Elongation of acyl-CoAs by microsomes from etiolated leek seedlings

Long chain fatty acid synthesis was studied using etiolated leek seedling microsomes. In the presence of ATP, 2-¹⁴C]malonyl-CoA was incorporated into fatty acids of C₁₆C₂₆. The omission of ATP, even in the presence of acetyl-CoA, led to a complete loss of activity, which was restored by addition of exogeneous acyl-CoAs. Comparison of acyl-CoA (C₁₂C₂₄) elongation showed that stearoyl-CoA, in the presence of 2-¹⁴C]malonyl-CoA, was the more efficient precursor leading to the formation of fatty acids having a chain length of C₂₀C₂₆. 1-¹⁴C]C₁₆CoA and 1-¹⁴C]C₁₈CoA were elongated in the presence of malonyl-CoA, without degradation of the acyl chain. The time-course and the malonyl-CoA concentration curves showed that 1-¹⁴C]C₁₈CoA was a better primer than 1-¹⁴C]C₁₆CoA. Acyl-CoA elongation was also studied over the concentration range 4.5–45 μM 1-¹⁴C]C₁₈CoA. Comparison of the radioactivity incorporated into the fatty acids formed using 2-¹⁴C]malonyl-CoA in the presence of C₁₈CoA, on the one hand, and 1-¹⁴C]C₁₈CoA in the presence of malonyl-CoA, on the other, demonstrated clearly that the acyl chain of the acyl-CoA was elongated by malonyl-CoA.