Evidence is presented that liver from thyroidectomized rats has ornithine decarboxylase (ODC) in both mitochondrial and soluble fractions. The cytosolic activity was stimulated 7-fold and the mitochondrial activity 3-fold 15 min after the administration of triiodothyronine (T3). While the rapid transient stimulation of ODC could represent a direct intracellular response to T3, an incidental effect on a very-rapidly-turning-over enzyme seems more likely; this suggests that in vivo ODC may be controlled by a demand for polyamines.

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