Characterization of plasma unsaturated lysophosphatidylcholines in human and rat

Unsaturated lysophosphatidylcholines (lysoPtdCho) bound to albumin circulat e in blood plasma and seem to be a novel transport system for carrying poly unsaturated fatty acids (PUFA) to tissues that are rich in these fatty acid s, such as the brain. The potential of these lysoPtdCho as a significant so urce of PUFA for cells has been assessed by comparing their plasma concentr ation with that of unsaturated non-esterified fatty acids (NEFA) bound to a lbumin. In humans, the PUFA concentration was 25.9+/-3.1 nmol/ml for these lysoPtdCho, compared with 33.4+/-9.6 nmol/ml for NEFA; in rats the equivale nt values are 14.2 +/- 0.6 and 13.1 +/- 1.1 nnol/ml respectively (means +/- S.E.M.). The lysoPtdCho arachidonic acid content was 2-fold (human) and 5- fold (rat) higher than that of NEFA. In human and rat plasma, unsaturated l ysoPtdCho were associated mainly with albumin rather than lipoproteins. The rate and extent of the acyl group shift from the sn-2 to sn-1 position of these lysoPtdCho were studied by the incubation of 1-lyso,2-[C-14]C(18:2)n - 6-glycerophosphocholine (GPC) with plasma. The rapid isomerization of thi s lipid occurred at pH 7 (20 % isomerization within 2 min) and was not prev ented by its association with albumin. The position of the acyl group in th e lysoPtdCho circulating in plasma was studied by collecting blood directly in organic solvents containing 1-lyso,2-[C-14]C(18:2)n - 6-GPC as a marker of isomerization that occurred during sampling and analysis. Approx. 50% o f the PUFA was located at the sn-2 position, demonstrating that substantial concentrations of 2-acyl-lysoPtdCho are present in plasma and are availabl e for tissue uptake, where they can be reacylated at the sn-1 position to f orm membrane phospholipids.