Hi José

You have made a question with a relative answer. because this is dependent of level what the target compound (glutathione) is present in matrix. I think good option for your case is LC-MS( with superficial knowledge about target compound), due complexity of your matrix, but it is necessary more details about samples.

Igree with Dr Marcos Steola

this is how I measure GSH and GSSG in a yeast cell. Obviously producing a cell free extract might differ. hope this helps.

For the estimation of intracellular glutathione, cells were washed twice with ice-cold phosphate-buffered saline (pH 7.4) resuspended in ice-cold 1.3% (wt/vol) 5-sulfosalicylic acid and 8 mM HCl, and broken using a 3000 mp ultrasonic homogenizer (Biologics, Inc, Cary, NC, US) using 10 s amplitude pulses for 1 min with the sample on ice at all times. The resulting cell-free extract was clarified by centrifugation (10 min, 13,000× g at 4 °C), and glutathione levels were measured in the resulting supernatant using a microplate method. Briefly, the reduction of 1.7 mM 5,5′-dithio-bis (2-nitrobenzoic acid (DTNB) in the presence of GSSG-reductase (3.3 U/mL) was measured for 5 min at 412 nm, the experiment was initiated by the addition of 0.9 mM β-NADPH. For quantification of oxidized glutathione, samples (including GSSG standards) were pretreated with 5% (vol/vol) 2-vinylpyridine for 1 h at room temperature before analysis. Glutathione levels are expressed as nanomoles per OD600, where OD600 of 1 corresponded to a cell density of 1.0 × 107 cells mL−1.