Using a five step protocol, you have purified an enzyme to the point where the specific activity remains constant when you perform any additional purification steps. When you run the purified enzyme on a gel filtration chromatography column, you find it has a molecular mass of 50 kD. However, when you run the purified enzyme on a SDS-PAGE gel, you obtain two different sized peptides one 30 kD and the other 20 kD. What could be the possible explanations for this result?