IN VITRO FORMATION OF POLYUNSATURATED FATTY ACIDS BY DESATURATION IN RAT BRAIN: SOME PROPERTIES OF THE ENZYMES IN DEVELOPING BRAIN AND COMPARISONS WITH LIVER

Abstract— In vitro desaturation of [1‐C¹⁴]linolenic, linoleic, oleic, and icosatrienoic acids was determined using homogenates and subcellular fractions of developing rat brain and liver. Linolenic, linoleic, and oleic acids were desaturated in the δ6‐position and activity was optimal in the presence of CoA, ATP, MgCl₂, and NADH in a citrate‐phosphate buffer at pH 6.0. Icosatrienoic acid was desaturated in the δ5‐position with a much broader pH optimum. The unstable desaturation systems of brain were protected by reduced glutathione and niacinamide and markedly inhibited by dithiothreitol, p‐chloromercuribenzoate, sodium cyanide or bathophenanthroline sulfonate. With brain homogenate of neonatal rats, the relative rates of desaturation of these substrates were 18:3(n ‐ 3) > 18:2(n ‐ 6) > 20:3(n ‐ 6) > 18:l (n ‐ 9). Specific activity of brain enzymes was greatest in neonatal rats with fluctuations in activity between 3 and 6 days of age. During this period, liver enzyme appeared to alter in a reciprocal manner. Total desaturation capacity of brain was maximal and fairly constant between 4 and 20 days of age, whereas liver activity increased dramatically after weaning. The activity of crude microsomal preparations from neonatal brain, like that of liver microsomes, was stimulated by a heat‐labile component of the cytosolic fraction. These results demonstrate that brain has a high capacity for desaturation of the essential fatty acids during crucial stages of brain development when liver activity is relatively low.