A common affinity tag used to express and purify fusion proteins is glutathione S-transferase However many researchers have reported difficulty eluting GST-tagged proteins from the affinity matrix This report demonstrates that the problem likely is due to the propensity of glutathione S-transferase to dimerize combined with a propensity of the tagged protein to oligomerize which results in formation of large oligomers of fusion protein that are chelated by the affinity matrix The solution to the problem is to use S-butylglutathione instead of glutathione to elute as S-butylglutathione binds more tightly to glutathione S-transferase and overcomes the chelate effect Moreover in contrast to glutathione S-butylglutathione has no reducing capability that might inactivate a tagged protein (C) 2010 Elsevier Inc All rights reserved