On the basis of earlier studies of rabbit pulmonary alveolar macrophages, the incorporation of 14C-labelled
polyunsaturated fatty acids into the
lipids of human fibroblasts from patients with various phenotypes of
Niemann-Pick disease was examined in order to define further the disturbance in metabolism of
bis(monoacylglycero)phosphate occurring in these disorders.
Docosahexaenoic acid, which had not been studied previously, was found to be incorporated by macrophages into
bis(monoacylglycero)phosphate in a highly selective fashion and was therefore used along with
arachidonic acid for studies of fibroblasts. Following incubation of fibroblasts in serum-free medium for 60 min, the distribution of
arachidonic acid label in
lipids was:
phosphatidylcholine, 51%;
phosphatidylethanolamine, 12%;
phosphatidylinositol, 9.5%; and
bis(monoacylglycero)phosphate, 2.3%; and of
docosahexaenoic acid label was 36, 20, 2.6 and 10.3% respectively.
Phosphatidylinositol had the highest specific activity of
arachidonic acid label and
bis(monoacylglycero)phosphate of
docosahexaenoic acid label. Prolongation of incubation to 21 h, with or without removal of label remaining in the medium at 1 h, resulted in proportional redistributions with
phosphatidylcholine decreasing and
phosphatidylethanolamine increasing. In
bis(monoacylglycero)phosphate and
phosphatidylinositol, the proportions of
arachidonic acid label decreased and increased respectively, whereas the proportions of
docosahexaenoic acid label in these
lipids were unchanged. As virtually all label taken up by cells was esterified, these redistributions are taken to reflect transacylations. In Niemann-Pick cells, the expected redistribution of
arachidonic acid label in
bis(monoacylglycero)phosphate failed to occur with cell types A and B which are deficient in
sphingomyelinase-
phospholipase C, and excess label accumulated after a 21-h incubation. Excess
docosahexaenoic acid label also accumulated in the
bis(monoacylglycero)phosphate of these cells. The highly selective incorporation of
docosahexaenoic acid in two cell types suggests a special role for
bis(monoacylglycero)phosphate in the metabolism of n-3
polyunsaturated fatty acids. A high specific activity found early in incubations of macrophages suggests that
polyunsaturated fatty acids may be incorporated into
phospholipids during de novo synthesis of
phosphatidic acid.