STUDIES OF A THYROID HORMONE AND ANDROGEN DEPENDENT PROTEIN IN RAT LIVER CYTOSOL

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Abstract
Sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis of the liver cytosol of euthyroid male rats revealed a prominent band (MW 26,000 daltons), designated protein II, which was virtually absent in the cytosol of hypothyroid animals. Injection of 500 .mu.g triiodothyronine (T3)/100 g body wt resulted in a maximal increase in the level of protein II, reaching 90% of the euthyroidal level 3 days after hormone administration. Concomitant studies with the liver mitochondrial enzyme .alpha.-glycerophosphate dehydrogenase (.alpha.-GPD) indicated that this T3 dose also resulted in a maximal enzyme response in this time period. Since it was estimated that 500 .mu.g of T3 will saturate nearly all nuclear T3 binding sites, the synthesis of both proteins is probably limited by nuclear binding. Protein II was absent in the liver cytosol of female rats but could be induced in ovariectomized female rats by androgens. Treatment of male rats with estradiol resulted in disappearance of protein II. Since administration of testosterone to hypothyroid male rats only minimally increased the amount of protein II, the absence of the protein in hypothyroid animals was not due to androgen deficiency. Similarities in the molecular weight and the response to hormonal manipulation of protein II and of the urinary .alpha.2uglobulin [.alpha.2-microglobulin] previously reported by Roy (1973) raise the possibility that these proteins are the same. The high concentration of protein II in male rat cytosol and the relative ease in its identification by SDS polyacrylamide gel electrophoresis make it a potentially useful model protein for the study of thyroid hormone action at the cellular level.