Covalent Addition of H2O, Enzyme Substrates and Activators to Pyrrolo‐quinoline Quinone, the Coenzyme of Quinoproteins

Abstract

The fluorescence excitation and the absorption spectrum of 2,7,9‐tricarboxy‐1H‐pyrrolo [2,3‐f]quinoline‐ 4,s‐dione (pyrrolo‐quinoline quinone, PQQ), measured in the pH range 7.0‐ 10.0, are quite different. However. when the temperature of the solution is lowered, the shape and maxima of these spectra become more similar. 1H‐NMR in 2H2/0 revealed a temperature‐dependent equilibrium between PQQ and a hydrated form. Evidence is presented that low temperature favours the formation of the fluorescing species which is PQQ, hydrated at the C‐5 position. Even further hydration is possible since absorption, fluorescence and NMR spectroscopy of PQQ in borate buffer pH 10.0 reveal additional hydration at the C‐4 position, pointing to a dihydrate. PQQ also reacts with quinoprotein enzyme substrates and activators. Spectroscopic measurements showed the existence of 5‐alkoxy‐5‐hydroxy‐PQQ and 5‐amino‐5‐hydroxy‐PQQ in the presence of alcohols and 2 M NH4CI, pH 9.0, respectively. In the latter case, the existence of 5‐imino‐PQQ could also be demonstrated. Addition compounds with amines appear to be unstable. The amines become probably oxidized because pyrrolo‐quinoline quinol (PQQH2) was found as the reaction product. On the other hand, an addition compound containing an imino bond could be isolated after addition of urea to a PQQ solution. Spectral characteristics of PQQ and its addition compounds are presented since these data are necessary for the spectral analysis of quinoproteins and the quantitative estimation of coenzyme. Copyright © 1982, Wiley Blackwell. All rights reserved