Intestinal Absorption of Polyunsaturated Phosphatidylcholine in the Rat

Abstract

The mechanism of intestinal absorption of polyunsaturated phosphatidylcholine in an oil medium was studied with 1,2-di-[9,10,12,13-3H4]linoleoyl-sn-glycero-3-phospho-[N-14CH3]-choline, 1-[1-14C]linoleoyl-2-[9,10,12,13-3H4]-linoleoyl- and 1-[9,10,12,13-3H4]linoleoyl-2-[1-14C]linoleoyl-sn-glycero-3-phosphocholine, especially with regard to the stability of the ester bonds in position 1 and 2 of the phospholipid molecule. The absorption rate, as measured by the disappearance from the gastro-intestinal tract, was comparatively rapid in the first 6 - 8 h, but then became considerably slower. After 24 h more than 90% of the applied radioactivity was absorbed from the intestinal tract. Respiratory 14CO2 from the degradation of the unsaturated acyl moiety in position 2 is produced much more rapidly than that from the acyl group attached to the 1-position of the glycerophosphocholine backbone. Analyses of the liver phosphatidylcholine by specific enzymatic hydrolysis with phospholipase A2, 6 h after the application, showed that in the isolated PC 9 times more labelled fatty acids from the original 1-position were present than from the corresponding 2-position. In rats with lymph fistula it was shown that more than 90% of the acyl moieties of the administered 1,2-di-[9,10,12,-13-3H4]linoleoyl-[N-14CH3] glycerophosphocholine was transported by the chylomicrons. About one half of the 14C choline radioactivity of the glycerophosphocholine backbone was found in the chylomicrons and the other half in the liver. The 3H radioactivity distribution in the chylomicrons amounted to 25% in the phosphatidylcholine fraction and 75% in the neutral lipids. Positional specific analyses of the phosphatidylcholine present in chylomicrons confirmed the fact that the 1-position remained practically intact while the 2-position underwent considerable exchange with unlabelled fatty acids. Analysis of the liver of the animals with lymph fistula indicated that it was practically free of the 3H radioactivity derived from the acyl moieties but contained a high percentage of the 14C radioactivity of the choline group. The methyl groups of choline were oxidized only to a very small extent. These results demonstrate that during the absorption process, about one half of the absorbed polyunsaturated phosphatidylcholine is hydrolyzed to 1-acyl-lysophosphatidylcholine and reacylated again to phosphatidylcholine upon entering the mucosa cell, while the other half is completely hydrolyzed to free fatty acids and glycerophosphocholine or its hydrolysis products. The fatty acids released are utilized for the reassembly of triacylglycerides and phosphatidylcholine found in the chylomicrons.