In vivo turnover of 1,2-dipalmitoylphosphatidylcholine and sphingomyelin in rabbit erythrocytes.

The in vivo turnover of both 1,2-dipalmitoylphosphatidylcholine (DPPC) and sphingomyelin (SM) in rabbit erythrocytes was studied. DPPC, either 14C-labelled in the fatty acyl chain at the 2-position of the glycerol moiety or 3H-labelled in the choline's methyl group, and [N-methyl-14C]SM (bovine) were introduced into the membrane of freshly isolated rabbit erythrocytes by using phospholipid transfer proteins. Thereafter, the labelled erythrocytes were reinjected into the bloodstream of the animal. Analysis of blood samples shows that both labels disappear from the circulating cells with the same rate, resulting in a half-time value of about 6.4-6.6 days. This result demonstrates that the loss of DPPC from the cells is due to transfer of intact molecules to the plasma and that a deacylation process is of no or minor importance as mechanism of renewal of DPPC. Labelled sphingomyelin, introduced into the rabbit erythrocyte membrane in a similar way, disappears from the circulating red cell with a half-time value of 15.5 days. This accounts for a daily replacement of the total SM pool by 3.2%.