Where can I find the extinction coefficient of glutathione (GSH)? - (Jul/19/2012 )

I've tried to quantify the GSH level in liver tissue. my calibration curve didn't work so i'm trying to calcuate with epsilon instead.
any ideas?

Are you assaying the glutathione directly or using an indirect assay (e.g. compound that reduces glutathione, measure colour change of said compound)?

I'm using nature protocol: http://www.nature.com/nprot/journal/v1/n6/full/nprot.2006.378.html

So you are actually measuring the change in a compound that is not glutathione - are you sure that you shouldn't be looking for the extinction coefficient for DTNB or TNB?