Biosynthesis of medium-chain triacylglycerols and phospholipids by HepG-2 cells

In an attempt to understand the metabolism by the liver of fatty acids (FA) of different chain length, we have studied the incorporation of [1(-14)C]-labeled C2, C8, C10, C12, and C16 into cellular lipids by HepG-2 cells. Over 90% of the radiolabeled FA were detected in phospholipids (PL) and triacylglycerols (TAG). The incorporation of C12 and C16 was three to four times higher than that of C8 and C10 (and reached 35 nmoles per mg protein after 1.5 h). The radioactivity of C2, C8, and C10 was recovered mainly in PL. C12 and C16 were incorporated at approximately equal amounts into PL and TAG. The radioactivity of both C2 and C8 was recovered exclusively in long-chain FA, suggesting oxidation of C8 into C2 units prior to FA synthesis. C10 likewise yielded mainly long-chain FA. However 10% of unchanged C10 was found in PL and up to 30% in TAG. 14C-C12 was largely incorporated unchanged. Under these conditions, the presence of C10 and C12 in PL and TAG was shown also by gas-liquid chromatography. In the presence of either C2, C8, or C10, up to 30% of 14C-monounsaturated FA were detected in PL and TAG. With C12 and C16, the fraction of 14C-monounsaturated FA was much smaller suggesting that extensive desaturation occurred during de novo synthesis.