Further Studies on the Localization of Peroxidase in Pig Thyroid Cells

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Previously the authors reported that thyroid peroxidase is bound mainly on the membranes of rough-surfaced endoplasmic reticulum. In the present paper, the question whether the enzyme is also contained in other subcellular organelles in the cells or not was investigated. Attempts to isolate plasma membranes and Golgi membranes were made, being monitored by following Na+, K+-ATPase and galactosyltransferase activities, respectively, through differential centrifugation and twice-repeated sucrose gradient centrifugation. The final preparations showed properties characteristic of these membranes and specific activities comparable to those of reported preparations of liver, but revealed practically no peroxidase activity. The buoyant densities of the Na+, K+-ATPaseand galactosyltransferase-associated particulates were around 1.136 and 1.154, respectively, while that of peroxidase-associated particulates (rough-surfaced endoplasmic reticulum) was about 1.192. Isolated nuclei did not show any peroxidase activity and the behaviour of both mitochondria and lysosomes was different from that of the peroxidaseassociated particulates. Thus, all the results obtained in the present experiments are not favorable to the view that peroxidase is located in nuclei, mitochondria, lysosomes, Golgi apparatus and plasma membranes (microvilli) of thyroid cells.
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