Incorporation of [14c] arachidonate in pig thyroid lipids and prostaglandins.

In pig and sheep thyroid, the arachidonate content of phosphatidylinositol (14.3--17.5%) is much higher than in the other phospholipids. In the thyroid, phosphatidylinositol seems to play an important role in the biosynthesis of prostaglandins. In neutral thyroid lipids, the arachidonate content is much higher in diacylglycerols (15.8%) than in monoacylglycerols (2.9%) or triacylglycerols (4.2%). However, the most important pool of esterified arachidonate is triacylglycerols (1030 nmol of arachidonate/g tissue). Arachidonate represents a very small part of total free fatty acids measured in the presence of albumin and indomethacin (0.65% or 16.4 nmol/g tissue). Thyrotropin (50 munits/ml) causes after 1 h a 2-fold increase in the level of free arachidonate (37.3 nmol/g tissue). Pig thyroid slices rapidly take up [14C]arachidonate and incorporate it into neutral lipids and phospholipids. Specific activity of phosphatidylinositol is 3-fold higher than that of phosphatidylcholine after an hour incubation. Specific activity of diacylglycerols is 8-fold higher than that of triacylglycerols. Thyrotropin (50 munits/ml) causes a significant decrease (32--33%) of incorporation of radioactivity in lipids as compared with standard incubation. This result is compatible with the isotopic dilution of the labeled [14C]arachidonate by nonradioactive arachidonate liberated in the incubation medium under thyrotropin action. Slices and homogenates of pig thyroid weakly convert [14C]arachidonate to prostaglandins E2 and F2alpha (1--2%. Thyrotropin (50 munits/ml) always diminishes the conversion of radioactivity to prostaglandins as compared with standard incubation. This result is compatible with the above-mentioned hypothesis.