Selenium binding to human hemoglobin via selenotrisulfide.
ABSTRACT
Selenotrisulfide (e.g., glutathione selenotrisulfide (GSSeSG)) is an important
intermediate in the metabolism of selenite. However, its reactivity with
biological substances such as peptides and proteins in the subsequent metabolism
is still far from clearly understood, because of its chemical instability
under physiological conditions. Penicillamine (Pen) is capable of generating
a chemically stable and isolatable selenotrisulfide, PenSSeSPen. To explore
the metabolic fate of selenite in red blood cells (RBC), we investigated
the reaction of selenotrisulfide with human hemoglobin (Hb) using PenSSeSPen
as a model. PenSSeSPen rapidly reacted with Hb under physiological conditions.
From the analysis of selenium binding using the Langmuir type binding equation,
the apparent binding number of selenium per Hb tetramer almost corresponded
to the number of reactive thiol groups of Hb. The thiol group blockade
of Hb by iodoacetamide treatment completely inhibited the reaction of PenSSeSPen
with Hb. In addition, MALDI-TOF mass spectrometric analysis of the selenium-bound
Hb revealed that PenSSe moiety binds to the ƒÀ subunits of Hb. Overall,
the reaction of PenSSeSPen with Hb appears to involve the thiol exchange
between Pen and the cysteine residues on the ƒÀ subunit of Hb.