Problemas en la lectura de MTT

Los ensayos de citotoxicidad que hacemos involucran varias tecnicas, entre ellas la del MTT. Este articulo señala que el suero fetal bovino, por la cantidad de albumina, podria dar problemas en la lectura.
BioTechniques® August 2007
Volume 43, Number 2: pp 178-186
Serum albumin leads to false-positive results in the XTT and the MTT assay
Dorothee Funk, Hans-Hermann Schrenk, and Eva Frei

Abstract
Tetrazolium salts like 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) or sodium 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H- tetrazolium-5-carboxanilide (XTT) that form formazans after reduction are widely used to investigate cell viability. Besides cellular enzymes, some constituents of cell media and other substances reduce tetrazolium salts, thereby interfering with these assays. We describe here that different preparations of serum albumin from bovine or human origin can lead to a concentration-dependent increase in the signals of the XTT assay; therefore leading to an overestimation of cell numbers and to an underestimation of potential cytotoxic effects of compounds to be tested. The same effect was seen in the MTT assay with human serum albumin (HSA). We demonstrate that this reductive activity cannot be inactivated by proteolytic digestion, but that it is due to the free cysteine residue in albumin, and is also observed when cysteine or glutathione (GSH) are used. Binding of N-ethylmaleimide (NEM) to the free cysteine residue leads to a decrease of the albumin interference in the XTT assay.

"...reduced XTT and MTT to the corresponding formazan and might explain the observation of Zhang and Cox who observed a 20% higher formazan signal when cells were assayed in 10% fetal calf serum (FCS) rather than in 5% FCS (16).Based on these findings, we strongly recommend to be careful with data from XTT and MTT assays when estimating cytotoxicity when albumin or other proteins with free SH groups are present, because the artificial increase in the assay signal, which is not due to cellular reductases, could lead to an underestimation of the cytotoxicity of the compounds to be tested"