Electron-transport enzymes of calf thyroid

Electron transport enzymes, RNA, DNA, and iodinating activity have been assayed in subcellular fractions derived from calf tissue.

About 0.1 μequiv DPNH is oxidized per min by mitochondria prepared from 1 g thyroid in 0.25 M sucrose and assayed at 25°. Cytochrome c reductase system and cytochrome oxidase (EC 1.9.3.1) activities, confined largely to the “mitochondrial” fractions, are more than 10 times greater. DPNH oxidase system is antimycin A-sensitive; cytochrome c reductase system is antimycin A-insensitive. Reduced pyridine nucleotide-dichlorophenol-indophenol reductase (“DT diaphorase”), TPN+-specific isocitrate dehydrogenase (EC 1.1.1.42), and glucose-6-phosphate dehydrogenase (EC 1.1.1.49) are present in the soluble portion in cell and have activities comparable to cytochrome c reductase system and cytochrome oxidase. There is ample basis for formation of large amounts of TPNH within the cell, and its rate of oxidation appears to be low.

RNA was distributed throughout all cell fractions without marked concentration in “microsomes”, and almost 70% was in the cell supernatant.

Iodinating activity was most intense in fractions that may be classified as heavy microsomes, and its distribution did not coincide with the distribution of cytochrome c reductase system or cytochrome oxidase.

Electron-transport enzymes, iodinating activity, and nucleic acid content varied in parallel following physiologic alterations in rat-thyroid function.