the levels found in animals n the standard diet. Furthermore, actinomycin D did not cause the early rise in the specific activities of sucrase and lactase in jejunal homogenates seen with carbohydrate feeding. Actinomycin D added to homogenates and brush borders in vitro had no effect on enzyme activity. 2. 2. These results suggest that the mechanismsm controlling the intracellular levels of the individual disaccharidases function independently, and that in the presence of dietary carbohydrate actinomycin D effects a change in teh intracellular distribution of these enzymes which is abolished by carbohydrate starvation. The data also support hypothesis that an actinomycin D sensitive step is required for the transfer of disaccharides from sites of synthesis to the brush border.