In 1983, the American company Eli Lilly successfully produced human insulin using recombinant DNA (rDNA) technology. They chose the bacterium Escherichia coli (E. coli) as the host organism because it grows rapidly and its genetics are well-understood. They synthesized DNA sequences corresponding to the A and B chains of human insulin, inserted them into plasmids of E. coli, and produced the chains separately. These chains were later extracted and combined by creating disulfide bonds to form the final human insulin, known commercially as Humulin.

1. History of Insulin Therapy

Before the advent of biotechnology, insulin for diabetic patients was extracted from the pancreas of slaughtered cattle and pigs. However, animal insulin differed slightly in chemical structure from human insulin, leading to allergic reactions or immune responses in some patients. This necessitated the creation of a "human-identical" insulin source.

2. Molecular Structure of Insulin

Human insulin is a peptide hormone consisting of 51 amino acids arranged in two short polypeptide chains:

• Chain A: Consists of 21 amino acids.
• Chain B: Consists of 30 amino acids.

In humans, insulin is synthesized as a pro-hormone (pro-insulin), which contains an extra stretch called the C-peptide. This C-peptide is removed during maturation, making the insulin functional. A major challenge in rDNA technology was producing insulin without this C-peptide or ensuring its proper removal.

3. The Eli Lilly Breakthrough (1983)

The core technique involved bypassing the pro-insulin stage entirely. Eli Lilly scientists prepared two different DNA sequences coding for Chain A and Chain B. These sequences were then separately introduced into E. coli host cells via plasmids. The bacteria acted as "biological factories," churning out the specific peptide chains. The chains were then purified and linked in a laboratory setting using disulfide bridges to achieve the correct 3D folding of functional insulin.

4. Steps in Recombinant Insulin Production

• Isolation: Identification of the human genes for Insulin A and B chains.
• Vector Insertion: Ligating these genes into a pBR322 or similar plasmid vector.
• Transformation: Inserting the recombinant plasmids into E. coli bacteria.
• Fermentation: Growing the transformed bacteria in large bioreactors to produce the protein.
• Purification: Extracting the A and B chains from the bacterial mass.
• Bonding: Joining the chains via disulfide bonds (Cys-S-S-Cys).

5. Why E. coli?

Escherichia coli is the preferred organism in molecular biology because of its simple structure, rapid generation time (doubles every 20 minutes), and the ease with which its genome can be manipulated. While yeast (Saccharomyces cerevisiae) is also used for protein production (and is used for modern insulin analogs), the original landmark production by Eli Lilly specifically utilized bacteria.

6. Impact of Humulin

Humulin was the first ever genetically engineered drug to be approved by the FDA. It solved the supply shortage of animal-derived insulin and eliminated the risk of cross-species disease transmission and immunological rejection.

7. Other Applications of Biotechnology in Medicine

• Gene Therapy: Correcting defective genes (e.g., ADA deficiency).
• Molecular Diagnosis: Using PCR and ELISA for early disease detection.
• Transgenic Animals: Creating animals that produce human proteins in their milk.