Insulin is made two ways: in the body by beta cells of the pancreatic islets, and in pharmaceutical factories by bacteria or yeast engineered with the human insulin gene. Both processes produce the same hormone that moves glucose from the bloodstream into cells.
Where the Body Makes Insulin
The pancreas is both a digestive and an endocrine organ. Its endocrine cells are grouped into roughly one million islets of Langerhans scattered among the digestive acinar tissue. Each islet contains several cell types, but beta cells, which produce insulin, make up about 70% of the islet population.
Beta cells sense rising blood glucose through a glucose transporter (GLUT1 in humans) and an enzyme called glucokinase. When glucose rises, beta cells generate ATP, close potassium channels, depolarize the membrane, open calcium channels, and release preformed insulin granules by exocytosis. This all happens within minutes of a meal.
From Gene to Hormone: Insulin Biosynthesis
The insulin molecule has two chains (A and B) joined by disulfide bonds. The body builds it in four steps.
- Preproinsulin — a single long chain of 110 amino acids made on the endoplasmic reticulum. The first 24 amino acids are a signal sequence that directs it into the ER.
- Proinsulin — the signal sequence is clipped, leaving an 84-amino-acid chain that folds and forms the three disulfide bridges.
- Insulin + C-peptide — enzymes (prohormone convertases 1 and 2) cut out the connecting peptide (C-peptide, 31 amino acids), leaving the A chain (21 amino acids) and B chain (30 amino acids) held together by two of the disulfide bonds.
- Secretory granules — insulin and C-peptide are packaged in granules alongside zinc; each granule contains roughly six insulin molecules arranged as a hexamer around two zinc ions.
Because beta cells release insulin and C-peptide in a 1:1 ratio, clinicians measure C-peptide as an indirect marker of endogenous insulin production. Our insulin resistance explainer covers how this signaling can break down.
A Brief History of Pharmaceutical Insulin
Frederick Banting and Charles Best first isolated insulin from dog pancreas at the University of Toronto in 1921. By 1923 Eli Lilly was producing insulin from slaughterhouse cattle and pig pancreases, and the university sold the patent to the world for one dollar. Animal-source insulin kept people with type 1 diabetes alive for six decades but could provoke allergic reactions and antibody formation.
In 1978 scientists at Genentech spliced synthetic human insulin genes into Escherichia coli, the first successful recombinant human protein. The U.S. Food and Drug Administration approved that insulin, marketed as Humulin by Eli Lilly, in 1982.
How Pharmaceutical Insulin Is Made Today
Step 1: Engineering the Host Cell
Manufacturers use either E. coli or baker’s yeast (Saccharomyces cerevisiae) as a production platform. The human insulin gene, or a modified version encoding an analog, is inserted into a plasmid along with regulatory sequences and introduced into the host.
Step 2: Fermentation
The engineered cells multiply in stainless-steel fermenters up to 50,000 liters, fed glucose and nitrogen under controlled pH, temperature, and oxygen. Over several days each cell produces thousands of proinsulin molecules.
Step 3: Harvest and Purification
Cells are lysed and centrifuged to collect inclusion bodies (E. coli) or secreted proinsulin (yeast). A series of chromatography steps — ion exchange, reverse-phase, and size exclusion — separates proinsulin from host-cell proteins, DNA, and endotoxins.
Step 4: Enzymatic Cleavage and Folding
Trypsin and carboxypeptidase cut out the C-peptide, leaving A and B chains joined by disulfide bonds. For some processes the chains are folded separately and then linked chemically.
Step 5: Formulation and Fill
Purified insulin is blended with buffers (phosphate or tris), stabilizers (zinc, glycerol), and preservatives (phenol, metacresol). Sterile filtration, vialing, cartridging, or filling into prefilled pens follows under clean-room conditions. Every lot is tested for potency, purity, endotoxins, and sterility.
Human Insulin Versus Analog Insulin
Human insulin has the identical sequence the pancreas makes. Analogs swap amino acids to alter how the hexamer dissociates after injection, changing onset and duration.
| Type | Example | Amino acid change | Onset | Duration |
|---|---|---|---|---|
| Rapid-acting analog | Lispro (Humalog) | B28 proline to lysine; B29 lysine to proline | 10-15 min | 3-5 hr |
| Rapid-acting analog | Aspart (Novolog) | B28 proline to aspartic acid | 10-20 min | 3-5 hr |
| Short-acting | Regular human (Humulin R) | None — identical to human | 30-60 min | 5-8 hr |
| Intermediate | NPH (Humulin N) | Human insulin with protamine | 1-2 hr | 12-18 hr |
| Long-acting analog | Glargine (Lantus) | A21 asparagine to glycine; two arginines added to B30 | 1-2 hr | 20-24 hr |
| Ultra-long analog | Degludec (Tresiba) | B30 removed; fatty acid attached to B29 lysine | 1 hr | Over 42 hr |
Quality Control and Regulation
Insulin is a biologic drug, regulated by the FDA in the United States and by the European Medicines Agency in Europe. Every batch must pass identity testing (HPLC, mass spectrometry), potency testing (rabbit or cell-based assays), and purity testing before release. Modern facilities operate under current Good Manufacturing Practice (cGMP) with continuous process monitoring.
Insulin at the Target Cell
Once injected or secreted, insulin binds to the insulin receptor on muscle, fat, and liver cells. The receptor activates a cascade that moves GLUT4 transporters to the cell surface, letting glucose in. For a broader look at insulin’s role, see our treatment hub.
The Bottom Line
Your pancreas and a stainless-steel fermenter make the same hormone by different routes. Beta cells synthesize preproinsulin, trim it to proinsulin, and store insulin in granules for minute-by-minute release. Pharmaceutical manufacturers borrow the same gene, drop it into a bacterium or yeast, and use fermentation and enzymatic cleavage to produce the millions of vials, cartridges, and pens that keep people with insulin-dependent diabetes alive.
