Occurrence and metabolism of arachidonic acid in the housefly, Musca domestica (L.)

Abstract

Gas-liquid chromatography (GLC) on polar, non-polar and capillary columns and chemical ionization-gas chromatography-mass spectrometry (CI-GC-MS) were used to verify the presence of arachidonic acid (20:4) and eicosatrienoic acid (20:3, n-6) at low levels in adult male and female houseflies. The phospholipid (PL) fraction contained the highest amounts of these fatty acids. An analysis of the distribution of radiolabelled 20:4, 20:3 (n-6), oleic acid (18:1) and palmitic acid (16:0) when injected into the insect showed that a large proportion of the radioactivity from 20:4 was incorporated into the PL fraction, whereas the radioactivity from the other fatty acids was recovered at higher levels in the free fatty acid and triacylglycerol fractions. When [ 3H]20:4 was injected into the haemolymph it was rapidly sequestered into the PL fraction. Over 80% of the recovered radiolabel from 20:4 was found in PL, 10 min after injection. Approximately one-half of the [ 3H]20:4 in the PL was recovered in phosphatidylethanolamine (PE), the major PL of the housefly. Liberation of the acyl moiety from the 2 position by phospholipase A 2 showed that over 80% of the sequestered 20:4 was in the 2-position. Radio-GLC showed that the injected [ 3H]20:4 was incorporated intact into PL. When large amounts of unlabelled 20:4 were injected into the insect or were included in the diet of adult insects prior to administration of [ 3H]20:4, then larger amounts of radioactivity were recovered in the TG and FFA fractions. These data document the presence of 20:4 in the housefly and demonstrate that injected [ 3H]20:4 was readily incorporated into PL specifically into the 2-position.