Human alpha-1-Microglobulin is Covalently Bound to Kynurenine Derived Chromophores

Alpha-1-Microglobulin carries a set of covalently linked
chromophores that give it a peculiar yellow-brown
color, fluorescence properties, and both charge and size
heterogeneity. In this report it is shown that these features
are due to the adducts with the tryptophan metabolite,
3-hydroxykynurenine, and its autoxidation products
and that the modification is more pronounced in
the protein isolated from urine of hemodialyzed patients.
The light yellow amniotic fluid alpha -1-microglobulin
acquires the optical properties and charge heterogeneity
of the urinary counterpart following incubation with
kynurenines. The colored amino acid adducts of urinary
and amniotic fluid alpha -1-microglobulins were separated
by chromatography after acid hydrolysis and analyzed
by mass spectrometry. Human serum albumin samples,
native and treated with 3-hydroxykynurenine in the
presence of oxygen, were used as a control. The retention
times and mass fragmentation products were compared,
and a lysyl adduct with hydroxantommathin was
identified in the urinary alpha -1-microglobulin and in the
modified albumin samples. The more extensive modification
of the urinary protein appears to be correlated
with uremia, a condition in which the catabolism of
tryptophan via the kynurenine pathway is increased, and
the consequent rise in the concentration of its derivatives is
accompanied by the oxidative processes due to the hemodialysis
treatment. The oxidative derivatives of 3-hydroxykynurenine,
which are known to act as protein crosslinking
agents, are the likely cause of the propensity of
urinary alpha -1-microglobulin to form dimers and oligomers.
This process, as well as the redox properties of these metabolites,
may contribute to the toxic effects of the kynurenine
species.