Interactions of phospholipids with thyroglobulin and their influence on the enzymic hydrolysis of this protein

Abstract

The interactions between purified [ 125I]thyroglobulin and various phospholipids were examined. Addition of either pea or thyroid phosphatidylinositol (pH range 2.7–4.0) caused precipitation of both interactants; however, pea phosphatidylinositol appeared to be more effective in complex formation. With limiting quantities of phosphatidylinositol, the degree of interaction was maximal at pH 4.0. Phosphatidylserine and phosphatidic acid formed complexes with thyroglobulin over a pH range 3.5–4.0, but the interaction decreased as the pH was lowered. Phosphatidylethanolamine was less effective than the above lipids in precipitating thyroglobulin, while phosphatidylcholine did not interact within the pH range studied. The formation of insoluble complexes with phosphatidylserine and phosphatidylinositol significantly increased the hydrolysis of [ 125I]thyroglobulin by purified thyroid acid proteinase at pH4.0. Complexes of [ 125I]thyroglobulin with phosphatidylinositol (formed at pH 4.0) were hydrolysed by mitochondrial/lysosomal particles at pH 5.0 to the same extent as thyroglobulin alone. By contrast, phosphatidylinositol not complexed prior to incubation inhibited the degree of hydrolysis of [ 125I]thyroglobulin by mitochondrial/lysosomal particles when mixed and incubated at pH 5.0, whereas phosphatidylserine tended to increase hydrolysis, but this effect was neutralized by simultaneous addition of phosphatidylinositol. The possible physiological significance of the results in relation to the hydrolysis of thyroglobulin in vivo is discussed.