However, the cells treated with 13C-ribitol showed clearly elevated levels of 13C-ribitol-5P and CDP-13C-ribitol, as well as 13C-ribitol, but only background levels of endogenous analogs (ribitol, ribitol-5P, and CDP-ribitol) as detected in the untreated cells (Supplementary Figure 2b).
Unfortunately, specific detection of the administered miR-143 and miR-145 in mouse intestine failed because the background levels of endogenous miR-143 and miR-145 after periodate oxidation were too high.
There is now extensive evidence that chemicals classified as estrogens can have markedly different activities at different times in life, in relation to different background levels of endogenous estradiol, and different activities in different tissues.
We also assumed a background level of total metabolites Y0 (micromolar) due to endogenous and dietary sources of the individual compounds, particularly phenol, hydroquinone, and catechol (Kim et al. 2006a; McDonald et al. 2001).
This result suggests some endogenous pyrophosphatase of E. coli probably contributed to the background level of isopentenols, given the expression of phosphatase didn't exhibit any enhanced production of isopentenols.
However, when the endogenous EcNudF gene of E. coli was knocked out, the background level of isopentenols was still observed.
The background level of constitutive activation of ATM and expression of γH2AX seen in untreated normal or cancer cells reports the ongoing DNA oxidative damage and replication stress induced by endogenous ROS [ 30- 32].
When the endogenous pyrophosphatase genes (EcNudF and yggV) that were capable of enhancing the hydrolyzation of the IPP and DMAPP were knocked out, the background level of isopentenols was still obtained.
Also, removing endogenous background levels of P i from a sample may be required prior to analysis if there is a low signal:noise ratio or minimal treatment effect.
Normal cellular metabolism is well established as the source of endogenous reactive oxygen species which account for the background levels of oxidative DNA damage detected in normal tissue.
