DELTA-6-DESATURASE AND DELTA-5-DESATURASE ACTIVITIES IN THE HUMAN FETAL LIVER - KINETIC ASPECTS

Delta 6- and Delta 5-desaturase activities were studied in human fetal liver microsomes obtained after legally approved therapeutic abortion . Enzyme activities were measured by a radiochemical method using reve rse-phase high performance liquid chromatography (HPLC), Free and phos pholipid fatty acids were assessed in each liver sample by a combinati on of thin-layer chromatography (TLC) and gas-liquid chromatography (G LC) procedures. The kinetic measurements showed higher Delta 6-desatur ase activity for the n-3 series than for the n-6 series. Apparent K-m of 6.5, 3.9, and 24.5 mu M and V-m of 7.5, 9.1, and 24.4 pmol.min(-1). mg(-1) were obtained, respectively, for 18:2n-6 Delta 6-, 20:3n-6 Delt a 5-, and 18:3n-3 Delta 6-desaturases. Beyond 30, 20, and 60 mu M of 1 8:2n-6, 20:3n-6, and 18:3n-3 concentration, respectively, the enzyme a ctivity deviated from Michaelis-Menten kinetics, suggesting an inhibit ion by excess substrate which is unlikely to occur in vivo as endogeno us substrate concentration is much lower. We observed a breakdown in l inearity between desaturase activity and microsomal protein concentrat ion beyond 4-5 mg microsomal protein, whatever the enzyme or substrate . Both this phenomenon and the inhibition due to excess substrate shou ld be taken into account in the determination of Delta 6- and Delta 5- desaturase activities, Comparison of concentrations of the respective endogenous substrates and the kinetic constants of each enzyme suggest ed that the higher Delta 6-desaturase activity observed for the n-3 se ries than for the n-6 series is not physiologically relevant in human fetal liver.