Biosynthesis of docosahexaenoic acid in human cells: Evidence that two different Delta(6)-desaturase activities may exist

It has been proposed that synthesis of docosahexaenoic acid (22:6(n-3)) in rat hepatocytes occurs by a route independent of Delta(4)-desaturase, which involves Delta(6)-desaturation and retroconversion (Voss A., Reinhart M., Sankarappa S. and Sprecher H. (1991) J. Biol. Chem. 266, 19995-20000). However, most cells exhibit these enzymatic activities and nevertheless synthesize low to undectectable amounts of 22:6(n-3). Moreover, there are few data on the occurrence of this pathway in human cells. In the present work, we have analysed the biosynthetic pathway of 22:6(n-3) in human Y-79 retinoblastoma and Jurkat T-cells. Y-79 cells were supplemented with 18:3(n-3) and 20:5(n-3) or incubated with [1-C-14]18:3(n-3) and [1-C-14]20:5(n-3) and lipids analysed by argentation TLC, reverse-phase TLC and GLC-mass spectrometry. Pulse-chase experiments revealed that synthesis of 22:6(n-3) from 20:5(n-3) in Y-79 cells occurred through two successive elongations, followed by a Delta(6)-desaturation of 24:5(n-3) to 24:6(n-3) and retroconversion to 22:6(n-3). Incubation of Y-79 cells with [1-C-14]18:3(n-3) in medium containing 50 mu M trans-9,12-18:2, a potent inhibitor of Delta(6)-desaturase, caused a reduction of 22:6(n-3) synthesis mainly by interfering with the desaturation of 18:3(n-3). However, when [1-C-14]20:5(n-3) was used as precursor, synthesis of 22:6(n-3) was depressed to a lesser extent and mainly by reduction of 24:6(n-3) retroconversion. Neuronal differentiation of Y-79 cells caused a great increase in Delta(6)-desaturase activity on 18:3(n-3), though the amount of 22:6(n-3) synthesized did not change or diminish, suggesting the existence of a particular Delta(6)-desaturase involved in the synthesis of 22:6(n-3). The existence of a distinctive Delta(6)-desaturase activity could also explain why Jurkat cells growing in serum-free medium showed a near 3-fold increase in the synthesis of pentaenes from 18:3(n-3) and, at the same time, a large decrease in the synthesis of 22:6(n-3). The verification of the involvement of two Delta(6)-desaturase activities in 22:6(n-3) synthesis would have important implications for the formulation of the nutritional requirements of this fatty acid during development.