Nuclear triiodothyronine receptor of human adipose tissue

L-Triiodothyronine induces malic enzyme in explants from human adipose tissue. Consequently, we looked for the presence of receptors for L-triiodothyronine in nuclei isolated from human adipose tissue. The binding of 125I-triiodothyronine by the nuclei was time- and temperature-dependent. At 37 degrees C binding reached a steady state after 60 minutes. Dithiothreitol enhanced total binding and suppressed nonspecific binding. Scatchard analysis showed the presence of a single class of binding sites. The apparent association constant, Ka, was 0.13 +/- 0.03 X 10(10) M-1, the maximal binding capacity 2.20 +/- 0.81 pmol/mg DNA (mean +/- SD, n = 7) and the number of binding sites 8,000/nucleus. L-Triiodothyronine and D-triiodothyronine had equal affinity to the nuclear receptor; triiodothyroacetic acid had three times higher affinity. L- and D-thyroxine had 8% and 12%, respectively, and tetraiodothyroacetic acid had 19% affinity compared to that of L-triiodothyronine. Reverse triiodothyronine was a weak competitor. Digestion of nuclei with micrococcal nuclease abolished specific binding. These results show that nuclei from human white adipose tissue possess high affinity receptors for L-triiodothyronine, which are associated with nuclear chromatin. It is likely that induction of malic enzyme in human adipose tissue by L-triiodothyronine is mediated by the nuclear receptors.