bigger band after reduction with dithithreitol - (Mar/16/2006 )

Hello, I've got problem with reducing SDS-PAGE. After reduction with dithiothreitol the band of my interest migrates slowly on SDS gel. It's bigger for about 5 kDa than in non-reduced sample. The other bands on the same gel in different lines didn't change their position or are smaller as is expected.
My band obviously doesn't have S-S, so there is nothing to be reduced with dithiothreitol. And there is no other protein in the sample that could bind to it. Anyway, i tried acetylation too, the result is the same. I just can't understand it.
Please, do you have any idea why it behaves like this?

thanks

Maybe something with the overall folding of the protein?