09.12.2012 Views

Food Lipids: Chemistry, Nutrition, and Biotechnology

Food Lipids: Chemistry, Nutrition, and Biotechnology

Food Lipids: Chemistry, Nutrition, and Biotechnology

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

Figure 13 Stereochemical configuration of triacylglycerols or structured lipids with sn<br />

notation indicating stereochemical numbering of the carbon atoms of glycerol moiety. When<br />

the carbon in the 2-position is in the plane of the page <strong>and</strong> the 1- <strong>and</strong> 3-carbons behind the<br />

plane of the page, if the �OH on the 2-position of glycerol is drawn to the left, the top<br />

carbon becomes 1 <strong>and</strong> the bottom becomes 3. Thus, a structured lipid with octanoic acid on<br />

the 1-position, <strong>and</strong> oleic acid on the 2-position <strong>and</strong> decanoic acid on the 3-position is named<br />

sn-glycerol-1-octanoate-2-oleate-3-decanoate.<br />

in the sn-1 <strong>and</strong> sn-3 positions (92,93,95,96). The sn-2 position of TAG is determined<br />

by pancreatic lipase hydrolysis of the fatty acids at the sn-1 <strong>and</strong> sn-3 positions,<br />

followed by GLC analysis of the 2-MAG fatty acid methyl or ethyl ester. Detailed<br />

stereospecific analysis of the fatty acids at all three positions of the glycerol molecule<br />

was excellently reviewed by Small (93) <strong>and</strong> is not discussed in detail here. 13 CNMR<br />

was used to determine acyl position of fatty acids on glycerol molecule (94).<br />

Grignard reagent or Grignard degradation (97,98) is useful in obtaining the<br />

complete stereochemical structure of any TAG following pancreatic hydrolysis. In<br />

general, phospholipid derivatives (phosphatidylcholine, PC) of 1,2-DAG <strong>and</strong> 2,3-<br />

DAG are made by reacting with phospholipase A 2 (PLA 2). Since the sn-2 fatty acid<br />

is known, chemical analysis of the 2,3-diacyl-PC PLA 2 hydrolysis product gives the<br />

fatty acid at the sn-3 position. Similarly, chemical analysis of the 1,2-DAG hydrolysis<br />

product of PLA 2 gives the fatty acid at the sn-1 position. Alternatively, pancreatic<br />

hydrolysis of the 1,2-DAG followed by chemical analysis can give the fatty acid at<br />

the sn-1 position, since this enzyme is sn-1,3 specific.<br />

III. ABSORPTION, TRANSPORT, AND METABOLISM OF<br />

STRUCTURED LIPIDS<br />

The influence of TAG structure on lipid metabolism has been the subject of recent<br />

reviews <strong>and</strong> research efforts (92,93,99–101). SLs may be targeted for either portal<br />

or lymphatic transport. In one widely accepted pathway, C2:0 to C12:0 fatty acids<br />

are transported via the portal system <strong>and</strong> C12:0 to C24:0 via the lymphatic system<br />

(2). There is growing evidence that MCFAs may indeed be absorbed as 2-MAG,<br />

especially if they are esterified to the sn-2 position of the SL. The rate of hydrolysis<br />

at the sn-2 position of TAG is very slow, <strong>and</strong> as a result the fatty acid at this position<br />

remains intact as 2-MAG during digestion <strong>and</strong> absorption. Indeed, close to 75% of<br />

sn-2 position fatty acids are conserved throughout the process of digestion <strong>and</strong> absorption<br />

(102).<br />

LCTs are partially hydrolyzed by pancreatic lipase <strong>and</strong> absorbed slowly as<br />

partial glycerides in mixed micelles (93). The resulting LCFAs are reesterified <strong>and</strong><br />

incorporated into chylomicrons in the enterocyte, whereupon they enter the lymphatics<br />

to reach the general circulation through the thoracic duct. However, MCTs<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!