# 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? Biochemists clearly demonstrated that fluoride is an enzyme disruptor. Tables 1 and 2 show that fluoride both inhibits and activates a wide range of isolated enzymes from various tissues in vitro. The introduction of CWF evoked a question of the safety of the consumption of fluoride, which is an enzyme poison. In 2006, a National Research Council (NRC) report stated that the available data were not sufficient to draw any conclusions about potential effects or risks to liver enzymes from low-level long-term fluoride exposures such as those seen with CWF. The concentrations of fluoride used in laboratory studies are several times greater than the concentration present in the body [27]. In agreement with this report, the American Dental Association (ADA) concluded that the recommended levels of fluoride in drinking water do not affect human enzyme activity in a living organism [28]. Such arguments have recently appeared in the comprehensive review of [PERSON] et al. [29]. These authors stated that most of the effects of fluoride remain to be established and many of the findings from in vitro studies were only observed in the millimolar range. The in vivo relevance of such concentrations in humans is questionable since fluoride plasma concentrations in healthy adults generally range between 0.4 and 3.0 \(\upmu\)M [29]. The findings of other studies found that plasma fluoride concentrations resulting from long-term ingestion of 1-10 ppm fluoride in drinking water range between 1-10 \(\upmu\)M. Fluoride plasma concentrations in persons in endemic areas may reach levels of 7.37-39.5 \(\upmu\)M [30]. The review of laboratory studies on the effects of fluoride on enzyme activities reveals that competing reactions might produce anomalous \"paradoxical\" effects. This means that the inhibitory or stimulatory impact of fluoride can differ with its concentration, i.e., inhibitory impact could be greater at a lower than at a higher concentration [31]. For example, [PERSON] et al. [32] found that the activity of lactate dehydrogenase in ram semen displayed a nine-fold decrease with 20-200 \(\upmu\)M fluoride, but at the concentration of millimolar fluoride, its activity is nearly 40% above that of the control. A stimulatory or beneficial effect of a subinhibitory concentration of a toxic substance is known as hormesis [33]. The existence of the paradoxical dose-responses further contributes to the various pattern of fluoride effects on enzymes both in vitro and in vivo. \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}{**REFEENCES**} \\ \hline \multirow{3}{*}{**acid phosphatase**} & ram semen & 20–200 \(\upmu\)M & [32] \\  & osteoblasts & mM & [34] \\  & osteoclasts & mM & [35; 36] \\  & kidney & 10 \(\upmu\)M & [37] \\ \hline \hline
**aconitase** & liver & mM & [38] \\ \hline \multirow{3}{*}{**adenylyl cyclase**} & liver & 5–10 mM & [39] \\  & fibroblasts & 5 mM & [40] \\ \hline \multirow{3}{*}{**acetylcholinesteraseterase**} & red blood cells & 0.01–10 mM & [41] \\  & brain & 5–50 mM & [42] \\ \hline
**amylase** & human saliva & 50–500 mM & [43] \\ \hline
**arginine** & liver, kidney & \textgreater{}4 mM & [44] \\ \hline
**Ca-ATPase** & sarcoplasmic reticulum & 5–10 mM & [26] \\ \hline \multirow{3}{*}{**catalase**} & red blood cells & \(\upmu\)M & [45] \\  & macrophages & 0.5–05 mM & [46] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Inhibitory effect of fluorides on enzymes in vitro.