[3H]GDP binding and thermogenin amount in brown adipose tissue mitochondria from cold-exposed rats.

Brown fat mitochondria were isolated from cold-exposed and control rats, and their content of the brown-fat-specific 32-kDa "uncoupling" protein thermogenin determined both by the traditional [3H]GDP-binding method and by the recently developed enzyme-linked immunosorbent assay (ELISA). In mitochondria isolated from both cold-acclimated (3 wk at 4 degrees C) and cold-exposed rats (24 h), an increase in thermogenin content was observable, both when estimated by the [3H]GDP-binding method and by the ELISA assay, and there was no statistically significant difference in the magnitude of these increases in the two methods. In 1 h cold-exposed rats there was no increase in [3H]GDP binding or in the ELISA reaction. When the amount of thermogenin was plotted against [3H]GDP binding in the different states, a relationship of 75,000 g thermogenin per mole GDP bound was obtained. Based on the resolution of these two methods, and under the three conditions investigated, it was concluded that there was no reason to postulate the existence of a "masked" form of thermogenin or of an "unmasking" process and that thermogenin in the mitochondria, as in the isolated state, has apparently one GDP binding site per dimer.