AMPEROMETRY AND CYCLIC VOLTAMMETRY OF TYROSINE AND TRYPTOPHAN-CONTAINING OLIGOPEPTIDES AT CARBON-FIBER MICROELECTRODES APPLIED TO SINGLE-CELL ANALYSIS

The amperometric and cyclic voltammetric detection of alpha-melanocyte stimulating hormone (MSH), beta-endorphin, and corticotropin-like int ermediate lobe peptide (CLIP), all proopiocortin (POC) derived peptide s secreted from melanocytes of the pituitary intermediate lobe, at car bon fiber microelectrodes was investigated. For amperometry, it was fo und that all of these peptides could be detected; however, fouling of the electrodes reduced the response of the electrode after successive application of the peptide in flow injection experiments. The fouling was apparently due to oxidation of tyrosine in the peptides as similar results were found for tyrosine but not tryptophan. The effect of fou ling could be reversed if the electrode was electrochemically treated by scanning from -1.0V to +1.0V at 300 Vis for 2 min between applicati on of the peptides. Using cyclic voltammtery at 800 V/s, it was possib le to distinguish MSH, which had a peak shaped voltammogram, from the other POC peptides, which had relatively flat voltammetric waves at th is scan rate. The scan rate dependence of the peak current for MSH rev ealed that the voltammetry was adsorption controlled. As a result, in a monitoring application, where voltammograms are continuously obtaine d with a fixed interval between them, decreasing the interval increase s the temporal resolution but decreases the sensitivity for MSH. It wa s found that when monitoring the current in the potential range of 0.9 0 to 1.00 V, the temporal response to MSH was dependent upon the poten tial window used for scanning. Using high scan rates and a potential w indow of 0 to 1.2V, it was possible to monitor exocytosis from single melanocytes and use the voltammogram to demonstrate detection of MSH f rom the cells.