Simultaneous voltammetric determination of free tryptophan, uric acid, xanthine and hypoxanthine in plasma and urine

Abstract

We have designed an improved electrochemical analysis of clinical samples in bodily fluids. The method allows reliable identification of several coexisting substances (tryptophan, uric acid, xanthine and hy- poxanthine) present on different physiological levels. We prepared in situ electrogenerated graphite oxides (GrO) directly on the surface of a graphite pencil electrode. GrO presence enhances electro- oxidation signals of small aromatic molecules and can shift their oxidation potentials. The oxidation potentials of tryptophan and tyrosine in their free and protein-bound forms, and free xanthine are very close to each other on common carbon-based materials. We found that free tryptophan’s oxidation signal is significantly more enhanced (about 11-times) than the free tyrosine signal (only negligibly in the given conditions). Distinguishing xanthine’s oxidation signal from that of the free tryptophan is allowed by their approximately 60mV separation, sufficient for parallel determination by signal deconvolution. Additionally, we show that proteins containing tyrosine and/or tryptophan behave as electro-inactive and do not interfere with free amino acid determination on the GrO-modified graphite electrodes, while they strongly influence all species’ detection on other carbon-based materials. Determining all individual compounds was possible at corresponding physiological values after sole dilution of human plasma (urine) and direct measurement with no pre-separation steps. ©