K Kirsch

Fat Absorption During Inhibition of Protein Synthesis: Studies of Lymph Chylomicrons

The effect of protein synthesis inhibition on the absorption of oleic acid from micellar solution was studied in mesenteric lymph fistula rats. A micellar solution of oleic acid labeled with tracer doses of oleic acid-(14)C was administered by intraduodenal infusion to rats with indwelling mesenteric lymph cannulas. Protein synthesis was inhibited by intraperitoneal acetoxycycloheximide (ACH), 0.25 mg/kg, 1 hr before lipid infusion. Lymph chylomicrons labeled with oleic acid-(14)C were collected from control and protein inhibited animals at various times after lipid infusion and subjected to sucrose density gradient centrifugation to determine changes in size. In control animals there was a transient increase in chylomicron size during maximal triglyceride absorption; however, in protein-inhibited animals there was a marked and sustained increase in chylomicron size as late as 4 hr after lipid infusion. Triglyceride and phospholipid determinations on washed chylomicrons from both groups indicated a greater triglyceride/phospholipid ratio after protein synthesis inhibition supporting a greater chylomicron size. Electron microscopy of lymph from both groups further confirmed a markedly increased chylomicron size after protein synthesis inhibition. It is proposed that an increase in size conserves chylomicron surface components, i.e. apoprotein, during conditions of inhibition of protein synthesis. These studies clearly demonstrate that the intestinal inhibition of protein synthesis is associated with an increase in the size of intestinal lymph chylomicrons and support the concept that protein synthesis is important in the formation and transport of chylomicrons from the mucosal cell into the lymph.

Intestinal Glycoprotein Synthesis and the Redistribution of Glycoproteins into Different Parts of the Surface Membrane

Publisher Summary This chapter describes an experiment to study the process of plasma membrane glycoprotein biosynthesis and the redistribution of these glycoproteins within the plasma membrane. In the experiment, newly synthesized glycoproteins were labeled in vivo by the intraperitoneal injection of radioactive fucose. The experiment was conducted by isolating Golgi, lateral-basal, and microvillus membranes at different times after radioactive fucose injection, focusing the attention on the kinetics of label incorporation and on the distribution of the label among different membrane glycoproteins in the various fractions. It has been shown, on the basis of autoradiographic evidence, that, shortly after injection, the label is concentrated in the Golgi complex. At later times, first the lateral and then the luminal membranes of the villus cell are labeled. Intracellularly, uncharacterized dense bodies and multilamellar vesicles, possibly involved in secretion, are the only other cell components that appear to be labeled.