Spectral studies of the interaction of the substrate ‘quinonoid’ 6‐methyl dihydropterine and the coenzyme NADH used as marker in the dihydropteridine reductase assay

In dihydropteridine reductase assay the substrate quinonoid dihydropterine is reduced again to tetrahydropterine, concomitantly oxidizing NADH, the indicator of the enzyme assay. Because of the strong oxidizing capacity of quinonoid dihydropterine, the degree of spontaneous oxidation of NADH by the various substances used in the dihydropterine reductase assay was studied spectrally. A high degree of spontaneous oxidation of NADH by the substrate itself was found, which can be regulated by dithiotreitol, dependent on its concentration. The absorbance increase at 336 nm, due to the non‐quinonoid dihydropterine formed spontaneously from its quinonoid form, strongly interferes with the absorbance decrease at 340 nm, due to the disappearance of NADH. The interference results in a shift of the absorbance maximum of NADH from 340 nm up to higher wavelengths. This phenomenon, expressing itself in various ways in blank and sample, is discussed with relevance to the validity of the current enzyme assays used in a further classification of hyperphenylalaninaemic patients.