Intracellular movement of triacylglycerols in the intestine

Abstract

The intestine can vary its triacylglycerol output rate depending on differing physiological conditions. The rate-limiting step in the complex process from fatty acid and monoacylglycerol entry to triacylglycerol export is unknown but suggested to be the transport of triacylglycerol from the endoplasmic reticulum to the Golgi. The present studies were carried out to test this hypothesis. The conversion rate of absorbed fatty acid to mucosal triacylglycerol was studied in rats infused intraduodenally with trioleoylglycerol, 135 mol/h, for 6 h followed by [ 3 H]oleate. In 30 sec, 79% of the mucosal 3 H-labeled fatty acid was esterified to [ 3 H]triacylglycerol. The increase in the 3 H specific activity of triacylglycerol in the endoplasmic reticulum and Golgi was studied in similarly prepared rats except that the radiolabel was [ 3 H]trioleoylglycerol. The endoplasmic reticulum triacylglycerol specific activity was always less than that of the Golgi with a steady state not reached until 60 min of [ 3 H]trioleoylglycerol infusion. The steady state of [ 3 H]triacylglycerol in the lymph was not reached until 70 min of infusion. We conclude that the data are consistent with the rate-limiting step in intestinal triacylglycerol export being the movement of triacylglycerol from the endoplasmic reticulum to the Golgi as the conversion of absorbed fatty acid to triacylglycerol is rapid and the movement of triacylglycerol from the Golgi to the lymph is rapid as well.-Mansbach II, C. M., and P. Nevin. Intracellular movement of triacylglycerols in the intestine.

Effect of increasing lipid loads on the ability of the endoplasmic reticulum to transport lipid to the Golgi

“…At the highest rate of infusion, 135 mol/h, not only was the attainment of the steady state further delayed as compared to the 90 mol/h input rate, but the radiolabeled TG did not enter a common TG pool. This can be shown by the fact that the TG in the Golgi had a higher specific activity than that of the ER, indicating that a TG-pool containing exogenous TG is preferentially transported from the ER to the Golgi as we have previously suggested (4).…”

“…Further, chylomicron-TG is derived primarily from dietary TG, utilizing MG as the glyceride-glycerol precursor (16,17,26). There is direct delivery of newly synthesized TG to the developing chylomicron (4,16,26). Even when chylomicron output is blocked by the non-ionic detergent, Pluronic L-81 (L81), the TG that is blocked in the ER from further progression (10), when released into the lymph after the removal of the L81, shows little evidence of hydrolysis (17).…”

“…Non-fasting Sprague-Dawley rats, 250-350 g, had a PE 50 (PE 50, Clay Adams, Parsippanny, NJ) cannula placed in the duodenum through a right sub-costal incision and in the jugular vein as previously described (4). The tip of the intraduodenal (ID) cannula was at the level of the entry of the common bile duct.…”

Isolated rabbit enterocytes as a model cell system for investigations of chylomicron assembly and secretion

“…The newly synthesized TAG was secreted only slowly by isolated enterocytes with a lag of ~30 min. A similar delay occurs in vivo (22). During the 120 min incubation period, only about 5% of the newly synthesized apoB-48 and TAG was secreted by isolated enterocytes.…”

“…The most probable explanation for this discrepancy lies in the markedly different experimental protocols. Mansbach and Nevin (22) infused lipid in vivo into the duodenum of rats for 6 h followed by a bolus of radiolabeled triolein. They then scraped the mucosa and prepared subcellular fractions for analysis.…”