Help with Question for Cell Bio Take Home Exam - (Mar/04/2015 )

Hey all, im new to this forum. I have a take home exam for cell bio and there is one question that has been driving me nuts. The question is as follows:

You are trying to purify adenyl cyclase from the brain. The assay that you use to identify the enzyme is based on conversion of α32P-ATP to cAMP. You can easily detect activity in crude brain homogenates stimulated by isoproteronol, which binds to B-adrenergic receptors, but the enzyme loses activity when low molecular weight cofactors are removed by dialysis. What single molecule do you think you could add to the dialyzed homogenate to restore activity?

Some answers I found ranged from Calcium to Magnesium and even GTP. But I'm not sure which is the right one and the explanations for it.

Anyone?

Moved to homework...bob.

The key is in the question... what are typical cofactors for enzymes? Which of those are low MW?