BpaB25 insulins. Photoactivatable analogues that quantitatively cross-link, radiolabel, and activate the insulin receptor.

Benzoylphenylalanine (Bpa), a photoactivatable amino acid was incorporated into the 25 position of the insulin B-chain by a combination of chemical synthesis and enzymatic semisynthesis. BpaB25 insulin binds specifically to the insulin receptor with an affinity approximately 40% that of native insulin. Addition of biotin or epsilon-amino-hexanoic acid-iminobiotin at the epsilon-LysB29 position to form BpaB25,B29 epsilon-biotinyl (BBpa) or BpaB25,B29 epsilon-(Aha-iminobiotin) analogues had little adverse effect on receptor binding affinity and provided a convenient handle for affinity purification, gel shift assays, and blotting of cross-linked complexes with avidin. The analogues were readily 125I-iodinated with the majority of 125I being incorporated at the TyrA14 position; the monoiodo-A14 derivative was easily separated from other forms by high performance liquid chromatography. Photolysis of a complex of the insulin receptor and either BpaB25 insulin or [125I]iodo-TyrA14,BpaB25 insulin yields a covalent insulin-receptor complex. The efficiency of cross-linking with these reagents was unusually high, ranging from 60 to 100%. Furthermore, cross-linking to the insulin receptor results in kinase activation in vitro, and in intact cells insulin receptor phosphorylation and internalization were both activated. Notably, even at saturating concentrations one molecule of BpaB25 insulin covalently cross-linked each alpha 2 beta 2 receptor, demonstrating that holoreceptor activation occurs with one high affinity insulin binding site occupied and, if a second insulin binds with lower affinity it must be in a different orientation. Bpa insulin analogues form a new class of photoaffinity reagents which facilitate studies relating insulin-insulin receptor structure and function.