# Mechanisms of Fluoride Toxicity: From Enzymes to Underlying Integrative Networks
## 3 Mechanisms of Fluoride Toxicity on Cellular Level
### What Do the Studies of Fluoride Effects on Enzymes In Vitro Tell Us?
\begin{table}
\begin{tabular}{l l l l} \hline \hline \multicolumn{1}{c}{**ENZYME**} & \multicolumn{1}{c}{**SOURCE**} & \multicolumn{1}{c}{**FLUORIDE Concentration**} & \multicolumn{1}{c}{**REFERENCES**} \\ \hline
**cytochrome-c-oxidase** & liver, heart & 0.01–10 mM & [47] \\ \hline  & red blood cells & 1–50 mM & [48; 49] \\
**enolase** & hepatocytes & 3 mM & [50] \\  & embryonic cells & 50 \(\mu\)M & [51] \\  & oral bacteria & 16–54 \(\mu\)M & [52] \\ \hline
**F(1)F(o)-ATP synthase** & mitochondria & mM & [53] \\ \hline
**glucose-6-phosphatase** & liver & \(\mu\)M & [19; 54] \\ \hline
**glutathione peroxidase** & red blood cells & \(\mu\)M & [45] \\ \hline
**glycogen synthase** & hepatocytes & 2–15 mM & [55] \\ \hline  & ram semen & 20–200 \(\mu\)M & [32] \\
**lactate dehydrogenase** & fetal osteoblast & 6–60 \(\mu\)M & [56] \\  & bone marrow & \(<\)0.5 mM & [36] \\ \hline
**lipase** & pancreas, liver & 5 \(\mu\)M–5 mM & [57; 58] \\ \hline
**L-Ca2+channels** & heart & 10 mM & [59] \\ \hline
**Na+/K+ ATPase** & plasma membrane & 1–10 mM & [25] \\ \hline  & liver & 10–50 mM & [60] \\  & bone & \(\mu\)M & [24] \\ \hline
**pyrophosphatase** & yeast & 5 mM & [24; 61] \\ \hline
**pyruvate kinase** & red blood cells & 10–50 mM & [62] \\ \hline
**succinate dehydrogenase** & heart, liver, kidney & 1–15 mM & [63; 64] \\ \hline
**superoxide dismutase** & red blood cells & \(\mu\)M & [45] \\ \hline
**urease** & human & 250 mM & [58; 65] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._ \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**ENZYME**} & \multicolumn{1}{c}{**SOURCE**} & \multicolumn{1}{c}{**FLUORIDE Concentration**} & \multicolumn{1}{c}{**REFERENCES**} \\ \hline
**acid phosphatase** & ram semen & 100 mM & [32] \\ \hline  & heart, liver, brain & 1–10 mM & [47; 66; 67; 68] \\
**adenylyl cyclase** & smooth muscle & 10 mM & [60] \\  & lymphoma cell & 10 mM & [69] \\  & kidney & 5 mM & [70] \\ \hline
**alkaline phosphatase** & bone cells & 10–100 \(\mu\)M & [71] \\ \hline
**aspartate transaminase** & ram semen & 20–200 \(\mu\)M & [32] \\ \hline
**Ca2+-ATPase** & sarcoplasmic reticulum & 1–10 mM & [72] \\ \hline
**glutamyl S-transferase** & ram semen & 20–200 \(\mu\)M & [32] \\ \hline
**K+[ACh]M channel** & heart & \(>\)1 mM & [73] \\ \hline
**K+ATP channel** & heart & mM & [74] \\ \hline
**lactate dehydrogenase** & hepatocytes & 1–30 mM & [50] \\  & ram semen & 100 mM & [32] \\ \hline
**L-type Ca2+ channel** & rabbit femoral artery & 10 mM & [60] \\ \hline
**glycogen phosphorylase** & hepatocytes & 1–50 mM & [39; 75] \\ \hline
**tyrosine kinase** & osteoblasts & 1–10 mM & [76] \\  & & 10–200 \(\mu\)M & [71; 77] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Stimulatory effect of fluorides on enzymes in vitro. Fluoride Effects on Transmembrane Signaling: A Link to Understanding Integrative Networks in Fluoride Toxicity  
Fluoride played an important role as a tool in the understanding of hormone signaling cascades in the second half of the last century. The liver membranes and multi-receptor fat cell systems provided the first insight that adenylyl cyclase (AC) is involved in the transduction of hormonal signals (Tables 1 and 2). [PERSON] and [PERSON] discovered that AC synthesizes the cyclic AMP (cAMP) from ATP. This observation led to the discovery of G proteins and intracellular second messengers' cascades [66; 78]. [PERSON] and [PERSON] were awarded The Nobel Prize in Physiology or Medicine 1994 for their discovery of G-proteins and their role in signal transduction. [PERSON] suggested in his Nobel speech that the system may contain a Mg-ATP complex at the catalytic site and fluoride markedly reduced the concentration of Mg\({}^{2+}\) necessary for stimulation of AC activity (Figure 2) [79].