A protein has a molecular mass of 400 kDa when measured by gel filtration. When subjected to SDS-PAGE in the absence of dithiothreitol (DTT), the protein gives three bands with Mr values of 180,000, 160,000 and 60,000. When SDS-PAGE is performed in the presence of DTT, three bands are again formed, this time with Mr values of 160,000, 90,000 and 60,000. What is the simplest explanation for these results?

The protein can be separated by different techniques like SDS PAGE, by the use of urea, and dithiothreitol. The urea and dithiothreitol acts as denaturant that can break the disufide bonds present in the protein.

The total molecular pass of protein is 400,000 Da. In absence of SDS three bands are observed with the mass of  180,000, 160,000 and 60,000. But in presence of dithiothreito These values are observed 160,000, 90,000 and 60,000. This might happen that protein consists of four subunits with 160, 90, 90, and 60 kDa. Without DTT 180,000 subunit is unable to reduce into 90K Da and linked through the disulfide bonds. In presence of DTT the 90,000 units are identical and visible at single lane.