Preproinsulin is translated in the beta-cell cytoplasm after which it is cleaved into proinsulin in the ER and then transported to the Golgi apparatus. Proinsulin processing occurs in maturing secretory granules and thus, insulin and C-peptide are secreted simultaneously, in equimolar amounts, in response to various stimuli, including glucose.
Insulin is made up of 51 amino acids and is one of the smallest proteins in the body. It is comprised of two polypeptide chains linked by two disulfide bonds, connecting cysteine molecules. There is a third disulfide bond that connects two amino acids within Chain A. Chain A consists of 21 amino acids and chain B contains 30 amino acids.
The amino acid sequence of insulin is highly conserved among vertebrates. Virtually all primates and monkeys share the same sequence. Rat and mouse insulins only differ from primate insulin by 4 amino acids. Even the positions of the three disulfide bonds are the same for most species. The similarities in amino acid sequences and conserved positions of the disulfide bonds lead to a three dimensional conformation of insulin that is very similar across species. Thus, insulin from one species is often biologically active in other species. For example, pig insulin has historically been used to treat human patients. Insulin from some species of fish can also be similar enough to human insulin to be clinically effective in humans. In addition, insulin in some invertebrates is quite similar to human insulin, and has similar physiological effects.
Unlike insulin, C-peptide structure differs greatly between species. For example, the amino acid sequence of rat C-peptide is approximately 30% different than that of primates. The structural differences across species and increased in vivo stability make C-peptide measurements more suitable (than insulin) when performing studies of xenotransplantation or subjects treated with exogenous insulin.
Although insulins from certain species are very similar, when it comes to crystal structure and amino acid sequence, the small differences can be of great importance depending on where they are located. For example, the insulin from Atlantic hagfish is very similar to pig insulin, yet hagfish insulin displays only a relative biologic potency of 5%, when compared to pig insulin, in rat adipocytes.
Mercodia knows the importance of having assays optimized for various species. In addition to our human insulin assays, we developed insulin assays specifically calibrated for rodent (rat/mouse) and large animals (cat/dog/horse/pig/cow/sheep) samples. A variety of our insulin assays have also been validated (in-house or by clients) for use with other animals (monkeys/rabbit/hamster).