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 C ₂, C ₈, C ₁₀, C ₁₂, and C ₁₆ into cellular lipids by HepG-2 cells. Over 90% of the radiolabeled FA were detected in phospholipids (PL) and triacylglycerols (TAG). The incorporation of C ₁₂ and C ₁₆ was three to four times higher than that of C ₈ and C ₁₀ (and reached 35 nmoles per mg protein after 1.5 h). The radioactivity of C ₂, C ₈, and C ₁₀ was recovered mainly in PL. C ₁₂ and C ₁₆ were incorporated at approximately equal amounts into PL and TAG. The radioactivity of both C ₂ and C ₈ was recovered exclusively in long-chain FA, suggesting oxidation of C ₈ into C ₂ units prior to FA synthesis. C ₁₀ likewise yielded mainly long-chain FA. However 10% of unchanged C ₁₀ was found in PL and up to 30% in TAG. ¹⁴C- C ₁₂ was largely incorporated unchanged. Under these conditions, the presence of C ₁₀ and C ₁₂ in PL and TAG was shown also by gas liquid chromatography. In the presence of either C ₂, C ₈, or C ₁₀, up to 30% of ¹⁴C-monounsaturated FA were detected in PL and TAG. With C ₁₂ and C ₁₆, the fraction of ¹⁴C-monounsaturated FA was much smaller suggesting that extensive desaturation occurred during de novo synthesis.