In the cold winter nights of the 1870s, along the rivers of Switzerland, a young biochemist named Friedrich Miescher worked by the open window. He was chasing the secrets of the cell nucleus. Salmon sperm, rich and plentiful in the cold season, became his unlikely material. From those tiny cells he isolated two remarkable substances: an acidic one he called “nuclein” (what we now know as DNA) and a powerfully basic protein that clung tightly to it. He named the protein protamine.

For decades it remained a scientific curiosity — a simple, arginine-rich protein that nature used to package DNA inside the heads of fish sperm. Then, in the 1930s, medicine found a use for it.

Doctors had just begun treating diabetes with the new miracle drug insulin, but the hormone worked too quickly and too briefly. A Danish physician, Hans Christian Hagedorn, remembered the sticky, binding nature of protamine. When he mixed a little of the salmon-derived protein with insulin, the hormone was released slowly into the body. The result was NPH insulin — Neutral Protamine Hagedorn — still used by millions of people today.

Around the same time, another discovery waited. Scientists experimenting with the blood thinner heparin noticed something unexpected: when they added protamine, the anticoagulant effect vanished almost at once. The positively charged protamine locked onto the negatively charged heparin and neutralized it completely. Suddenly surgeons had an antidote. After open-heart operations, when large doses of heparin had kept the blood from clotting in the heart-lung machine, a careful injection of protamine sulphate could reverse the thinning and allow the patient’s blood to clot normally again.

From the sperm of salmon, extracted and purified, came a medicine that both prolonged the life-saving action of insulin and safely reversed the powerful effects of heparin. Today much of it is still sourced from fish, though recombinant versions are appearing. Yet the story remains the same: a humble protein once hidden inside fish sperm became an essential guardian in operating theatres and diabetes clinics around the world.