Alkaptonuria (AKU) is a rare inborn error of metabolism associated with a deficient activity of
homogentisate 1,2-dioxygenase (HGO), an
enzyme involved in
tyrosine and
phenylalanine metabolism. Such a deficiency leads to the accumulation of
homogentisic acid (HGA) and its oxidized/polymerized products in connective tissues, where
melanin-like pigments accumulate (
ochronosis).
Ochronosis involves especially joints, where an ochronotic
arthropathy develops. Little is known on the molecular mechanisms leading to
ochronosis and ochronotic
arthropathy in AKU. Previous works of ours showed that HGA in vitro propagates oxidative stress through its conversion into
benzoquinone acetate (BQA). We hence used an in vitro model consisting of human serum treated with HGA and evaluated the activities of
glutathione related
anti-oxidant enzymes and levels of compounds indexes of oxidative stress. Proteomics and redox-proteomics were used to identify oxidized
proteins and
proteins more likely able to bind BQA. Overall, we found that the production of ochronotic pigment in HGA-treated serum is accompanied by lipid peroxidation, decreased activity of the
enzyme glutathione peroxidase and massive depletion of
thiol groups, together with increased protein carbonylation and
thiol oxidation. We also found that BQA was likely to bind
carrier proteins and naturally abundant
serum proteins, eventually altering their chemico-physical properties. Concluding, our work points towards a critical importance of
thiol compounds in counteracting HGA- and BQA- mediated stress in AKU, so that future research for disease
biomarkers and pharmacological treatments for AKU and
ochronosis will be more easily addressed.