Guanine-nucleotide-binding proteins Gi and Gs in fat-cells from normal, hypothyroid and obese human subjects.

In human adipocyte plasma membranes, pertussis toxin catalysed the ADP-ribosylation of an apparently single 40 kDa protein. The same protein was also observed in Western blots by using an antibody which identifies the C-terminal decapeptide of Gi alpha (alpha subunit of Gi). In analogous experiments, cholera toxin and an antibody raised against the C-terminal decapeptide of Gs alpha (alpha subunit of Gs) were used to identify two proteins of 42 and 45 kDa, the former of which was more prominent. A method was developed to estimate the relative amounts of Gi and Gs in crude adipocyte plasma membranes in a single immunoblot by using the two antisera. In animal models, changes in the amounts of G-proteins have been suggested to explain alterations in hormone-responsiveness in hypothyroidism and obesity. However, the amounts of Gi and Gs were unaltered in thyroidectomized papillary-carcinoma patients who had been without hormone substitution for 4 weeks. In adipocyte plasma membranes prepared from markedly obese subjects, the amounts of both Gi alpha and Gs alpha as calculated per mg of protein were decreased, but the Gi/Gs ratio remained unaltered in comparison with control subjects.