Insulin polymorphism: some physical and biological properties of rat insulins

Although rat insulins I and II show no significant differences in their biological activities and receptor binding on isolated fat cells, X-ray studies and circular dichroism indicate that they have differences in their structures. Rat insulin II forms zinc insulin hexamers in an identical manner to bovine insulin, but insulin I, which has a unique proline substitution at B9, forms hexamers less easily. Rat insulin I can form zinc insulin hexamers given higher zinc concentrations, as indicated by the formation of rhombohedral 2Zn insulin crystals. On the other hand, rat insulin II forms cubic crystals of space group $\text{P4}{}_2\text{32}\text{with}\text{a = 67}\text{A}\text{?}$ under similar conditions. Model building indicates that these crystals contain a tetrahedral arrangement of zinc hexamers. They have a higher solvent content and are less stable than rhombohedral insulin crystals. The relation of these observations to the rat insulin storage granules and the importance of polymorphism to the physiology and evolution of insulin are discussed.