The species of acyl-CoA in subcellular fractions of type II cells isolated from adult rat lung and their incorporation into phosphatidic acid.

Microsomes and cytosol were prepared from type II cells isolated from adult rat lung. Upon determination of the acyl-CoA composition in the microsomes, we found 49% palmitoyl-CoA, 2% myristoyl-CoA, 21% stearoyl-CoA, 5% palmitoleoyl-CoA, 16% oleoyl-CoA, 5% linoleoyl-CoA and 2% arachidonoyl-CoA. The acyl-CoA composition of the cytosol was very similar. Upon incubation of type II cell microsomes with [U-14C]glycerol 3-phosphate and with acyl-CoA species mixed in the proportions in which they were found in this cell fraction, approx. 40% of the synthesized phosphatidic acid was disaturated. Of the two quantitatively most important acyl-CoA species, the palmitoyl species was incorporated 4-times faster into total and disaturated phosphatidic acid than the stearoyl species. These two species were distributed very similarly among the phosphatidic acid species synthesized de novo. In newly formed disaturated phosphatidic acid, the palmitoyl groups were distributed approximately equally between the 1- and the 2-position. From these data, it can be estimated that of the phosphatidic acid molecules synthesized by type II cell microsomes, approx. 26% contain two palmitoyl moieties. Assuming that both phosphatidic acid phosphatase and cholinephosphotransferase are non-selective with regard to the substrate species that they convert, this would mean that 26% of the phosphatidylcholine molecules synthesized de novo would be dipalmitoylphosphatidylcholine. As in surfactant, approx. 60% of the phosphatidylcholine is constituted by the dipalmitoyl species, this would mean that approx. 45% of the surfactant dipalmitoylphosphatidylcholine would be made via de novo synthesis.