Myristate is selectively incorporated into surfactant and decreases dipalmitoylphosphatidylcholine without functional impairment

Abstract

Lung surfactant mainly comprises phosphatidylcholines (PC), together with phosphatidylglycerols and surfactant proteins SP-A to SP-D. Dipalmitoyl-PC (PC16:0/16:0), palmitoylmyristoyl-PC (PC16:0/14:0), and palmitoylpalmitoleoyl-PC (PC16:0/16:1) together comprise 75-80% of surfactant PC. During alveolarization, which occurs postnatally in the rat, PC16:0/14:0 reversibly increases at the expense of PC16:0/16:0. As lipoproteins modify surfactant metabolism, we postulated an extrapulmonary origin of PC16:0/14:0 enrichment in surfactant. We, therefore, fed rats (d19-26) with trilaurin (C12:0(3)), trimyristin (C14:0(3)), tripalmitin (C16:0(3)), triolein (C18:1(3)) or trilinolein (C18:2(3)) vs. carbohydrate diet to assess their effects on surfactant PC composition and surface tension function using a captive bubble surfactometer. Metabolism was assessed with deuterated C12:0 (ω-d(3)-C12:0) and ω-d(3)-C14:0. C14:0(3) increased PC16:0/14:0 in surfactant from 12 ± 1 to 45 ± 3% and decreased PC16:0/16:0 from 47 ± 1 to 29 ± 2%, with no impairment of surface tension function. Combined phospholipase A(2) assay and mass spectrometry revealed that 50% of the PC16:0/14:0 peak comprised its isomer 1-myristoyl-2-palmitoyl-PC (PC14:0/16:0). While C12:0(3) was excluded from incorporation into PC, it increased PC16:0/14:0 as well. C16:0(3), C18:1(3), and C18:2(3) had no significant effect on PC16:0/16:0 or PC16:0/14:0. d(3)-C14:0 was enriched in lung PC, either via direct supply or via d(3)-C12:0 elongation. Enrichment of d(3)-C14:0 in surfactant PC contrasted its rapid turnover in plasma and liver PC, where its elongation product d(3)-C16:0 surmounted d(3)-C14:0. In summary, high surfactant PC16:0/14:0 during lung development correlates with C14:0 and C12:0 supply via specific C14:0 enrichment into lung PC. Surfactant that is high in PC16:0/14:0 but low in PC16:0/16:0 is compatible with normal respiration and surfactant function in vitro.