The importance of sample preparation and storage in glutathione analysis.

There is little consistency in the literature regarding the procedures for sample preparation employed for the measurement of glutathione (GSH) in biological tissues. Most procedures use an acid to homogenize tissue samples to precipitate proteins from the mixture and to minimize oxidative changes. Others employ 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) in the homogenization, which serves only to block thiol groups. The present studies were undertaken to critically compare the two methods of sample preparation on resulting GSH values determined by the Tietze method in numerous organs of mice. It was found that kidney, liver, and pancreas GSH levels were seriously underestimated when DTNB was used instead of acid to prepare the tissue samples. This discrepancy was eliminated when the animals were pretreated with AT-125, confirming the participation of gamma-glutamyltranspeptidase, the enzyme responsible for the first step in the degradation of GSH. GSH added to kidney homogenates in DTNB was degraded rapidly and continuously in a time-dependent fashion. In contrast, GSH added to acid homogenates produced stable GSH values up to 8 h after sample preparation in most cases. Storage of acid homogenates at -70 degrees C for 12 months gave results identical to original measurements, within 10% error, for 9 of 10 samples tested.