Insulin initiates its physiological effects by binding to a
high affinity specific receptor located on the plasma
membrane. The receptor is saturable, and both the
binding capacity and the biological activity of insulin
are maximal at a plasma insulin concentration of 20 to
30 μU/ml. Insulin is not altered during the binding
process, and reaction of the disulfide bonds is not
involved. After binding to the receptor, insulin
transmits its signal to the interior of the cell through a
second messenger that influences enzymatic processes.
Thus, the hormone probably carries out its actions
without entering the cell