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Short description of key information on bioaccumulation potential result: 
Doses of up to 5 g phosphorous/animal/day in the form of 4 different sources (disodium hydrogen phosphate, tetrasodium pyrophosphate, sodium tripolyphosphate or Graham salt) resulted in an equilibrated balance and were well tolerated. With longer P chains the intestinal resorption appears to decrease. No guideline studies are available and therefore discussion of the toxicokinetics is based on all available data.

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential

Additional information

TOXICOKINETIC ASSESSMENT

TEST MATERIAL: Group 2i: Diphosphates (with Sodium and potassium cation)

 

Group 2i, diphosphates also known as pyrophosphates. The diphosphate/pyrophosphate ion is the simplest form of a condensed phosphate group. A condensed phosphate anion has one or several P-O-P bonds. As the group contains only two phosphate groups, both of the phosphorus ions are classified as “terminal phosphorus”. The diphosphate can undergo ionisation with loss of H+ from each of the two –OH groups on each P and therefore can occur in the -1, -2 -3 or -4 state. The degree of ionisation is dependent upon the associated cations and the ambient pH (if in solution).

 

The ionic form of the Group 2i alkali metals sodium and potassium is M1+and they are ubiquitous and essential to all known living organisms.

 

The Inorganic diphosphates were grouped based on the structure of the phosphate ions firstly, then grouped accordingly to their cation identity. Group 2i includes the following substances: Sodium acid pyrophosphate (SAPP, Na2H2P2O7, MW 221.9 g/mol), tetrasodium pyrophosphate (TSPP, Na4P2O7, MW 265.9 g/mol), tetrapotassium pyrophosphate (TKPP, K4P2O7, MW 330.3 g/mol) and trisodium diphosphate (TriSPP, Na3HP2O7, 243.9 g/mol)

 

No partition coefficient value was determined for Group 2i Substances as they are inorganic diphosphates that are highly ionic (depending on ambient pH). Because of this ionic nature the passive passage across biological membranes will be negligible. However as sodium and potassium are key elements in various cellular processes their import and export over cell membranes is regulated via pore systems and usually tightly regulated. Diphosphate is an anion that occurs in all living cells and is formed mainly by the synthesis of DNA from Nucleotide triphosphates (DNAn + Deoxyribonucleotide triphosphate → DNAn+1 + diphosphate). Usually it is cleaved into two orthophosphate molecules by one of the different members of the alkaline phosphatase family which are present in all tissues. Diphosphate nevertheless is generally relatively stable against uncatalyzed hydrolysis (half-life = 10 d in autoclaved Flat branch sediment (Blanchar RW and Riego DC, 1975, Tripolyphosphate and diphosphate hydrolysis in sediments, Soil sci. soc. Am. J 40: 225-229)).

As the substances are of ionic nature and dissociate readily into the cations and anions in water, they will subsequently be discussed as separate substances under aqueous conditions.

 

Diphosphates are registered as food additives under the No. E 450 and are used in the food chemistry mainly as emulsifiers but also as parting agent, baking agent preservative agent and anti-oxidising agent. It is used also as carrier for pharmaceuticals.

 

Absorption

 

General:

The cations Sodium and potassium are essential for cellular life; therefore regulated uptake into cells will take place via the typical cellular uptake mechanisms specific for the respective ions. As a tightly regulated equilibrium of these ions is crucial for the functioning of normal cells (co-transport of specific substance across cell membranes, neuronal signal transduction, regulation of water uptake in the intestine and formation of mucus in the lungs and sweat and generally the formation of the membrane potential of all living cells) the amount of uptake is generally tightly regulated. Diphosphate on the one hand is negatively charged hence a passive diffusion across cell membranes is not possible. A pyrophosphate specific export protein is described for certain cells in kidneys and joints (see below).

Oral / intestinal absorption:

- Cations: Sodium ion fluxes in the intestine are complex and the key mechanism for the uptake of different substances like glucose, chorine or orthophosphate via specific co-transport trans-membrane proteins from the intestinal lumen into the brush-border epithelial cells. Na+/K+ ATPase then pumps sodium ions out again into the lumen while importing potassium ions under consumption of energy in form of ATP thereby keeping up a steep electrochemical gradient. In addition passive transport of Sodium occurs largely through tight junctions and the lateral spaces and is paracellular. Pinocytosis and other vesicle transport systems can also have an influence on sodium homeostasis and fluxes.

Transfer of Sodium and potassium to the blood circulation system and homeostasis of these ions in other tissues are well regulated and similarly complex as the above stated uptake mechanisms and are broadly described in the general biochemical and medical literature.

Recommended daily intakes of Sodium are 1500 mg/d/person per day and of Potassium 4700 mg/d/person for young adults (Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. www.nap.edu. The,).

- Diphosphate: As stated above diphosphate is rapidly transferred into orthophosphate by intestinal alkaline phosphatase. So the majority of diphosphate is probably absorbed as orthophosphate. In addition direct uptake of diphosphate via diffusion or pinocytosis might add to the total uptake. Recent publications report that specific transmembranal transport proteins exist for diphosphate (Am J Hum Genet. 2002 Oct;71(4):985-91. Epub 2002 Sep 17. Autosomal dominant familial calcium diphosphate dihydrate deposition disease is caused by mutation in the transmembrane protein ANKH.) But whether comparable proteins are also involved in intestinal uptake of diphosphate is not clear at the moment.

