Studies on the hormone action at the target tissues: Activation of receptor and metabolism of DNA

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Cytosolic 3,5,3'-triiodo-l-thyronine(T_3)-binding protein(CTBP) was purified from rat kidney. The molecular weight of CTBP was estimated by gel exclusion study, sucrose density gradient centrifugation and SDS-polyacrylamide gel electrophoresis, and gave a result that MR was 58,000,which was different from that of nuclear T_3 receptor. The purified CTBP was activated by NADPH or by NADP in the presence of -SH residue. The maximal binding capacity was calculated and gave a result that one molecule of CTBP can bind one molecule of T_3. The affinity of T_3 binding to CTBP was identical to that of nuclear T_3 receptor, although nuclear T_3 receptor had a different molecular size.These pyridine nucleotides increased the maximal binding capacity without changes in the affinity constant. NADPH-activated CTBP did not bind to nuclei. However, the NADP-activated CTBP could bind to the nuclei. By extraction with 0.5 M NaCl, acceptor protein for NADP-activated CTBP was isolated. The molecular weight of the acceptor protein was 200,000. NADPH-activated CTBP, therefore, may play a role as a reservoir for intracellular T_3. And NADPH-activated CTBP may play a role in the regulation of gene expression induced by T_3 binding to its specific receptor in nuclei. NADP and NADPH take play as counterregulatory factors for T_3-induced gene expression.