[Involvement of electron transfer system on microsomal fatty acid chain elongation]

It has been demonstrated that liver microsomal electron transfer system participates in lipid metabolism, fatty acid desaturation or cholesterol biosynthesis. Recent reports have shown the participation of cytochrome b5 and NADPH-cytochrome P-450 reductase (FPT) on NADPH-initiated microsomal fatty acid chain elongation. However, it remains unclear in which reductive step of the elongation system they participate. In the present study, the author examined carefully the composition of microsomal chain elongation products in the presence or absence of anti-cytochrome b5 or anti-FPT IgG. The reaction products from [2-14C] malonyl-CoA and palmitoyl-CoA by rat liver microsomes were mainly 18 : 0 and 18 : 1. However, when anti-b5 or anti-FPT IgG was included in this system, not only total chain elongation was suppressed but also a new radioactive product was appeared significantly (20-30%) besides elongated fatty acids. The new product was identified as 2-heptadecanone derived from beta-ketostearic acid, which increased depending upon the amounts of antibodies added. These results suggest that cytochrome b5 and FPT participate in the first reductive step, conversion from beta-ketostearoyl-CoA to beta-hydroxystearoyl-CoA, in the microsomal chain elongation system.