Intracellular events in the assembly of chylomicrons in rabbit enterocytes

The aim of this study was to determine the intracellular events in chylomic ron assembly in adult villus enterocytes. We have used novel methods for se paration of the intracellular components of the secretory compartment [roug h and smooth endoplasmic reticulum (RER and SER, respectively) and Golgi], and their membrane and luminal components, from villus enterocytes isolated from rabbit small intestine. The steady state composition of the component s of the secretory compartment and the intracellular pools of newly synthes ized apolipoprotein B-48 (apoB-48) and triacylglycerol (TAG) was determined . The observations indicate that the SER is the main site of TAG synthesis and of chylomicron assembly. Newly synthesized apoB-48 and TAG accumulate i n the SER membrane and are transferred into the lumen in a microsomal trigl yceride transfer protein-dependent step. In enterocytes isolated from chow- fed rabbits, in which fat absorption is relatively slow transfer of apoB-48 and TAG from the SER membrane into the lumen appears to be rate limiting. In enterocytes from fat-fed rabbits, TAG accumulates in the lumen of the SE R, suggesting that movement out of the SER lumen becomes rate limiting, whe n chylomicron secretion is markedly stimulated. In these cells, the cytosol ic TAG also increased to 450 mug/g enterocytes, compared with 12 mug/g ente rocytes from chow-fed rabbits, indicating that transfer of TAG from the SER membrane into the secretory pathway can become saturated, so that newly sy nthesized TAG moves into the cytosol.