Fig. 3.

Proposed pathway of the pyomelaninlike AKU pigment based on the formation of benzoquinone enzymatically or by auto-oxidation and then subsequent dimerisation as well as alternative metabolic routes to, HGA mercaptouric acid and HGA glucuronide and subsequent protein binding. NB the relative proportions of these processes are not known (Williams et al. 2012; Zannoni et al. 1969). NB the enzyme “polyphenol oxidase” cited in the figure is reported in protein database as the enzyme EC:1.10.3.1, catalysing the oxidation of ortho- but NOT para-diphenols. Polyphenol oxidases are enzymes generally found in plants but not in humans, as this enzyme does not exist in the sequenced human genome. An enzyme able to oxidise p-diphenols is the “laccase” (EC:1.10.3.2), acting both on o-quinols and p-quinols and often acting also on aminophenols and phenylenediamine