Protein produced with brown color? - I have some problem in protein expression (Mar/25/2005 )

I was trying to produce some protein in E.coli BL21(DE3). 37C 1mM IPTG 4hour.

However the pellet is brown(In contrast other protein is produced as yellow color). In the purification steps, there is always some absorption at 420nm(red-brown) associated with my protein no matter what separation methods were used! In the concentrating step, this is some brown color at the bottom of the concentrator (Amicon)

It is known there should be no heme group included in this protein.

What can this color be caused by? why it is always associated with my protein?

I had similar experiences with E. coli. I think it has something to do with media components. In Pichia the problem is even worse - if your protein gets secreted into the media you end up with a brown-green solution of your protein (before the final concentration step it looks colorless, but when you concentrate, the color becomes obvious).

If you isolate your protein from E. coli cytoplasm it may be a good idea to wash the cells (with e.g. PBS) before lysing them.