Problem Adjacent Channel Support

Hi Community, 

We have PMP 450, 450i and also Medusa, and in the 450 normal We cannot change to disable the option Adjacent Channel Support, if We change from enable to disable and then click to save, the option returns to enable again, and it is imposible to change to disable this option. I have tried in a few APs. Other question is that: Why We should activate to enable this option if We have 450b SMs?