a. Insulin from an animal source, though it caused some patients to develop allergy or other types of reactions to the foreign protein. Insulin consists of two short polypeptide chains : chain a and chain b, which are linked together by disulphide bridges. In mammals, including humans, insulin is synthesized as a pro-hormone, which contains an extra stretch called the C-peptide.
b. In 1983, Eli Lilly, an American company, prepared two DNA sequences corresponding to a and b chains of human insulin and introduced them in plasmids of E.coli to produce insulin chains. Chains a and b were produced separately, extracted and combined by creating disulphide bonds to form human insulin.
c. The insulin produced by human body is different from the insulin produced by the above mentioned company. Insulin is synthesized as a pro hormone which contains an extra stretch called the C peptide. This C peptide is not present in the mature insulin and is removed during maturation into insulin. The main challenge for production of insulin using rDNA techniques was getting insulin assembled into a mature form.