Factors Relevant in the Reaction of PQQ with Amino Acids

Abstract

In order to reveal the stability of PQQ in complex samples, its reaction on incubation with amino acids was followed spectrophotometrically, by monitoring oxygen consumption, and with a biological assay. For several aamino acids, the formation of a yellow coloured compound (λ_max = 420 nm) was accompanied by oxygen uptake and disappearance of biological activity from the reaction mixture. The yellow product appeared to be an oxazole of PQQ, the exact structure depending on the amino acid used. Besides the condensation reaction, there is also a catalytic cycle in which an aldimine adduct of PQQ and the amino acid is converted into the aminophenol form of the cofactor and an aldehyde resulting from oxidative decarboxylation of the amino acid. Addition of NH ⁺₄ -salts, and sometimes as well as that of certain divalent cations greatly stimulated both reactions. With basic amino acids, oxazole formation scarcely occurred. However, as oxygen consumption was observed (provided that certain divalent cations were present), conversion of these compounds took place. Oxazole formation also occurred under anaerobic conditions with concomitant formation of PQQH₂, suggesting that PQQ is able to oxidize the presumed oxazoline to an oxazole. A reaction scheme is proposed for the aerobic oxidation of glycine with PQQ. Although acid hydrolysis of PQQ-oxazoles was feasible, hydrolysis in the presence of amino acids did not lead to PQQ, since tryptophan appeared to have a deleterious effect on the cofactor under the conditions of acid hydrolysis. The results here described explain therefore why analysis methods for free PQQ can fail in case the samples contain amino acids.