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 [1-¹⁴C]-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.