Delivery of nitroxide spin label to cultured cells by liposomes

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Abstract
The positively charged nitroxide spin label, 2,2,6,6‐tetramethyl‐piperidine‐N‐oxyl‐4‐ trimethylammonium (Cat1), was encapsulated in two types of liposomes, phosphatidylserine/phosphatidylcholine (PS/PC) and phosphatidylserine/distearoylphosphatidylcholine/dipalmitoylphosphatidylcholine (PS/DSPC/DPPC). The liposomes were incubated with mouse thymus‐bone marrow (TB) cells to study the uptake and metabolism of nitroxides entrapped in Iiposomes. The effects of temperature, metabolic inhibitors, and fixation of cells were investigated. The results indicate that different mechanisms are involved in the uptake of these two types of liposomes. PS/PC liposomes interact predominately with the plasma membrane of TB cells and release Cat1 continuously, whereas the majority of PS/DSPC/DPPC liposomes are taken into the cells intact via endocytosis. These findings suggest that it may be possible to deliver nitroxides selectively, either to the membrane of cells or to their interior by manipulating the lipid com‐ position of the liposomes. This study also found that the rate of reduction of Cat 1 delivered using liposomes was increased under hypoxic conditions. Thus, the use of liposomes for in vivo delivery of nitroxides has the potential to provide NMR contrast that reflects different metabolic conditions. © 1988 Academic Press, Inc.