The second phase of chylomicron assembly takes place in the lumen of the endoplasmic reticulum. ApoB-48 continues to bind triacylglycerol, as well as cholesterol, cholesterol esters, and molecules of apolipoproteins A-I, A-II, and A-IV. The reaction is annotated here to involve small numbers of these molecules, but the true numbers in vivo are much greater - a nascent chylomicron entering the lymphatic circulation contains >200,000 molecules of triacylglycerol (TG), ~35,000 of phospholipid, ~11,000 of cholesterol ester, ~8,000 of free cholesterol, ~60 copies of apolipoprotein A-I, ~15 copies of apolipoprotein A-IV, and copies of apolipoprotein A-II (Bhattacharya and Redgrave 1981; Havel and Kane 2001).
The presence of MTP:PDI (microsomal triacylglycerol transfer protein:protein disulfide isomerase) is required for lipid addition both in vitro and in vivo, but its molecular role at this stage of chylomicron formation is unclear and may be indirect (Gordon et al. 1995; Hussain et al. 2003).