The bioavailability of orthophosphate from diphosphate has also been shown by Feldheim et Al, 1985 (Feldheim W and Hesselbach C, 1985, Untersuchungen zum Calcium- und Phosphorstoffwechsel beim Mensche/Studies on Calcium and Phosphorous Metabolism in Man. 3rd Communication: Absorption Behaviour in Phosphates of different Chain Length in Miniature Pigs, Aktuelle Ernährungsmedizin 10(1); 30 - 33, 1985, Institut für Humanernährung der Universität Kiel). In this study supplementation of a basic diet with 1 – 3 g of either ortho- or diphosphate led to comparable uptake and excretion of orthophosphate.

It is therefore considered appropriate to take into consideration an absorption rateof 50%.

 

Respiratory tract:

Absorption via inhalation of the substance is possible as particles at the size of < 10 µm are respirable and at the size of < 4 µm are able to reach the alveoli.

Based on the general considerations above, an uptake of the cations sodium and potassium is likely, though the amount is expected to be low as compared to oral uptake. Absorption of diphosphate is unlikely via the inhalation route.

Non-resorbed particles in the oral cavity, the thorax and the lungs will be transferred to the gastro-intestinal tract with the mucus and absorbed there. Therefore absorption from the gastrointestinal tract will contribute to the total systemic burden of the substance that is inhaled.

 

Dermal:

Sodium and potassium ions can penetrate the skin to some extent but the absorption is much lower than via the oral route. The diphosphate is, depending on the pH, highly to very highly ionised which reduces drastically the potential to penetrate the lipid rich environment of the striatum corneum. Therefore dermal uptake of diphosphate will probably be minimal.

 

 

Distribution

 

Sodium and potassium are natural components of blood as free ions and their distribution and circulations is as precisely regulated as their uptake. Diphosphate is not expected to the blood circulation but will be cleaved either by extracellular or intracellular alkaline phosphatases after uptake in the intestine (or in the lung). Nevertheless recent research revealed a transporter protein responsible for export of diphosphate from cell lumen to the extracellular matrix in joints and in kidneys probably in order to complex free calcium ions thereby inhibiting crystallisation of calcium apatite (growth regulation of bones) and calcium phosphate / calcium oxalate (inhibition of kidney stone formation) respectively (Cell Physiol Biochem. 2009;24(5-6):595-604. Epub 2009 Nov 4. The diphosphate transporter ANKH is expressed in kidney and bone cells and co-localises to the primary cilium/basal body complex.) Whether diphosphate is produced exclusively by pyrophosphate excreting cells or whether diphosphate is actively transported to these cells is not clear to date.

 

 

Metabolism

 

All three ions (Sodium, potassium and diphosphate) are inorganic and stable to reduction or oxidation in biological systems. Diphosphate is hydrolysed to orthophosphate by ubiquitous alkaline phosphate activity (different iso-enzymes in different tissues). Orthophosphate then takes part in various physiological processes including formation of Deoxyribonucleotide phosphates (e.g. AMP, cAMP, ADT, ATP).

 

 

Excretion

 

Assuming homeostasis of Sodium and potassium as indispensable nutrients in a healthy organism the same amount of the ions is excreted as taken up. Sodium, potassium and phosphate formed from diphosphate are generally excreted via kidneys but also via faeces and sweat (varying for the specific ion).

As stated above diphosphate is also excreted via specialized cell in the kidneys into the urine, probably in order to inhibit kidney stone formation from high urinary calcium concentrations. O’Brien et al reports a dose dependent rise of pyrophosphate excretion after feeding healthy and kidney stone forming human volunteers with defined diets that provided 1.5, 3.0 or 4.5 g/d/person orthophosphate in three successive weeks. Pyrophosphate excretion was comparable in the two groups and ranged from 3.5 - 13 mg/24 h in the 1.5 g diet phase to 15 – 40 mg/24 h in the 4.5 g diet phase (Urinary pyrophosphate in normal subjects and in stone formers. O'Brien MM, Uhlemann I, McIntosh HW. Can Med Assoc J. 1967 Jan 14;96(2):100-3.).

 

 

Toxicity data

 

The above stated assumptions are supported by the findings in the toxicological studies. Oral toxicity is generally around 2000 mg/kg bw, but mortality occurred at sufficiently high doses. Acute dermal toxicity was not found for any of the three substances tested (SAPP, TAPP and TKPP). All animals survived doses up to 7.96 g/kg bw of the respective diphosphate. This underlines the low potential of the three diphosphates to penetrate the skin. The skin irritation found for all four substances is probably caused by their basic nature and their high buffer capacity. The acute inhalation toxicity is difficult to assess as the nominal concentrations (which were the highest attainable) differ significantly from the gravimetrically derived values (35.14 mg/L vs. 0.58 ± 0.103 mg/L for SAPP, 28.8 mg/L vs. 1.10 ± 0.054 mg/L for TKPP). At these highest attainable concentrations 2/10 (SAPP) and 1/10 animals (TKPP) died.

The available repeated dose studies confirm that the kidneys are the primary target organ of subchronic oral toxicity of diphosphates. Both SAPP and TKPP induced tubulorrhexis and medullar and cortical calcification to different degrees in rats if administered sub-chronically at high concentrations of 1 – 10% in the feed. (Diphosphates might have a Janus-faced role in this process leading on the one hand to an increased phosphate burden if cleaved and taken up as orthophosphate but on the other hand might help to inhibit calcification by sequestering calcium ions.)