Copper Deficiency Increases Total Protein and Apolipoprotein A-I Synthesis in the Rat Small Intestine 1 2

Abstract

This study was designed to determine whether an enhanced intestinal synthesis of apolipoprotein (apo) A-I is associated with the hyperapolipoproteinemia observed in copper-deficient rats. Male weanling Sprague-Dawley rats were assigned to two dietary treatments, Cu deficient (0.6 ppm Cu) and Cu adequate (6.0 ppm Cu) for 6 weeks. In vivo studies were then performed after rats were injected with a flooding dose of 150 μM [ 3 H]phenylalanine (PHE, 50 μCi/ml/100 g body wt). Three rats from each treatment were sacrificed at 5, 10, 15, 30, and 60 min postinjection. The small intestine was rapidly rinsed and frozen in liquid N₂. In vitro studies were performed by labeling freshly isolated 6-cm segments from duodenum, jejunum, and ileum with [ 3 H]PHE (33 μCi/ml, 49.7 Ci/mmol) in PHE-free minimum essential medium for 7 and 14 min. In vivo and in vitro intestinal samples were sonicated, solubilized in 1% Triton X-100, and centrifuged to provide the detergent soluble fraction for the isolation of nascent apo A-I and total protein. Radioactivities associated with nascent apo A-I isolated by immunoprecipitation and SDS-PAGE, and with total protein precipitated by trichloroacetic acid, were measured to determine the influence of Cu deficiency on nascent apo A-I and total protein synthesis. In the Cu-deficient small intestine, the synthesis of total protein was measured only in the duodenum and was enhanced after 1 hr for the in vivo studies. Moreover, total protein synthesis was enhanced at both 7 and 14 min of the in vitro studies for all three small intestinal segments of the Cu-deficient rats. Apo A-I synthesis was measured only at the jejunum and was also enhanced by Cu deficiency in the in vitro studies. Thus, an increase in intestinal apo A-I synthesis may contribute to the elevated plasma apo A-I level in Cu-deficient rats.