Potassium (S)-lactate

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics, other

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

supporting study

Justification for type of information:

For justification of read-across please refer to the read-across report attached to IUCLID section 13.

Reason / purpose for cross-reference:

read-across source

Conclusions:

Potassium and chloride are essential constituents and two of the most abundant ions in all animal species. In adult humans, the total body potassium is approx. 3.5 mol (135 g). 98% of this is located intracellularly (150 mmol/L), the extracellular potassium concentration is approx. 4 mmol/L. Overall, potassium concentrations is body fluids are actively regulated by the mammalian body.

Executive summary:

In the published review document OECD SIDS (2001), the following information is provided on toxicokinetics, metabolism, mechanisms of action of potassium chloride:

Potassium and chloride are essential constituents and two of the most abundant ions in all animal species. In adult humans, the total body potassium is approximately 3.5 mol (135 g). 98 % of this amount is located intracellularly (150 mmol/L), the extracellular potassium concentration is approximately 4 mmol/L. Total body chloride in adult humans is approximately 2.1 mol (75 g). 80 % of this is located extracellularly (120 mmol/L), and the the intracellular concentration of chloride is approximately 3-4 mmol/L.

Metabolism, biotransformation and kinetics

About 90 % of the ingested dose of potassium is absorbed by passive diffusion in the membrane of the upper intestine. Potassium is distributed to all tissues where it is the principal intracellular cation. Insulin, acid-base status, aldosterone, and adrenergic activity regulate cellular uptake of potassium. The majority of ingested potassium is excreted in the urine via glomerular filtration. The distal tubules are able to secrete as well as reabsorb potassium, so they are able to produce a net secretion of potassium to achieve homoeostasis in the face of a potassium load due to abnormally high levels of ingested potassium. About 15 % of the total amount of potassium excreted is found in faeces. Chloride leaves the tubular lumen by secondary active transport of sodium, and also passive diffusion. Excretion and retention of potassium is mainly regulated by the main adrenal cortical hormones. Normal homoeostatic mechanisms controlling the serum potassium levels allow a wide range of dietary intake. The renal excretory mechanism is designed for efficient removal of excess potassium, rather for its conservation during deficiency. Even with no intake of potassium, humans lose a minimum of 585–1170 mg K per day. However, the distribution of potassium between the intracellular and the extracellular fluids can markedly affect the serum potassium level without a change in total body potassium.

Mechanisms of action

K+ is the principal cation mediating the osmotic balance of body fluids. In animals, the maintenance of normal cell volume and pressure depends on Na+ and K+ pumping. The K+/Na+ separation has allowed for evolution of reversible transmembrane electrical potentials essential for nerve and muscle action in animals, and both potassium and chloride are important in transmission of nerve impulses to the muscle fibers.

Potassium transport through the hydrophobic interior of a membrane can be facilitated by a number of natural compounds that form lipid-soluble alkali metal cation complexes. Potassium fulfils a critical role as counterion for various carboxylates, phosphates and sulphates, and stabilises macromolecular structures. Potassium and chloride is also important in the regulation of the acid-base balance of the body.

Reference 2

Endpoint:

basic toxicokinetics

Type of information:

other: expert statement based on literature data

Adequacy of study:

supporting study

Justification for type of information:

For details and justification of read-across please refer to the report attached in section 13 of IUCLID.

Reason / purpose for cross-reference:

read-across source

The role of lactic acid in metabolism has kept researchers occupied for a long time. For many years, lactic acid was considered a dead-end waste product of the glycolysis, the conversion of glucose into pyruvate (producing a relatively small amount of ATP), in the absence of oxygen. Recently, the role of lactic acid in metabolism was reconsidered, and L-lactate is considered as a functional metabolite and mammalian fuel. It was observed that lactate can be transferred from its site of production (cytosol) to neighbouring cells and other organs, as well as intracellularly, where its oxidation or continued metabolism can occur. This "lactate shuttle" results in the distribution of lactic acid to other cells, where it is directly oxidised, re-converted back to pyruvate or glucose, allowing the process of glycolysis to restart and ATP provision maintained.

Conclusions:

