Re-esterification of triacylglycerols in the enterocyte
Once LCFAs have entered the cell they are activated by acyl-CoA and are re-esterified with glycerol back into TAG and phospholipids by two distinct bio-chemical pathways, the 2-MAG and glycerol-3-phosphate (G-3-P) pathways. The difference between these two pathways lies in:
● their substrates of activation
● the former using 2-MAG and the latter α-glycero-3-phosphate
● their location within different cellular organelles: the 2-MAGs reside in the smooth endoplasmic reticulum and the G-3-P in the rough endoplasmic reticulum
● the periods during which they are most active.
The 2-MAG pathway is quantitatively of greater importance in the enterocyte of the intestine and thus predominates in the postprandial period, whereas the G-3-P pathway is more active in the postabsorptive phase in tissues such as liver, muscle, and adipose tissue. Following the absorption of a fatty meal and uptake of 2-MAG into the enterocyte, up to 90% of these molecules are rapidly acylated back to 1,2-dia-cylglycerol and finally TAGs by the sequential actions of three enzymes: CoA ligase, monoglycerol acyl-transferase, and diacylglycerol acyltransferase. In a similar fashion, lysophosphatidylcholine, produced by the action of pancreatic phospholipase ‘A’ on dietary phospholipids, is absorbed by the enterocyte and re-esterified back to phosphatidylcholine in the enterocyte by direct acetylation (Figure 6.7). The bulk of free cholesterol absorbed from the intestinal lumen is also re-esterified in the enterocyte by the enzyme ACAT.
Figure 6.7 Re-esterification of triacylglycerides in enterocytes. CM, chylomicron; FA, fatty acid; IDL, intermediate-density lipoprotein; LDL, low-density lipoprotein; TAG, triacylycerol; VLDL, very low-density lipoprotein. (Reproduced from Mangiapane EH, Salter AM, eds. Diet, Lipoproteins and Coronary Heart Disease. A Biochemical Perspective. Nottingham University Press, Nottingham, 1999, with permission of Nottingham University Press.)