Binding of thyroid hormone by heme proteins may signal protein age

Cellular aging is thought to include the accumulation of proteins with post-translational modifications that alter degradation rates or function or both. Protein catabolism has been regarded as a randomly occurring process, but recent observations have suggested mechanisms by which old and young cells may distinguish recently-synthesized protein molecules from "old" proteins. Hemoglobin, myoglobin and other heme proteins bind thyroid hormone progressively with time and in a poorly-dissociable manner. This cumulative acquisition of thyroid hormone is postulated to be an index of molecular aging.