METABOLISM OF 3-[H-3]SPHINGOSINE SPHINGOMYELIN LABELED WITH [C-14] PALMITIC OR [C-14] LINOLEIC-ACID BY HEP G2-CELLS AND RAT-LIVER INVIVO

The metabolism of sphingomyelin labeled with 3-[H-3]sphingosine and [C-14]16:0 or [C-14]18:2 fatty acid was studied in cultured Hep G2 cells or macrophages and after injection into rats. In pulse-chase experiments, the loss of H-3 and C-14-label was more rapid when the cells had been pulsed with 18:2 than with 16:0 sphingomyelin. At the end of 24 h chase, the labeled ceramide contained more [C-14]18:2 fatty acid than [C-14]16:0. In addition, the H-3-label derived from 3-[H-3]sphingomyelin was recovered also as free sphingosine. After injection in vivo, more [H-3]sphingosine-labeled sphingomyelin was present in the liver 3 and 24 h after injection of 16:0 than after injection of 18:2 sphingomyelin. The ratio of [H-3]ceramide derived from 16:0 sphingomyelin to that derived from 18:2 sphingomyelin as percent of injected dose was 1.84 3 h after injection and 1.31 after 24 h. The ratio of H-3/C-14 in liver ceramide was 6.4 3 h after injection of 18:2 sphingomyelin and 3.4 after 16:0 sphingomyelin. The present results show that 3-[H-3]sphingomyelin is metabolized quite extensively and that the fate of the sphingosine moiety is related to the type of fatty acid present in the phospholipid. These findings indicate that there is little or no reutilization of 18:2 ceramide for sphingomyelin formation and suggest that sphingosine derived from 18:2 sphingomyelin is channeled primarily for catabolism.