Validation of an LDH Assay for Assessing Nanoparticle Toxicity
We observed LDH stability in different media. We found good stability of LDH in either HBSS (Hank’s Balanced Salt Solution) or RPMI 1640 cell culture medium containing 1% FBS but not in water (Figure 1).
Therefore, LDH inactivation potentials of these NPs were evaluated in RPMI with 1% FBS, finding that TiO2-25 or Ag-40 NPs did not have significant inactivating potential in cell culture medium with 1% FBS while Cu-40 and Ag-35 were able to inactivate LDH in a dose-dependent manner (Figure 2).
The fact that TiO2-25 NPs do not inactivate LDH can help explain the dose-response curve of the released LDH upon exposure of R3/1 cells to TiO2-25. In such a dose-response curve (Figure 3), the LDH response increases over increasing doses and then decreases beyond a certain dose limit (100 μg/ml).
A comparison between Cu-40 NPs and Cu2+ found similar profiles of LDH inactivation of the two at equal molar concentration of Cu NPs and Cu2+ (Figure 4).
Curve fitting assuming one-phase exponential decay suggested that the half lives of LDH in 20 μg/ml of Cu-40 NPs (4.4% per hr) and Cu2+ (4.2% per hr) were very close. To validate that the LDH inactivating potential of CuSO4 was caused by Cu2+ and not by SO 2+4, a comparison between CuSO4 and Na2SO4 was compared at equimolar concentration of SO42+. Results showed that CuSO4 but not Na2SO4 could inactivate LDH, confirming our assumption that the LDH inactivation potential of CuSO4 was indeed due to the presence of Cu2+ but not SO42+ (Figure 5).
The similar profiles of LDH inactivation of Cu-40 and Cu2+ (Figure 4) seem to suggest that Cu-40 mainly inactivates LDH via the release of copper ions (Cu+ and/or Cu2+). To test this hypothesis, we measured the dissolution rate of Cu-40 in simulated lung fluids and found that Cu-40 has a high rate indeed (Figure 6). In clear contrast, Cu-40 NPs had a half life of about 9-10 hr in the fluid with a pH of 7.4 while less than 0.1% of Ag-40 NPs dissolved 30 hr later. However, we do not know whether the dissolved ions are Cu2+ or Cu+.