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Toxicological relevance of the counterion “sulfate”

The registrant is of the opinion that the toxicity of strontium sulfate is driven by the strontium moiety and that the sulfate anion does not contribute to the overall toxicity of the substance strontium sulfate to any relevant extent, for the following reasons:

Sulfate anions are abundantly present in the human body in which they play an important role for the ionic balance in body fluids. Sulfate is required for the biosynthesis of 3′-phosphoadenosine-5′-phosphosulfate (PAPS) which in turn is needed for the biosynthesis of many important sulfur-containing compounds, such as chondroitin sulfate and cerebroside sulfate. The Joint FAO/WHO Expert Committee on Food Additives (JECFA) concludes that the few available studies in experimental animals do not raise any concern about the toxicity of the sulphate ion in sodium sulphate. Sodium sulphate is also used clinically as a laxative. In clinical trials in humans using 2-4 single oral doses of up to 4500 mg sodium sulphate decahydrate per person (9000 – 18000 mg per person), only occasional loose stools were reported. These doses correspond to 2700 - 5400 mg sulphate ion per person. High bolus dose intake of sulphate ion may lead to gastrointestinal discomfort in some individuals. No further adverse effects were reported (JECFA 2000, 2002). This position was adopted by the European Food Safety Authority (EFSA 2004) without alteration.

Based on the above information, one can therefore safely assume that the sulfate anion in strontium sulfate does not contribute to the overall toxicity of strontium, sulfate. It is concluded that only the effect of “strontium” is further considered in the human health hazard assessment of strontium sulfate.

Read across from SrCl2 to SrSO4:

The toxicity of strontium substances such as strontium sulfate can reasonably assumed to be determined by the availability of strontium ions in solution. As a first surrogate for bioavailability, the water solubility of a test substance may be used. Strontium chloride is highly water soluble with ~538 g/L at pH ~ 7, whereas strontium sulfate is moderately soluble (~125 mg/L at pH ~ 6.5). Hence, any read across from strontium chloride to strontium sulfate is inherently very conservative.

Evidence for neurological activity of strontium was obtained from a number of in vitro investigations. In a calcium-free medium, strontium ions weakly supported the generation of excitatory postsysnaptic potentials following stimulation of guinea pig cervical ganglia (i.e., the release of acetylcholine was less efficient than when calcium was present) (s_McLachlan_1977). Incubation of vasa deferentia isolated from guinea pigs showed that both strontium and barium cations can substitute for calcium ions in the release of noradrenaline at adrenergic nerve terminals.


It was concluded that all these cations act through the same site at some stage in the process of potassium induced transmitter release (s_Nakazato_1980).Perfusion through acutely denerved cats´ adrenal glandsby strontium in calcium-free Locke´s solution resulted in an intense catecholamine secretion and restored the response to acetylcholine and excess potassium (s_Douglas_1964). Findings that the stimulating effect of strontium also persisted in glands perfused with EDTA led to the conclusion that strontium can substitute for calcium in the secretory process without liberation of endogeneous calcium from some intracellular binding sites. The sequestration of different divalent cations including strontium was examined, in comparison to calcium, by mitochondria and smooth endoplasmic reticulum from isolated presynaptic nerve terminals (s_Rasgado-Flores_1987). Strontium cations were less effective (by about 50 to 60%) than calcium ions.

Justification for classification or non-classification