Enzyme electrodes tyrosine

Enzyme electrodes for the amino acids tyrosine and lysine have been... 

For samples containing several substrates, a preliminary separation should be considered or the total must be determined. In the case of L-amino acid assay the use of decarboxylating enzymes acting selectively on different amino acids is an attractive possibility, and enzyme electrodes of this type are known for L-tyrosine, L-phenylalanine, and L-tryptophan. These sensors will be coupled with a carbon dioxide base sensor. 

Enzyme electrodes have been widely used for the assay of amino acids in clinical analysis since several amino acids (tyrosine, phenylalanine, tryptophan, methionine) are important diagnostic health indicators. 

Implanted relay modified-enzyme electrodes might find application also in selective and controlled electrochemical oxidation or reduction of biochemicals in a specific organ (e.g., the oxidation of L-tyrosine to L-Dopa). This would open a route to electrochemotherapy. 

Guilbault G.G. and Shu F.R. (1972) Enzyme electrodes based on the use of a carbon dioxide sensor. Urea and L-Tyrosine electrodes. Anal. Chem.,44, 2161-2165. 

L-Amino acid enzymes immobilized on O2 electrodes are used to determine methionine, leucine, phenylalanine, tyrosine, cysteine, lysine, and isbleucine... 

Decarboxylases of phenylalanine, tyrosine, and lysine and ammonia lyases of histidine, glutamine, and asparagine are also highly selective. Guilbault et al. (1988) described a potentiometric enzyme sensor for the determination of the artificial sweetener aspartame (L-aspartyl-L-phen-ylalanine methylester) based on L-aspartase (EC 4.3.1.1). The ammonia liberated in the enzyme reaction created a slope of 30 mV/decade for the enzyme-covered ammonia sensitive electrode. The specificity of the sensor was excellent however, the measuring time of 40 min per sample appears not to be acceptable. The measuring time has been decreased to about 20 min by coimmobilizing carboxypeptidase A with L-aspartase (Fatibello-Filho et al., 1988). 

A PNH3 electrode covered with this enzyme is used [123] for the determination of L-phenylalanine with very little interference fiom otho amino acids, notably L-tyrosine. A similar selectivity is obtained with phenylalanine decarboxylase [103] which, when attached to a pC02 electrode, enables the specific determination of phenylalanine. Other ammonia lyases are used te determine amino acids. Methionine lyase [124] and histidine ammonia lyase [12S] produce ammonia which is detected with a PNH3 electrode. 

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