Adsorption Isotherms of Insulin onto Various Materials

Insulin adsorption isotherms on various materials have been measured to begin to clarify the role of material surface energy in insulin aggregation. Using ¹²⁵I-insulin and direct γ-counting of the exposed material, more insulin (per unit area) was adsorbed to hydrophobic materials (Teflon, Silastic) than to the hydrophilic ones (polyacrylamide, glass) from insulin solutions ranging from 0.1 to 100 U/ml. For example, after 30 min at room temperature, Teflon disks adsorbed 4.0 × 10⁴ μU/cm² and glass beads adsorbed 3.0 × 10³ μU/ cm² from a phosphate-buffered, 100-U/ml solution (pH 7.4). The Teflon value exceeded, by a factor of six, an estimated plateau surface concentration based on the molecule area (750 Ų), suggesting the occurrence of multilayer adsorption. Changing the buffer to acetate and lowering the pH to 3.5 resulted in an increased surface concentration, while the addition of glutamic or aspartic acid at pH 3.5 reduced the surface concentration to be comparable with that observed on glass and less than that observed on Teflon from phosphate buffer. Increasing the temperature to 37°C resulted in a small decrease in adsorption, consistent with the exothermic adsorption of other proteins on similar materials. However, the significance of the adsorbed molecules in nucleating aggregate formation relative to other contributing factors remains to be assessed.