Specific labeling by 2-hydroxy-5-nitrobenzyl bromide (HNB) of tryptophan residues in rat diaphragms abolishes insulin stimulation of sugar and amino acid uptake, and of the incorporation of previously accumulated amino acid into muscle protein. A brief period of preincubation of tissues with insulin protects against the antiinsulin effects of the labeling reagent. The proposal is made that tryptophan residues of a polypeptide component in the cell membrane are involved in the recognition of the hormone. Complementary interaction of this component with insulin is regarded as primary to the latter's effects on muscle.
Rieser P, Maturo JM. Insulin, Substrate Transport, and Protein Synthesis in MuscleThe Problem of Hormonal Recognition. Arch Intern Med. 1969;123(3):267–271. doi:10.1001/archinte.1969.00300130049008