In our experiments no changes were observed in the nitrogen reducing pathway when nitrate was replaced by nitrite as electron acceptor.
Specifically, pathway improvements were identified, including reducing pathway enzyme expression levels and overexpressing nucleotide sugar biosynthesis genes, which enhanced production of lipid-linked Man3GlcNAc2 by nearly 50-fold to 13.9 μg/L.
This implies that other competition affecting factors, such as pH, sulfide concentrations, or the type and complexity of the electron donor, may have a decisive effect on the nitrate reducing pathway that dominates, rather than, as suggested by Kraft et al. (2014), via direct control of either nitrite or nitrate as electron acceptor.
Typically, activation of critical pathways is accompanied by subsequent activation of negative feedback loops reducing pathway activity.
Metabolic control analysis, indeed, suggests that it is just such a coordinated downregulation of expression of many enzymes in a pathway that is necessary for reducing pathway flux.
Increasing pathway activity stimulates quiescent HSCs to become myofibroblasts, whereas the reducing pathway activity in myofibroblasts restores the quiescent phenotype (23).
Consistent with our hierarchical clustering analysis, introduction of the R164Q mutation led to relatively few changes in gene expression or showed reduced pathway activation compared with the other mutants studied.
We have shown previously that a reducing pathway can be re-established by addition of GSH to SP cells resulting in re-equilibration of the PrxIV redox status to reflect that seen for intact cells (Tavender and Bulleid, 2010b).
Hence, during the preparation of semipermeabilized (SP) cells, factors required for maintaining reducing pathways within the ER are removed.
