Abstract
The lecithin-cholesterol acyl transferase (LCAT) activity in rat mesenteric lymph was examined as a possible source of chylomicron cholesteryl ester. Lymph activity was only 2-3% of rat serum activity. Removal of d less than 1.006 lipoproteins increased lymph LCAT activity, but only to 6-8% of that of serum. Relative to total cholesterol in the d greater than 1.08 g/ml fractions, lymph LCAT activity in lymph from fasting rats was less than serum, but in lymph from nonfasting rats the ratio LCAT/HDL-cholesterol reached levels greater than serum, suggesting a contribution of enzyme from the gut. Both LCAT activity and HDL concentration in mesenteric lymph increased during feeding. Subfractions of lymph that inhibited serum LCAT were: chylomicrons, VLDL, chylomicron lipid, VLDL apoprotein, and HDL apoprotein. In the rat, the low LCAT activity of mesenteric lymph was in part due to the low enzyme concentration present, and the activity was apparently lowered further by lipid-rich lipoproteins that inhibited the reaction. Enzyme inhibition due to the apoprotein fractions of lipoproteins is probably minor in the rat in vivo.
Highlights
T h e lecithin-cholesterol acyl transferase (LCAT) activity in rat mesenteric lymph was examined as a possible source of chylomicron cholesteryl ester
Cholesterol present in the intestinal lumen is absorbed entirely as unesterified cholesterol (UC) [2, 3] and is largely reesterified within mucosal cells [4].UC and cholesteryl esters (CE) are secreted by the mucosal cells and enter lymphatic channels in the triglyceriderich lipoproteins, chylomicrons, and very low density lipoproteins (VLDL)(5).T h e CE in mesenteric lymph has several potential sources. ( a )Since lymph is a filtrate of plasma, some plasma lipoprotein CE may enter the lymph directly via interstitial spaces
LCAT activity in fasting lymph was very low compared with blood serum and was reduced still further in lymph containing a high concentration of triglyceride-rich lipoproteins
Summary
T h e lecithin-cholesterol acyl transferase (LCAT) activity in rat mesenteric lymph was examined as a possible source of chylomicron cholesteryl ester. Relative to total cholesterol in the d > 1.08 g/ml fractions, lymph LCAT activity in lymph from fasting rats was less than serum, but in lymph from nonfasting rats the ratio LCAT/HDLcholesterol reached levels greater than serum, suggesting a contribution of enzyme from the gut Both LCAT activity and HDL concentration in mesenteric lymph increased during feeding. The low LCAT activity of mesenteric lymph was in part due to the low enzyme concentration present, and the activity was apparently lowered further by lipid-rich lipoproteins that inhibited the reaction. The experimental results show that the lymphatic LCAT activity is very low compared with plasma, and that both the lipid and the protein components of lymph lipoproteins may exert inhibitory effects of LCAT activity
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