METABOLISM OF CHYLOMICRON PHOSPHATIDYLINOSITOL IN THE RAT - FATE IN-VIVO AND HYDROLYSIS WITH LIPOPROTEIN-LIPASE AND HEPATIC LIPASE IN-VITRO

Phosphatidylinositol (PI), mainly stearoyl-arachidonyl PI, occurs as a minor phospholipid constituent in both chyle and plasma lipoproteins. The kinetics and the pathways by which plasma and chyle PI is metabol ized have not been investigated. The role of lipoprotein PI in the sup ply of arachidonic acid (20:4) and inositol lipid components to differ ent tissues is thus unknown. In this study we examined the fate of chy le PI in vivo and its hydrolysis by lipoprotein lipase (LPL), hepatic lipase (HL), and postheparin plasma in vitro. Chyle and chylomicrons w ere labeled in the PI portion by feeding [H-3]myo-inositol and in the phosphatidylcholine (PC) portion by feeding [C-14]choline in a linolea te-rich fat meal (Intralipid) to mesenteric duct-cannulated rats. Afte r intravenous injection of doubly labeled chyle into normal rats, [H-3 ]PI disappeared from plasma at a slower rate than [C-14]PC; after 60 m in 41.6 +/- 2.7% H-3 and 24.3 +/- 1.8% C-14(means +/- SEM, n = 4, P < 0.01) remained in plasma lipids. About 15% of both isotopes were in li ver lipids after 60 min. Previous injection of a blocking antiserum ag ainst rat HL did not significantly influence the serum and liver radio activity after 60 min. Radioactive PI was rapidly transferred to high density lipoproteins (HDL) during the metabolism of chylomicrons. Anal ysis of H-3 and C-14 in different molecular species of PI and PC in ch yle and in serum indicated that there was no significant difference in disappearance rates between Various species, stearoyl-arachidonyl PI thus disappearing at the same rate as total [H-3]PI. Both lipoprotein lipase (LPL) and HL catalyzed formation of lyse-PI in vitro, the rate being increased by the addition of serum. About 60% of the lyse-PI for mation catalyzed by postheparin plasma in 60 min could be blocked by a ntiserum to HL, which almost completely blocked the hydrolysis occurri ng after the first 10 min. The study thus shows that both LPL and HL h ydrolyze chylomicron PI in vitro. LPL and HL may, however, be of limit ed importance for the clearance of chyle PI in vivo, most of the chylo micron PI being transferred to HDL, and thereafter eliminated from pla sma at a slow rate mainly by other mechanisms.