Action of Phospholipase C on the Thyroid

Treatment of dog thyroid slices with an enzyme present in the growth medium of Clostridium perjringens makes them unresponsive to thyroid-stimulating hormone (TSH) as measured by the oxidation of glucose-1J4C to 14C02. This enzyme appears to be phospholipase C since (a) both the anti-TSH effect and the phospholipase activity are inseparable over a 240-fold purification, (b) both activities are inhibited by 5 X low4 M ethylenediaminetetraacetate and destroyed by boiling, and (c) the addition of excess phospholipid protects the thyroid from the enzyme. Phospholipase C appears to abolish the responsiveness to TSH by producing a specific alteration in the thyroid cell membrane, since thyroids treated with enzyme oxidize glucose at a normal rate in the absence of TSH and respond normally to acetylcholine, a substance previously shown to increase glucose oxidation in the thyroid.