Since insulin was discovered in 1921, it has become one of the most thoroughly studied molecules in scientific history.

Diabetes has been recognized as a distinct medical condition for at least 3,500 years, but its cause was a mystery until early this century. In the early 1920s, researchers strongly suspected that diabetes was caused by a malfunction in the digestive system related to the pancreas gland, a small organ that sits on top of the liver.

At that time, the only way to "control" diabetes was through a diet low in carbohydrate and sugar, and high in fat and protein. Instead of dying shortly after diagnosis, this diet allowed diabetics to live - but only for about a year.

Exactly what was wrong, or missing, in the sugar metabolism pathway of people with diabetes was unknown until a group of Canadian researchers purified insulin in 1921 and proved that diabetes is a disease of insulin deficiency.

As with most major scientific discoveries, the groundwork for the discovery of insulin, had been laid by several others before the Canadian researchers isolated it. In 1889, two European researchers, Minkowski and von Mering, found that when the pancreas gland was removed from dogs, they developed all the symptoms of diabetes and died soon afterwards. Minkowski and von Mering proposed that the pancreas was crucial for sugar metabolism.

Later experimenters narrowed the search to the Islets of Langerhans-clusters of specialized cells within the pancreas. In 1910, Sharpey-Shafer of Edinburgh suggested a single chemical was missing from the pancreas in diabetic people. He proposed calling this chemical "insulin," and later the successful Canadian researchers took him up on the suggestion.

Meanwhile, an American scientist E. L. Scott was partially successful in extracting insulin with alcohol. R. C. Paulesco, a Romanian, made an extract from the pancreas that lowered the blood glucose of dogs. Some claim Paulesco was the first to discover insulin.

The biggest breakthrough came in 1921 when Frederick Banting and Charles Best conducted a series of experiments one summer in the laboratory of J.J. R. Macleod at the University of Toronto. Like Minkowski and von Mering, they showed that removing the pancreas from dogs made them diabetic.

Then they went a step further and painstakingly took fluid from healthy dogs' Islets of Langerhans, injected it into the diabetic dogs and restored them to normalcy - for as long as they had the extract.With the help of a biochemist colleague named J. B. Collip, they were then able to extract a reasonably pure formula of insulin from the pancreas of cattle from slaughterhouses.

In January, 1922, a diabetic teenager in a Toronto hospital named Leonard Thompson became the first person to receive an injection of insulin. He improved dramatically, and the news about insulin spread around the world like wildfire. For their work, Banting and Macleod received the Nobel Prize in Medicine the very next year, in 1923. Banting shared his part of the prize money with Best, and Macleod shared his with Collip.

The University of Toronto immediately gave pharmaceutical companies license to produce insulin free of royalties. In early 1923, about one year after the first test injection, insulin became widely available, and saved countless lives.

Insulin was one of the first proteins to be crystallized in pure form, in 1926. The crystalline form allowed researchers to study its structure with a technique called x-ray crystallography and approximate its three-dimensional shape. Knowing a molecule's shape helps understand how it works in the body and since then scientists have tried to sort out how insulin acts and what other molecules it might interact with.

In 1955, insulin became the first protein to be fully sequenced. That work resulted in a 1959 Nobel Prize for Frederick Sanger. Through Sanger's work we now know that all human proteins have a unique sequence of any or all of 20 types of amino acids. The amino acids are strung together into chains called peptides, somewhat like letters combining into long words.

Many proteins have more than one chain, joined together in specific ways. Human insulin has two peptides. The A chain (for acidic) has 21 amino acids and B chain (for basic) has 30 amino acids. The two chains are connected by two disulfide bridges, bonds formed between the sulfur atoms in the amino acid cystine. The A chain also has a third internal disulfide bridge. The disulfide bridges hold the molecule together. Without them, the protein would probably not be active in the body.

Once a protein's sequence is known, it is possible, in theory, to recreate it synthetically. In fact, insulin was the first protein to be chemically synthesized in a laboratory, in 1963. But researchers were unable to produce much of it. For 60 years after Banting's group isolated insulin, diabetics relied on hormone purified from animals, primarily cattle and pigs. Animal insulin works well on the whole, but is not an exact match with the human hormone and sometimes causes adverse reactions, for example, skin rashes. In 1978 insulin became the first human protein to be manufactured through biotechnology. A team of researchers from the City of Hope National Medical Center and the fledgling biotechnology company Genentech managed to synthesize human insulin in the laboratory using a process that could produce large amounts.

The team inserted the gene for human insulin into bacterial DNA, and used the bacteria as miniature factories to make the A and B chains of the protein separately. In a second step, a chemical process combined them. The result was human insulin, without the problems animal insulin sometimes causes. Humulin, as the commercial product was called, revolutionized diabetes treatment when it became widely available in the early 1980s. Today, almost all diabetic people use recombinant human insulin instead of animal insulin.

The early diabetes researchers didn't know how lucky they were. It turns out that the amino acid sequence of insulin is almost exactly the same in different animal species, so insulin from cows and pigs also works in humans. Insulin's basic structure - two peptide chains with three disulfide bridges - is conserved in all the nearly 100 different species investigated so far.

Looking at the enzyme in more detail, the sequence of porcine (pig) insulin and human insulin is almost identical, but not exactly--it differs by one amino acid. Bovine (beef) insulin is different by three amino acids from human. This is why people were able to use insulin from cattle in the 1920s, although they didn't know it at the time.

In 1996 the Food and Drug Administration approved a modified human insulin called Humalog, which was specially developed to be active very quickly after injection.

What's next for insulin? Scientists are not sure, but one thing is certain: Beyond its well-known value to diabetics, insulin has also been on the forefront of science for more than 60 years.

By Deepinder Brar.

Deepinder Brar is a science writer based in Berkeley, California.