In this article we will discuss about the process of digestion and absorption of lipids.
Digestion of Lipids:
Whereas previous hydrolyzing action of enzymes is the determining factor for the absorption of carbohydrates and proteins, absorption of lipids depends not only on the presence of lipolytic enzymes, but also on the degree of emul-sification of lipids in the intestine.
In fact, it has even been possible to achieve absorption without hydrolysis, of lipids emulsified in extremely fine droplets thanks to a synthetic detergent, much more efficient than the natural emul-sifiers.
These natural agents are the bile salts (especially sodium glycocholate) which, with the help of other emulsifiers like the phospholipids and the mono- and diglycerides (resulting from the partial hydrolysis of triglycerides) will permit the absorption of a mixture of partially hydrolyzed lipids.
But in the first instance, the action of bile salts already manifests itself during digestion; they stabilize the pancreatic lipase and prevent its inactivation in the intestinal lumen.
a) The Pancreatic Lipases:
They are esterases which specifically hydrolyze the glycerol esters. The bonds in position 1 and 3 are the first to be broken, thus transforming a triglyceride into a 2-monoglyceride. The 2-monoglycerides are isomerized spontaneously to 1-monoglycerides which are then hydrolyzed. The pancreatic lipase does not act on lipids in solution, but only on micelles of triglycerides.
b) The Cholesterol-Esterase:
This enzyme hydrolyzes the ester linkage of sterides, form in which dietary cholesterol is generally found, thus liberating sterol and the fatty acid.
c) The Phospholipases:
There are 4 types.
Phospholipase A1 detaches the fatty acid in the position 1 from glycerophos- pholipids.
Phospholipase A2 detaches the fatty acid in the position 2 from glycerophos- pholipids.
In both cases one obtains a phospholipid having only one fatty acid, called lysophosphatide (in the case of a lecithin, the degradation by a phospholipase A1 or A2 gives lysolecithin).
Lysophosphatides are very powerful surface active agents capable of causing serious hemolysis. This may explain, at least partially, the toxic action of certain snake venoms which contain high concentrations of phospholipase A.
Phospholipase A2 plays a very important role in the biosynthesis of eicosanoids. This enzyme liberates from phosphatides, arachidonic acid, precursor of eicosanoids. The activators of the synthesis of these hormones are all activators of phospholipase A2. The same enzyme can hydrolyze alkyphos-phatidylcholines to alkyl-2-lyso derivatives. The alcohol group liberated is acetylated yielding PAF acether.
The successive action of phospholipases A1 and A2 (or in the reverse order) detaches the 2 fatty acids from the molecule of the phospholipid to give a glyceryl-phosphoryl-derivative (glyceryl-phosphoryl-choline in the case of a lecithin).
Phospholipase C hydrolyzes the ester linkage between phosphoric acid and glycerol, thus liberating the prosphorylated nitrogenous base (e.g., phosphoryl- choline) and a diglyceride.
Phospholipase D splits the ester linkage between the nitrogenous base (choline, ethanolamine, etc.) and phosphoric acid, and thus liberates the base and phosphatidic acid.
d) The Phosphodiesterases:
As indicated by their name, these enzymes are specific of the phosphodiester linkages and by acting on glyceryl-phosphoryl-choline for example (obtained, by action of phospholipases A on a lecithin) they will:
1. Either break the ester linkage between phosphoric acid and choline, thus liberating choline and glycerophosphate,
2. Or break the ester linkage between phosphoric acid and glycerol, thus liberating glycerol and phosphorylcholine (see fig. 5-3).
e) The Phosphomonoesterases (or Phosphatases):
They complete the hydrolysis of the products liberated by the phosphodiesterases; for example, from phosphorylcholine they liberate choline and phosphoric acid.
Absorption of Lipids:
The product passing through the intestinal mucosa is therefore a mixture of lipids more or less hydrolyzed (in fact, an emulsion containing fatty acids and also mono-, di-, triglycerides, phospholipids in more or less hydrolyzed state, sterols, etc.).
Glycerol and the short-chain fatty acids (up to 10 carbon atoms) will proceed to the liver through the blood vessels (particularly the portal vein).
All other long-chain fatty acids as well as the mono- and diglycerides contribute to the reconstitution of triglycerides in the intestinal mucosa; these neosynthesized triglycerides will be brought to the liver through the lymphatic vessels and then the blood stream, in the form of lipoprotein particles, called chylomicrons; the latter are of about 1 µ diameter and consist of 98% lipids (mainly triglycerides) and 2% proteins.
The lipids which are not absorbed are found in the faeces, together with lipids (especially steroids) discharged into the intestine by the bile, and lipids excreted through the intestinal wall.
The transport by the blood stream, of the lipids synthesized in the liver to various tissues, particularly the adipose tissue (storage site of triglycerides) or the muscle (where fatty acids are utilized for energy production) takes place in the form of lipoproteins. The latter are complexes whose protein and lipid contents vary according to the particles, which enables a separation of the various lipoproteins according to their density.
After reaction with specific cellular receptors, the lipoproteins are re-arranged partly thanks to the action of the lipoprotein-lipase which catalyzes the hydrolysis of the triglycerides present in the lipoprotein, with liberation of fatty acids; these can then be metabolized in the cells as explained below.