Enzymatic rotating biosensor for cysteine and glutathione determination in a FIA system

The high sensitivity that can be attained using an enzymic system and mediated by catechols has been verified by online interfacing of a rotating biosensor and continuous flow/stopped-flow/continuous-flow processing. Horseradish peroxidase, HRP, [EC 1.11.1.7], immobilized on a rotating disk, in presence of hydrogen peroxide catalyzed the oxidn. of catechols, whose back electrochem. redn. was detected on glassy carbon electrode surface at -150 mV. Thus, when L-cysteine (Cys) or glutathione (GSH) was added to the soln., these thiol-contg. compds. participate in Michael addn. reactions with catechols to form the corresponding thioquinone derivs., decreasing the peak current obtained proportionally to the increase of its concn. Cys was used as the model thiol-contg. compd. for the study. The highest response for Cys was obtained around pH 7. This method could be used to det. Cys concn. in the range 0.05-90 micro M (r = 0.998) and GSH concn. in the range 0.04-90 micro M (r = 0.999). The detn. of Cys and GSH were possible with a limit of detection of 0.7 and 0.3 nM, resp., in the processing of as many as 25 samples per h. Current response of the HRP-rotating biosensor is not affected by the oxidized form of GSH and Cys (glutathione disulfide, GSSG, and L-cystine, resp.), by sulfur-contg. and alkyl-amino compds. such as methionine and lysine, resp. The interferences from easily oxidizable species such as ascorbic acid and uric acid are lowest. [on SciFinder (R)]