ONLY one rat liver ribosomal protein (S6) is phosphorylated in vivo1. During hepatic regeneration the phosphorylation of S6 is increased by an order of magnitude, and derivatives are generated (in some experiments as many as five) which contain increasing numbers of phosphoserine residues1. Very similar changes in the phosphorylation of S6 are caused by administration to animals of glucagon or cyclic AMP (A.M.G. and I.G.W., unpublished). Indeed, a number of hormones including adrenocorticotrophic hormone (ACTH)2 and thyroid hormone3 increase the phosphorylation of ribosomal protein, although in the latter instances, the identity of the phosphoprotein has not been established. These observations have led us to consider the possibility that cyclic AMP is the mediator of the phosphorylation of S6 (ref. 4). The concentration of cyclic AMP in the liver is increased in diabetic animals and restored to normal by insulin5, although it is still moot whether the effect of the hormone on the level of the cyclic nucleotide accounts for all (or even any) of its actions on hepatic cells6. We have tested the effect of experimental diabetes and of insulin administration on the phosphorylation of rat liver ribosomal protein: the phosphorylation of S6 was markedly increased by diabetes, and reduced towards the normal level by insulin administration.
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