ABSTRACT
Guanosine diphosphate binding to the uncoupling protein of isolated mitochondria of brown adipose tissue in newborn rabbits was measured as an index of thermogenic activity. The binding was 0.281 +/- 0.022 nmol GDP/mg mitochondrial protein at 1 day of age, 0.214 +/- 0.017 at 3 days, 0.428 +/- 0.038 at 5 days, and 0.208 +/- 0.016 at 7 days. The increase in binding between 3 and 7 days of age suggests that the brown fat has an increased thermogenic capacity at that age. In addition, the potential for synthesis of the uncoupling protein was investigated in 1- to 5-day-old newborn rabbits by probing the total cellular ribonucleic acid for the messenger that codes for uncoupling protein. The amount of uncoupling protein messenger was highest at 1 day of age and declined at least until 5 days of age. Because the amount of uncoupling protein messenger decreased as the GDP binding increased, the results suggest that either the initially translated uncoupling protein was unmasked at about 5 days of age or there was a delay in the incorporation of uncoupling protein into the mitochondrial inner membrane, or both.
