This is a question about SDS-PAGE, being performed on a mixture of albumin and insulin

This is a question about SDS-PAGE, being performed on a mixture of albumin and insulin.In experiment 1, the proteins are treated with radioactive NEM --> then DTT--> then NEM In experiment 2, the proteins are treated with NEM--> DTT--> then radioactive NEM at the end

NEM = N-ethyl maleimide, reacts with free SH groups but not with S-atoms in disulfide bonds

DT = Dithiothritol: breaks disulfide bonds

Could you please explain the purpose of a radioactive NEM?

When the proteins were treated in experiment 1 --- the bands were lighter compared to proteins in experiment 2 -- why is that the case?

I understand the SDS-PAGE, that it works to separates proteins based on molecular weight.