Oral delivery of insulin: Novel approaches

Insulin is a hormone that is synthesized in the β-cells of the pancreas as a proinsulin precursor and is converted to insulin by enzymatic cleavage. The resulting insulin molecule is composed of 51 amino acids arranged into two polypeptide chains - the A and B chains - which are connected by two interchain disulphide bridges. In the A chain, there is an additional intrachain disulphide linkage [1]. The primary structure of human insulin is shown in Figure 1 a. In the secondary structure, chain A consists of two antiparallel α-helices (A2 to A8 and A13 to A20), while chain B forms a single α-helix “B9 to B19” followed by a turn and a β strand “B21 and B30” [2]. The folding of insulin into a tertiary structure is essential for its biological activity (Figure 1b). Insulin has an isoelectric point (pI) of 5.3 and a charge of -2 to -6 in the pH range 7-11. Another intrinsic property of insulin is its ability to readily associate into dimmers, hexamers and higher-order aggregates. At the low concentrations found in the blood stream (< 10-3 µM), insulin exists as a monomer, which is its biologically active form. Following biosynthesis, insulin is stored as crystalline zinc-bound hexamers in vesicles within the pancreatic β-cells from which secretion occurs in response to elevated blood glucose levels [3]. The biological actions of insulin are initiated when insulin binds to its cell surface receptor. Insulin is an anabolic hormone and when binding to its receptor begins, many protein activation cascades occur. These include: the translocation of the glucose transporter to the plasma membrane and the influx of glucose, glycogen synthesis, glycolysis and fatty acid synthesis. Insulin has been observed as promoting the transport of some amino acids and potassium ions. Insulin also inhibits the liberation of free fatty acids and glycerol from the adipose tissue [3].