Nicolae Paulescu, a Romanian professor of physiology at the
University of Medicine and Pharmacy in Bucharest, was the first to isolate insulin. He did that in 1916. He called it pancrein. He isolated it by developing an
aqueous pancreatic extract which, when injected into a diabetic dog, proved to have a normalizing effect on blood sugar levels. He had to interrupt his experiments because the World War I. In 1921 he wrote four papers about his work carried out in Bucharest and his tests on a diabetic dog. Later that year, he detailed his work by publishing an extensive whitepaper on the effect of the pancreatic extract injected into a diabetic animal, which he called: "Research on the Role of the Pancreas in Food Assimilation".
Dr. Frederick Banting and Dr.
Charles Best also did similar experiments while trying to find a cure for diabetes. In these experiments they used dogs.
They first knew insulin would control diabetes in a person when they injected some insulin into a 14 year old boy named Leonard Thompson who was dying of diabetes. After the injection he survived. Banting won the 1923 Nobel Prize in Physiology or Medicine, along with John Macleod, for the discovery of insulin. Best and Paulescu were not honored at that time.
The first genetically-engineered, synthetic "human" insulin was produced in a laboratory in 1977 by Herbert Boyer using E. coli.