Determination of PQQ in quinoproteins with covalently bound cofactor and in PQQ‐derivatives

Abstract

Application of the so‐called hexanol extraction procedure for PQQ determination, originally based on detachment of the cofactor from quinoproteins and conversion into PQQ‐5,5‐dihexyl ketal, leads in several cases to a number of products due to uncontrollable esterification. The present modified procedure, detaching the covalently bound cofactor and converting it into 4‐hydroxy‐5‐hexoxy‐pyrroloquinoline, was tested on a number of proteins. Only the expected product was obtained for the known quinoproteins, in a quantitative yield, as revealed by comparison with the values determined with the hydrazine method. Thus this independent method confirmed that bovine serum amine oxidase, porcine kidney diamine oxidase, dopamine β‐hydroxylase from bovine adrenal medulla, methylamine dehydrogenase from Thiobacillus inversutus, glutamate decarboxylase from Escherichia coli, and 3,4‐dihydroxyphenylalanine decarboxylase from pig kidney are really quinoproteins. Quantitative conversion was also achieved for condensation and addition products of PQQ (PQQ‐acetone, PQQH₂, PQQ‐oxazole, PQQ‐dinitrophenylhydrazone, and PQQ‐tryptophan). In view of this conversion and the fact that catalytic activity of PQQ is not required, the method seems suited to investigate the distribution of the cofactor in eukaryotes, especially in mammals where it is almost certain that PQQ occurs only in derivatized form. Finally, just like the hydrazine method, the hexanol extraction procedure seems unable to keep the structure of the cofactor as it exists in the active site, intact, as demonstrated for the pro‐PQQ cofactor of methylamine dehydrogenase.