Δ6- and Δ5-desaturase activities in the human fetal liver: kinetic aspects

Δ6- and Δ5-desaturase activities were studied in human fetal liver microsomes obtained after legally approved therapeutic abortion. Enzyme activities were measured by a radiochemical method using reverse-phase high performance liquid chromatography (HPLC). Free and phospholipid fatty acids were assessed in each liver sample by a combination of thin-layer chromatography (TLC) and gas–liquid chromatography (GLC) procedures. The kinetic measurements showed higher Δ6-desaturase activity for the n–3 series than for the n–6 series. Apparent Km of 6.5, 3.9, and 24.5 μm and Vm of 7.5, 9.1, and 24.4 pmol·min-1·mg-1 were obtained, respectively, for 18:2n–6 Δ6-, 20:3n–6 Δ5-, and 18:3n–3 Δ6-desaturases. Beyond 30, 20, and 60 μm of 18:2n–6, 20:3n–6, and 18:3n–3 concentration, respectively, the enzyme activity deviated from Michaelis-Menten kinetics, suggesting an inhibition by excess substrate which is unlikely to occur in vivo as endogenous substrate concentration is much lower. We observed a breakdown in linearity between desaturase activity and microsomal protein concentration beyond 4–5 mg microsomal protein, whatever the enzyme or substrate. Both this phenomenon and the inhibition due to excess substrate should be taken into account in the determination of Δ6- and Δ5-desaturase activities. Comparison of concentrations of the respective endogenous substrates and the kinetic constants of each enzyme suggested that the higher Δ6-desaturase activity observed for the n–3 series than for the n–6 series is not physiologically relevant in human fetal liver.—Rodriguez, A., P. Sarda, C. Nessmann, P. Boulot, C. L. Leger, and B. Descomps. Δ6- and Δ5-desaturase activities in the human fetal liver: kinetic aspects.