CHARACTERIZATION OF THE LIPID SURROUNDING THE DELTA(6)-DESATURASE OF RAT-LIVER MICROSOMES

The Delta(6)-desaturase system was isolated from rat liver microsomes by high hydrostatic pressure (1500 bars) and the enzyme components wer e then separated by size chromatography. The lipids extracted by organ ic solvents from the pressure shed fractions were phosphatidylcholine (PC) and cholesterol at a mole ratio of 4:1. The acyl chains of the sh ed PC were 56% saturated and 21% polyunsaturated resulting predominant ly from 13% higher and 15% lower contents of palmitic and arachidonic acid, respectively, as compared to those of microsomal PC. The weight ratio of phospholipids to protein in the shed desaturase fraction was 0.2 which corresponds to an average of 31 phospholipid molecules aroun d each desaturase molecule. Differential scanning calorimetry of the l ipids associated with the desaturase system showed a phase transition at 41 degrees C. Fluorescence anisotropy studies of the desaturase sur rounding lipids indicated the same transition point. We concluded that the Delta(6)-desaturase has an associated lipid surrounding of PC and cholesterol at an approx. 4:1 mole ratio that constitutes a gel phase at physiological temperature. We suggest that this state is essential for optimal desaturase activity and that the specific acyl chains of the lipid annulus provide a regulatory sensor of the Delta(6)-desatura se activity.