Dopamine determination using electrochemical

Because LCEC had its initial impact in neurochemical analysis, it is not, surprising that many of the early enzyme-linked electrochemical methods are of neurologically important enzymes. Many of the enzymes involved in catecholamine metabolism have been determined by electrochemical means. Phenylalanine hydroxylase activity has been determined by el trochemicaUy monitoring the conversion of tetrahydro-biopterin to dihydrobiopterin Another monooxygenase, tyrosine hydroxylase, has been determined by detecting the DOPA produced by the enzymatic reaction Formation of DOPA has also been monitored electrochemically to determine the activity of L-aromatic amino acid decarboxylase Other enzymes involved in catecholamine metabolism which have been determined electrochemically include dopamine-p-hydroxylase phenylethanolamine-N-methyltransferase and catechol-O-methyltransferase . Electrochemical detection of DOPA has also been used to determine the activity of y-glutamyltranspeptidase The cytochrome P-450 enzyme system has been studied by observing the conversion of benzene to phenol and subsequently to hydroquinone and catechol... 

Biochemical analyses of 6-OHDA-injected animals revealed a 93 percent depletion of dopamine. The tissue was assayed using electrochemical detection following separation by high-pressure liquid chromatography (Felice et al. 1978). recorded as ng/mg protein in the nucleus accumbens and compared to control rats with sham lesions (sham=65.5 4.4, lesion=4.9 1.5 t(39)=23.4). A lesion was defined as complete if 75 percent or more of the dopamine was determined to be depleted from the nucleus accumbens compared to mean sham group values. 

Neurotransmitters have also been detected on microchips using electrochemical detection, eliminating the need for on-chip reactions or derivitization. Amperometric detection, a current change when an analyte passes the detection electrodes, was demonstrated on a microchip for the determination of dopamine concentrations in standard solutions [10], The microdevice developed in this... 

The determination of catecholamines requires a highly sensitive and selective assay procedure capable of measuring very low levels of catecholamines that may be present. In past years, a number of methods have been reported for measurement of catecholamines in both plasma and body tissues. A few of these papers have reported simultaneous measurement of more than two catecholamine analytes. One of them utilized Used UV for endpoint detection and the samples were chromatographed on a reversed-phase phenyl analytical column. The procedure was slow and cumbersome because ofdue to the use of a complicated liquid-liquid extraction and each chromatographic run lasted more than 25 min with a detection Umit of 5-10 ng on-column. Other sensitive HPLC methods reported in the literature use electrochemical detection with detection limits 12, 6, 12, 18, and 12 pg for noradrenaline, dopamine, serotonin, 5-hydroxyindoleace-tic acid, and homovanillic acid, respectively. The method used very a complicated mobile phase in terms of its composition while whilst the low pH of 3.1 used might jeopardize the chemical stability of the column. Analysis time was approximately 30 min. Recently reported HPLC methods utilize amperometric end-point detection. 

In 1976, Adams published a pioneering article on the voltammetric technique of in vivo determination of electrochemically active compounds in the brain [33]. It describes notably successful measurements on neurotransmitters such as dopamine (DA) and seretonin (5-HT) using carbon paste electrodes with tip diameters from 50 to 200 m implanted in the brain tissue of a rat. Thereafter, in vivo voltammetric technique attracted much attention from neuroscientists and electrochemists, and many papers have been published in the field [30, 34-47]. In this section microelectrodes suitable for the detection of neurotransmitters, the operation techniques for positioning electrodes, and some results are described. 

Electrochemical detection was easily employed for the determination of uric acid in urine, abnormal concentrations of which have been linked to several disease states [14]. A glass/PDMS hybrid device with an off-chip platinum electrode was used to evaluate standard samples for both dopamine and uric acid. The linear responses for dopamine and uric acid were 1-165 and 15-110 pM, respectively, with a 1 pM limit of detection for both. Normal concentrations of uric acid in urine are 800-8000 pM, thus a 50 to 75-fold dilution was used with the urine samples analyzed to place them within the linear range of the detection method. Uric acid concentrations in these urine samples were confirmed using the clinically accepted method. This new method should allow clinical detection of both abnormally high and abnormally low uric acid concentrations in urine samples on a microdevice. 

Wagner, J., Palfreyman, M., and Zraika, M., 1979, Determination of dopa, dopamine, DOPAC, epinephrine, norepinephrine, a-monofluoromethyldopa, and a-difluorome-thyldopa in various tissues of mice and rats using reversed-phase ion-pair liquid chromatography with electrochemical detection, y. Chromatogr. 164 41-54. 

In recent years, polyirnide-modified electrodes are widely used in the field of sensor and biosensor. Among the electroactive species, dopamine (DA) has been of interest to neuroscientists and chemists. A loss of DA containing neurons may result in some serious diseases such as Parkinsonism. Therefore, the determination of the concentration of this neurochemical is important. Dopamine in central nervous system coexists with ascorbic acid, whose oxidation peak potential is close to that of dopamine. Therefore, a significant problem faced in electrochemically determination of dopamine is the presence of electroactive ascorbic acid, which reduces the selectivity and sensitivity [28-30]. 

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