A denaturation/renaturation (similar to the one carried out by Anfinsen with ribonuclease) experiment was carried out using insulin. However, in contrast to Anfinsen's results, only less than 10% of the activity of insulin was recovered when urea and BME were removed by dialysis. (This is the level of activity you would expect if the disulfide bridges paired randomly). In contrast, if the experiment is repeated with proinsulin, full activity is restored upon renaturation. Explain these observations.