[21 ‐ Arginine ‐ A] Insulin

An analog of sheep insulin which differs from the parent molecule in that the C‐terminal amino acid residue of the A chain, asparagine, is replaced by arginine, has been synthesized and isolated in highly purified form. The [Arg 21 ] A chain of sheep insulin was synthesized by the fragment condensation approach and isolated as the S‐sulfonated derivative. Conversion of the latter into the sulfhydryl form and interaction with the S‐sulfonated B chain of bovine (sheep) insulin yielded [Arg 21 ‐ A] sheep insulin, which was purified by chromatography on a carboxymethylcellulose column with an exponential sodium chloride gradient. The [Arg 21 ‐ A] sheep insulin shows potencies of 10.5–12.5 IU/mg when assayed by the mouse convulsion method and 8.6 IU/mg by the radioimmunoassay method (cf. 23–25 IU/mg for the natural hormone). It has been suggested that in the insulin molecule the A 21 asparagine participates in salt bridge‐ and hydrogen bond‐forming interactions which are critical in the biological activity of the hormone. Although the [Arg 21 ‐ A] analog still retains these interactions, it is only ca. 50% as active as the natural hormone. It is speculated that other factors than the abovementioned interactions come into play, which involve the side chain of the A 21 amino acid residue and affect the biological activity of the hormone.