Elucidation of the pathways for the metabolism of 22- and 24-carbon (n-6) polyunsaturated fatty acids

It is generally accepted that the conversion of 22:4(n-6) to 22:5(n-6) is by an acyl-CoA-dependent 4-desaturase. However, when rat liver microsomes were incubated with (1-{dollar}sp{lcub}14{rcub}{dollar}C) 22:4(n-6) under standard conditions for measuring acyl-CoA desaturases, it was not possible to detect the synthesis of 22:5(n-6). When malonyl-CoA and NADPH were included in the incubation, 22:4(n-6) was chain-elongated to 24:4(n-6), which was then desaturated to 24:5(n-6). The acid, 24:5(n-6), could serve as a precursor for 22:5(n-6) by moving to a site for partial {dollar}beta{dollar}-oxidation and undergoing one cycle of {dollar}beta{dollar}-oxidation. Rat hepatocytes metabolized (1-{dollar}sp{lcub}14{rcub}{dollar}C) 22:4(n-6), (3-{dollar}sp{lcub}14{rcub}{dollar}C) 24:4(n-6), and (3-{dollar}sp{lcub}14{rcub}{dollar}C) 24:5(n-6) to yield esterified radioactive 22:5(n-6). These results demonstrate that 22:4(n-6) is the precursor of 22:5(n-6) but the pathway is independent of an acyl-CoA-dependent 4-desaturase.; The site for partial {dollar}beta{dollar}-oxidation is most likely peroxisomes. When (3-{dollar}sp{lcub}14{rcub}{dollar}C) 24:5(n-6) was incubated with rat liver peroxisomes, radioactive 22:5(n-6) was formed. The rate of {dollar}beta{dollar}-oxidation of (3-{dollar}sp{lcub}14{rcub}{dollar}C) 24:5(n-6) and (1-{dollar}sp{lcub}14{rcub}{dollar}C) 22:5(n-6) was less compared to (3-{dollar}sp{lcub}14{rcub}{dollar}C) 24:4(n-6) and (1-{dollar}sp{lcub}14{rcub}{dollar}C) 22:4(n-6) suggesting double bond removal may play a role in regulating {dollar}beta{dollar}-oxidation. When microsomes and 1-acyl-GPC were included in the incubations, the preferred fate of fatty acids with their first double bond at position 4 was esterification rather than continued {dollar}beta{dollar}-oxidation.; When 23,23,24,24-{dollar}dsb4{dollar}-24:4(n-6) was incubated with peroxisomes, microsomes, and acceptor, the amount of deuterated arachidonic esterified in 1-acyl-GPC was equivalent to the acid-soluble radioactivity from (3-{dollar}sp{lcub}14{rcub}{dollar}C) 24:4(n-6). This result suggests that once produced, after two cycles of {dollar}beta{dollar}-oxidation, the primary fate of arachidonate is esterification rather than continued {dollar}beta{dollar}-oxidation.; Both (n-6) and (n-3) families of fatty acids require that 18- and 24-carbon acids serve as substrates for desaturation at position-6. Competitive studies between 18:2(n-6) and 18:3(n-3) plus 24:4(n-6) and 24:5(n-3) showed preferential desaturation of the (n-3) acids. Rates of desaturation of 24:4(n-6) and 24:5(n-3) were similar. These results are consistent with the existence of a single 6-desaturase, though further studies are required to determine if one or multiple forms exist. Rates of chain elongation were also measured. Similar rates were observed for 20:4(n-6) and 20:5(n-3) as well as 22:4(n-6) and 22:5(n-3). These results cannot explain why there is selective synthesis and acylation of 22:6(n-3) rather than 22:5(n-6) in membrane lipids.