The reversible and irreversible autophosphorylations of insulin receptor kinase

When the catalytically active, tyrosyl-phosphorylated form of insulin receptor was isolated from human placenta and treated with ADP, only partial dephosphorylation was observed. This observation suggests the existence of two distinct classes of phosphotyrosyl residues of the phosphorylated insulin receptor: one in which the phosphoryl groups undergo reversible transfer to ADP and one in which they do not. There were 8.8 +/- 2.2 phosphorylation sites per tetrameric insulin receptor, of which 2.4 +/- 0.5 were irreversible (mean +/- S.D., n = 6). The reversible sites were determined to be equivalent and noninteracting and to have an equilibrium constant for the transfer of a phosphoryl group from ATP to a site of reversible phosphorylation of 8.7. Surprisingly, both phosphorylation and dephosphorylation at the reversible sites were relatively insensitive to the presence of insulin. Since only minimal autophosphorylation of insulin receptor was detected in the absence of insulin, the results suggest that the incorporation of phosphate into the two to three irreversible phosphorylation sites is insulin dependent. Phosphorylation of the irreversible phosphorylation sites then renders the insulin receptor relatively insensitive to the continued presence of insulin and facilitates rapid reversible phosphorylation of a second group of tyrosyl residues. The dependence of the degree of phosphorylation of insulin receptor on the ATP:ADP ratio may provide a mechanism for modulating the cellular response to insulin.