Only 1-3 % of secreted insulin from the islets of Langerhans is in the form of proinsulin but due to its long half-life, it accounts for 5-30 % of the insulin-like molecules circulating in the blood. Additionally, because of similarities in the structure of proinsulin and insulin, proinsulin can bind with some affinity to the insulin receptor, producing up to 5-10 % of the metabolic activity induced by insulin (Weiss et al., 2000[130]).