Thyrotropin-stimulated phosphatidylinositol-specific phospholipase A2 in pig thyroid, a re-examination.

Haye et al. [ 1,2] have described previously a phosphatidylinositol-specific phospholipase A1 in thyroid homogenates which is stimulated by thyrotropin, and have suggested a role for it in the release of arachidonic acid for prostaglandin synthesis. They did not, however, clearly identify all the products formed after PI degradation. The release of free fatty acid from PI can also be effected by the combined action of PI-phosphodiesterase (EC 3 .1.4.10) and diacylglycerol lipase; we have suggested that this latter route might be of prime importance in releasing arachidonic acid in vivo in a number of physiological systems [3-51, and experimental evidence that this is so has been obtained in aggregating platelets [6,7]. We have, therefore, attempted to repeat the experiments of Haye et al. [l] using [3H]oleoyl-PI and [32P]PI as substrates, both to confirm the thyrotropin stimulation of fatty acid release, and to identify lyso-PI as a product. We have been unsuccessful in both attempts: we can find no evidence of an enzymic activity hydrolysing PI in thyroid homogenates other than the PI-phosphodiesterase (with or without thyrotropin) and, although the release of free fatty acid from [3H]oleoyl-PI is stimulated by thyrotropin, addition of the BSA + lactose solution in which the hormone is dissolved has an identical stimulatory effect.