2.2. Insulin as a Hormone
2.2.1. Activity at the Cellular Level
Insulin binds to a site called the insulin receptor on the cell surface of liver and other cells, and in so doing changes the receptor shape and triggers a cascade of events which cause the cells to be more receptive to glucose uptake. The liver stores the glucose as glycogen, a polysaccharide. In situations where the glucose level is below optimum, the glycogen is broken down and released back into the body as glucose. The insulin receptor is itself a protein and it is made up of two alpha subunits on the outside of the cell membrane, each composed of 719 amino acids, and two beta subunits with 620 amino acids apiece, which straddle the cell membrane and are attached to the alpha units by disulfide bridges. Once again we see the importance of being able to deliver the insulin to the cell receptor in the active form so that it can bind properly.
Figure 2.2.1. Insulin attached to Insulin receptor
2.2.2. Natural Delivery Profile
The control of glucose levels in the blood of non diabetic people is exquisite. It takes place very rapidly, and on two different levels, one at the gene that expresses insulin in the cells of the pancreas, and the other at storage vesicles in the same cells. The net result, which the ideal insulin delivery system should mimic, is that when the blood glucose level rises, insulin is released into the blood, and when it drops back to normal, the insulin supply should be stopped. We would like to produce a responsive drug delivery system.