Association and metabolism of exogenously-derived lysophosphatidylcholine by cultured mammalian cells: kinetics and mechanisms

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Abstract
The association and metabolism of exogenously-derived lysophosphatidylcholine (lysoPC) with cultured mammalian cells from a variety of sources was studied, and a mechanism was defined by computer modeling for Vero cells. Cell monolayers were incubated with radiolabeled lysoPC, and the kinetics of disappearance from the medium, association with the cells, and metabolism by the cells of lysoPC were monitored both in the absence and in the presence of fetal bovine serum. Exogenously-supplied lysoPC first associated with cell membranes, followed by an almost complete conversion to phosphatidylcholine (PC). The kinetics of partitioning and metabolism were identical regardless of whether the exogenously-supplied lysoPC was labeled with [methyl-3H]choline or with [1-14C]palmitate. A two-step mechanism, consisting of a reversible partitioning of exogenous lysoPC into the cell membrane followed by enzymatic reacylation of PC, was found to adequately describe the observed kinetics in the presence of 0 or 0.5% fetal bovine serum. The effect of temperature on the individual rate constants and on the overall process was examined. An Arrhenius plot indicated an acute temperature sensitivity between 15 and 23 degrees C, consistent with a dependence on the lipid phase of the membrane and a regional phase transition temperature characteristic of mammalian cells. The acute temperature sensitivity was almost entirely due to the temperature dependence of reacylation. A multistep mechanism was established by combining the kinetic constants determined under conditions of low exogenous protein with the binding constant between lysoPC and serum protein. This mechanism accurately predicts the rates of uptake of exogenously-derived lysoPC with cultured cells in the presence of serum concentrations between 0 and 10%. A survey of a variety of cultured cells indicated that the kinetics of association and metabolism of exogenously-derived lysoPC is cell-type specific.

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