The lumen of the endoplasmic reticulum (ER) is usually considered an oxidative environment;
however, oxidized thiol-disulfides and reduced pyridine nucleotides occur there parallelly,
indicating that the ER lumen lacks components which connect the two systems. Here,
we investigated the luminal presence of the thioredoxin (Trx)/thioredoxin reductase
(TrxR) proteins, capable of linking the protein thiol and pyridine nucleotide pools
in different compartments. It was shown that specific activity of TrxR in the ER is
undetectable, whereas higher activities were measured in the cytoplasm and mitochondria.
None of the Trx/TrxR isoforms were expressed in the ER by Western blot analysis. Co-localization
studies of various isoforms of Trx and TrxR with ER marker Grp94 by immunofluorescent
analysis further confirmed their absence from the lumen. The probability of luminal
localization of each isoform was also predicted to be very low by several in silico
analysis tools. ER-targeted transient transfection of HeLa cells with Trx1 and TrxR1
significantly decreased cell viability and induced apoptotic cell death. In conclusion,
the absence of this electron transfer chain may explain the uncoupling of the redox
systems in the ER lumen, allowing parallel presence of a reduced pyridine nucleotide
and a probably oxidized protein pool necessary for cellular viability.
