Enzymatic isomerization and epimerization of d ‐erythrose 4‐phosphate and its quantitative analysis by gas chromatography/mass spectrometry

An enzyme preparation from beef liver catalyzed the isomerization and epimerization of d ‐erythrose 4‐phosphate to d ‐erythrulose 4‐phosphate and d ‐threose 4‐phosphate. The presence of d ‐erythrulose 4‐phosphate and d ‐threose 4‐phosphate was demonstrated by several analytical methods. After dephosphorylation, the presence of d ‐erythrulose and d ‐threose was confirmed by thin‐layer chromatography, gas‐liquid chromatography and an enzymatic method depending upon d ‐erythrulose reductase. The enzymatic products were also identified and simultaneously quantitated by a new procedure using gas chromatography/mass spectrometry. Each of three tetroses was distinguished by the combination of the reduction with sodium borodeuteride and the determination of relative intensities of the ion pairs m/z 379 and 380 of sugar tetritol trifluoroacetate. By gas chromatography/mass spectrometry, we observed that d ‐threose 4‐phosphate was also converted into d ‐erythrulose 4‐phosphate and d ‐erythrose 4‐phosphate. At the equilibrium, about 90 % of the tetrose 4‐phosphate existed in the form of d ‐erythrulose 4‐phosphate. On the basis of gas chromatography/mass spectrometric evidence together with gas chromatographic and thin‐layer chromatographic patterns, it is suggested that the single enzyme of the beef liver catalyzed both reactions of isomerization and epimerization of aldotetrose 4‐phosphate.