Phenylalanine microbiological assay

Figure 10.16 The Guthrie test. The similarity in structure between phenylalanine and its metabolic antagonist, /3-2-thienylalanine, provides the basis for a microbiological assay for phenylalanine.

The principle of microbiological assay is simple. For instance, in order to determine, say, phenylalanine, one selects a microorganism that needs phenylalanine. In a medium which contains an abundance of all other nutrients, the extent of bacterial growth will be determined by the amount of phenylalanine added to the medium. The analyst will set up a series of tubes, standards and unknowns, and after inoculation put them in the incubator. After a specified period—overnight or several... 

The phenylalanine concentration may be measured in blood either by chemical estimation or by microbiological assay (the Guthrie test, q.v.). The latter technique can be used as a screening procedure. 

Provisional values for the amino acid composition of crystalline botulinus toxin have been determined by Buehler et al. (19), using microbiological assay methods. The following amino acids were shown to be present aspartic acid, valine (4.0%), serine (3.0%), threonine (7.3%), arginine (3.2%), lysine (7.9%), histidine (1.0%), glutamic acid (14.9%), leucine (8.9%), isoleucine (11.0%), cysteine (0.48%), methionine (0.83%), phenylalanine (1.08%), and tyrosine (9.5%). The total is 73%. 

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