Kinetics of spontaneous and plasma-stimulated sphingomyelin transfer

The mechanism of transfer of a pyrene-labeled sphingomyelin (PySM) between different lipid compartments was studied by a fluorescence technique. The first-order kinetics are independent of donor and acceptor concentration and the identity of the acceptor; the rates are accelerated by 'structure-breaking' solutes and inhibited by 'structure-making' solutes. These observations are consistent with the transfer of PySM occurring via the aqueous phase that separates the donor and acceptor compartments. We have partially purified a plasma factor that stimulates the transfer rate. Our in vitro results suggest that both spontaneous and stimulated transfer might contribute to the redistribution of sphingomyelin in vivo.