Introduction to Human Nutrition

96 Introduction to Human Nutrition

Re-esterifi cation of triacylglycerols in
the enterocyte

Once LCFAs have entered the cell they are activated
by acyl-CoA and are re-esterifi ed 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 fi nally 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-esterifi ed 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-esterifi ed in the enterocyte by the enzyme

ACAT.

Lipoprotein assembly and secretion

Plasma lipoproteins are a family of spherical, macro-

molecular complexes of lipid and protein, the princi-

pal function of which is to transport endogenous

lipids (synthesized in the liver) and exogenous

lipids (synthesized in the gut from dietary fats)

from these sites of production and absorption to

peripheral sites of utilization (e.g., oxidation in

muscle, incorporation in membranes, or as precur-

sors of biologically active metabolites) and storage

(e.g., adipose tissue).

Via lymph

FA

FA

Energy in

muscle

TAG in

adipose tissue

VLDL

VLDL

IDL

LDL

50% 50%

80% 20%

Capillary

Exogenous lipid transport pathway Endogenous lipid transport pathway

Intestine

Liver Liver Other tissues

Liver

CM

CM

Energy in

muscle

TAG in

adipose tissue

CM

remnant

Lipoprotein

lipase

Lipoprotein

lipase

Hepatic

lipase

Figure 6.7 Re-esterifi cation 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.)