A comparative study of the metabolism of n-9, n-6 and n-3 fatty acids in testicular cells from immature rat

Dietary 18 and 20-carbon fatty acids of the n-6 and the n-3 families are metabolized to 22:5,n-6 and 22:6,n-3 by a sequence of specific desaturases and chain elongation via 24-carbon intermediates. This pathway is regulated so that more 22:6,n-3 than 22:5,n-6 is found in the tissues. Rat testis is an exception since 22:5,n-6 is present in large proportions in this organ. Therefore rat testis appears to be interesting for studies of the detailed synthesis of 22:5,n-6 compared with that of 22:6,n-3. By using fresh preparations of rat testicular cells from 19-day-old rats enriched in Sertoli cells, we compared the metabolism of 1-¹⁴C-labelled n-3, n-6 and n-9 fatty acids. The testicular cells actively synthesized 22:6,n-3 and 22:5,n-6, but not 22:4,n-9 from the 18 and 20-carbon precursors. Of 200 mol ¹⁴C-labelled C₁₈ and C₂₀ fatty acids added initially, approximately 20-40 mol were found as 24-carbon intermediates after 24 h of incubation. This indicates that the balanced capacity of elongation, desaturation and chain shortening favours the accumulation of 24-carbon intermediates in these cells. One exception was [1-¹⁴C]20:3,n-9 which was efficiently elongated to 22:3,n-9 but not to C₂₄ fatty acids. Our data suggests that the poor elongation of n-9 fatty acids from C_22⁻ to C₂₄ may be an important hindrance in the synthesis of 22:4,n-9. The efficient synthesis of 22:5,n-6 may also partly explain why this is the major 22-carbon fatty acid in rat testis.