5.3.7. pH Dependent solubility
An approach which has features of both of the previous two systems is one that attempts to produce a glucose sensitive device by linking the drop in pH caused by the reaction of glucose on glucose oxidase, with the fact that protein solubility if highly pH dependent. Here a modified insulin is again used, this time trilysyl insulin, that is insulin with three lysine groups attached to it. Lysine is a basic amino acid and it renders the insulin high at around its new isoelectric point between pH 7 and 8. When insulin is at physiological pH, that is pH 7.4, it is insoluble. When the pH is lowered when glucose reacts at the catalyst glucose oxidase, to form gluconic acid, then it becomes soluble and can diffuse out of the devise into the body. The solubility curves with respect to pH are shown in figure 5.3.15.
Figure 5.3.15. Solubility dependence of (A) Regular insulin. and (B) Trilysyl. insulin (after Godshial)