Activationof Aminotransferasesby Pyridoxal 5 - Phosphate : Importance of ExperimentalDetails

1512 CLINICALCHEMISTRY,Vol. 26, No. 10, 1980 lution containing 1.25 smolfL was degraded after 1 mm. When exposed to light of wavelength 254 nm, tryptophan was alsorapidlydegraded,although less quickly than in bright sunlight: 64% in the 5 itmol/L solution was destroyed after 15 mm exposure,whileatthelower concentration no detectable tryptophan remained after 30-mm exposure. Tryptophan in the supernate of deproteinized plasma is also unstable to ultraviolet light or bright sunlight, but decomposes appreciably slower than is the case for solutions of pure standard: 60% of the tryptophan was lost after a 60-mm exposure to either bright sunlight or 254-nm light. When the solutions of tryptophan were allowed to stand on the laboratory bench in bright daylight, there was 13% loss after 5 mm from a 5 mol/L solution, and no measurable tryptophan remained after 60 mm. At the lower concentration, the tryptophan was completely degraded after only 15 mm. In the case of the plasma samples there was a 22% loss after 60 mm. Thus tryptophan in solution is extremely labile under many conditions of light exposure, and this amino acid in dilute solution may be rapidly and completely destroyed by light, even in the absence of ferric chloride. Plasma extracts containing tryptophan are also labile, although the rate of decomposition is somewhat slower than that for pure standards. In contrast, no measurable tryptophan was lost from solutions stored in the dark at room temperature. Evidently, in any assay procedure for measuring plasma tryptophan, the assay mixture should be carefully protected from light at all times until a stable derivative of the amino acid has been formed.