Based on the following information, answer the questions given below:
Insulin used for diabetes was earlier extracted from pancreas of slaughtered cattle and pigs. Later, In 1983, Eli Lilly an American company prepared Humulin by r DNA technology. Insulin consists of two short polypeptide chains: chain A and chain B, that are linked together by disulphide bridges. In mammals, including humans, insulin is synthesised as a pro-hormone. The pro-hormone also needs to be processed before it becomes a fully mature and functional hormone. 

Question:

What was the major problem faced with the insulin developed from animal source?