How did you stain it? Coomassie? Silver stain? Anti-His Blot?
Are you saying the lambda max is 260 or you have *no* absorbance at 280?
If the later I would check your spectrophotometer.
If the former you may have some contamination with nucleotides. Is your
protein likely to bind to DNA/RNA?
Have you done a bradford or BCA titration to check whether concentration
calculated from OD 280 corresponded to that calculated with different
That may not work but how about staining your SDS gel with ethidium
bromide if you're convinced you have contaminating DNA?