Effect of Vitamin B₆ Deficiency on Linoleic Acid Desaturation in the Arachidonic Acid Biosynthesis of Rat Liver Microsomes

The effect of vitamin B₆ deficiency on the desaturation of linoleic acid (18:2n-6) to γ-linolenic acid (18:3n-6) in rat liver microsomes was studied. Rats fed with a vitamin B₆-deficient 70% casein diet for 5 weeks displayed a striking decrease mainly in the activity of terminal z!6-desaturase in the 18:2n-6 desaturation system, although the activity of the electron-transport enzyme, NADH-cytochrome b₅ reductase, was also significantly decreased. The 18:2n-6 desaturation index, the arachidonic acid (20:4n-6)/18:2n-6 ratio in the liver microsomal total lipids and in their phosphatidylcholine (PC) fraction, decreased together with the microsomal /16-desaturase activity, these changes being quite faithfully reflected in the plasma lipid. Among the microsomal lipid components, the PC content significantly decreased. In addition, a highly positive correlation was found between the PC content and J6-desaturase activity in liver microsomes.In an in vitro experiment, the J6-desaturase activity decreased linearly with increasing rate of PC hydrolysis by phospholipase C. These results suggest that the altered PC content in microsomes induced by vitamin B6 deficiency in vivomay affect the 18:2n-6 desaturation and thus decrease the biosynthesis of20:4n-6.