Human insulin is a two-stranded protein of 51 amino acids produced by the beta-cells in the islets of Langerhans in the pancreas. Since the insulins of all vertebrates are more or less similar, traditionally insulin was extracted from cadaveric pancreases of cows and pigs. At first, these insulins contained many impurities which would give rise to painful injection site reactions, rashes, antibody formation, allergies and even anaphylactic shock. Repeating the purification steps (which included crystallisation of the insulin) several times yielded far cleaner insulins and these highly pure animal insulins were used for many years. However, by the early 1980's biosynthesis became a possibility and human insulin was the first product to be produced in this way on a large scale, gradually replacing the animal insulins. For both the animal and human insulins, several variants existed. The first animal insulin preparations had a relatively short duration of action, which triggered a search for ways to prolong the action. Zinc-insulins were used for many years, but the most successful way to prolong action was the addition of protamin, which is used to this day as NPH insulin. Other variants, such as histone insulin and globin insulin proved rather less useful.
The human insulin molecule
The primary structure of the insulin molecule was elucidated by Sanger in 1951. Human insulin is a protein consisting of a A-chain with 21 amino acids, and a B-chain with 30 amino acids. The chains are linked by two disulfide bridges between the cystein residues at positions A7 and B7, and A20 and C19. An additional disulfide-bridge connects the cystein residues at A6 and A11, which is important for the tertiary structure and the physiological efficacy of the molecule. The formal chemical formula of insulin is C257H383O77N65S6 which results in a molecular weight of 5808. Its iso-electric point (point of least ionisation/ water solubility) is at an pH of 5.4. Particularly in the presence of Zinc ions, human insulin molecules aggregates to dimers, hexamers and more complex crystalline structures.
Fig. 1. Amino-acid compositions of human, cow and pork insulinThe structure of the insulin molecule is remarkably conserved across vertebrate species. Thus, beef insulin differs from human insulin by only 3 amino acids (A8:Thr>>Val, A10:Ile>>Val and B30:Thr>>Ala), and porcine insulin differs only by one amino acid (B30:Thr>>Ala) (figure 1). This made it possible to use insulin derived from slaughtered animals in humans, which was the common practice up to around 1980.