Interference with ammonia determination amino acids

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. 

The manifold design allows for the determination of ammonia concentration in the range 0.2-20 xg/l as NH4 over a salinity range 35- 10%o, with negligible interference from amino acids. 

Petty et al. (29) developed an FI A procedure for the determination of total amino acids in seawater. Total dissolved amino acids can be determined with a detection limit of 0.01 yM and a sampling rate of 150 per hour with the manifold shown in Figure 8. The amino acids are determined as fluorescent isoindoles (41), which are formed by reaction of primary amines with 1,2-benzenedicarbaldehyde (o-phthaldialdehyde) and mer-captoethanol. The fluorescence quantum yields of the isoindoles formed from most amino acids are similar 41, 42), and the sensitivity of the FI A analysis is similar for most amino acids (29). Ammonia also forms a fluorescent product with 1,2-benzenedicarbaldehyde, but its fluorescence quantum yield is about a factor of 15 lower (29, 42). The ammonia interference is low, therefore, in most areas of the ocean. 

One problem of prime importance is the reliable determination of the number of residues of ,/3-unsaturated amino acids in proteins. Direct amino acid analysis subsequent to total hydrolysis of proteins is not feasible. The ,/3-unsaturated amino acids are subject to degradation with the formation of amide (ammonia) and -keto-acid. The numbers and types of ,/3-unsaturated amino acids in nisin (1) and subtilin (10) and in the fragments of the two peptides were, nevertheless, determined by amino acid analysis, only, however, after the addition of mercaptan across the double bonds of dehydroalanine and dehydrobutyrine (19). Using benzylmercaptan, the addition products are S-benzylcysteine (from dehydroalanine) and /3-methyl-S-benzylcysteine (from dehydrobutyrine). The two thioether amino acids are eluted from ion exchange columns of the amino acid analyzer free from interference by other amino acids... 

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