Potassium-S-lactate rapidly dissociated into lactate and potassium ions. The metabolism of lactic acid is well understood. In the evaluation of the use of lactic acid as the active substance in biocidal products, the natural occurrence of lactic acid in human food and the human body, as well as the role of the compound in human metabolism and physiology should be taken into account. This means that, when the risk for its use in biocidal products is assessed, the natural exposure to lactic acid in food and via endogenous sources, as well as exposure via the use of lactic acid as a food additive should be considered.
In the present report it is concluded that lactic acid can no longer be considered as a “dead-end” waste product of human metabolism, but should instead be seen to play an important role in cellular, regional, and whole body metabolism. Lactic acid has been detected in blood, several other body fluids and tissues. Concentrations of lactic acid increase significantly during intense exercise. At rest, blood concentrations have been reported of 1-1.5 mMol/L (90.1-135.12 mg/L), which can increase up to 10 mMol/L (900.8 mg/L) during exercise.
External human exposure to lactic acid can occur via its natural presence in food, for example in fruit, vegetables, sour milk products, and fermented products such as sauerkraut, yogurt and beer. Based on the available information on concentrations of lactic acid in some of these products, an estimate of the daily consumption of lactic acid due to its natural presence in food was made using the ‘FAO/WHO standard European diet’. A (minimum) daily intake of 1.175 g/person/day was calculated using the available information. Another source of external exposure is its use as food additive; as such it is authorized in Europe (E270) and the United States (generally recognized as safe = GRAS). A daily intake of 1.65-2.76 g/person/day was estimated using the “Per Capita times 10” method, based on the amount of lactic acid put onto the market (EU and USA) as a food additive by Purac. Based on the high levels of lactic acid in the human body and in human food, and its use as food additive, the evaluation of the human health effects of lactic acid should first and for all be based on a comparison of this background exposure and the potential contribution of lactic acid in biocidal products to these levels. Therefore, a risk assessment should not be based on the comparison with effects of exposure, but on the comparison with the total daily intake of lactic acid via food, both naturally and as food additive, which was estimated to be 2.8 g/person/day. When the application of Purac’s products will not result in a systemic exposure that contributes substantially to the total systemic exposure, many of the standard human toxicological studies dealing with systemic effects are deemed superfluous.

Executive summary:

The natural occurrence of lactic acid in human food and the human body, as well as the role of the compound in human metabolism and physiology is of primary importance in the understanding of the metabolism and toxicology of lactic acid. This means that, in risk assessment, the natural exposure to lactic acid in food and via endogenous sources, as well as exposure via the use of lactic acid as a food additive should be considered.

In the present report it is concluded that lactic acid, unlike believed previously, can no longer be considered as a “dead-end” waste product of human metabolism, but should instead be seen to play an important role in cellular, regional, and whole body metabolism. Lactic acid has been detected in blood, several other body fluids and tissues. Concentrations of lactic acid increase significantly during intense exercise. At rest, blood concentrations have been reported of 1–1.5 mMol/L (90.1–135.12 mg/L), which can increase up to 10 mMol/L (900.8 mg/L) during exercise.

External human exposure to lactic acid can occur via its natural presence in food, for example in fruit, vegetables, sour milk products, and fermented products such as sauerkraut, yoghurt and beer. Based on the available information on concentrations of lactic acid in some of these products, an estimate of the daily consumption of lactic acid due to its natural presence in food was made using the ‘FAO/WHO standard European diet’. A (minimum) daily intake of 1.175 g/person/day was calculated using the available information.

Another source of external exposure is its use as food additive; as such it is authorized in Europe (E270) and the United States (generally recognized as safe = GRAS). A daily intake of 1.65–2.76 g/person/day was estimated using the “Per Capita times 10” method, based on the amount of lactic acid put onto the market (EU and USA) as a food additive.

Based on the high levels of lactic acid in the human body and in human food, and its use as food additive, the evaluation of the human health effects of lactic acid should first and for all be based on a comparison of this background exposure and the potential contribution of lactic acid in biocidal products to these levels. Therefore, a risk assessment should not be based on the comparison with effects of exposure, but on the comparison with the total daily intake of lactic acid via food, both naturally and as food additive, which was estimated to be 2.8 g/person/day.

This information is used in a read-across approach in the assessment of the target substance. For justification of read-across please refer to the read-across report attached to IUCLID section 13.

Reference 3

Endpoint:

basic toxicokinetics in vitro / ex vivo

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

supporting study

Justification for type of information:

For details and justification of read-across please refer to the read-across report attached to IUCLID section 13.

Reason / purpose for cross-reference:

read-across source

Duration and frequency of treatment / exposure:

Single application. Incubation for 5, 10, 20, 40, 60 and 120 minutes.

Preliminary studies:

The protein content of the nasal epithelium homogenate was determined to be 15.3 mg per mL. The increase of absorbance towards p-nitro-phenylbutyrate of the nasal epithelium homogenate using 15.3 µg of protein was 0.191 ± 0.006 (n= 2), corresponding to an esterase activity of 0.71 ± 0.02 µmol per min per mg of epithelial protein. The rate of chemical hydrolysis was investigated with the three smaller lactate esters methyl, ethyl and isopropyl-S-lactate in 0.05 M phosphate buffer pH 7.0 at 37 °C. The lactate esters were incubated at a concentration of 500 µM for 5 and 20 minutes, and 1, 3 and 19.5 hours. The rate of chemical hydrolysis was very low. Liberated L-lactic acid was only detected for methyl lactate and after 19.5 hours (6.1 mol corresponding to 1.2 % of the total amount of ester). Metabolism of L-lactic acid (buffered) was not observed when L-lactic acid was incubated with nasal epithelium homogenate. The use of inhibitors to prevent enzymatic oxidation to pyruvate was therefore not necessary. The recovery of L-lactic acid, as added to the incubation mixture in the absence of esters, was 91-93%. Using a set of enzymatic incubations of ethyl-S-lactate without addition of buffer, the effect on the pH was measured as a function of incubation time. At the last time point (150 min), the amount of protons detected was approximately 20-fold less than the amount of lactic acid produced. An initial lag phase was observed in the detection of protons: an increase in proton concentration was only measured after 60 minutes of incubation. This lag phase cannot be completely explained by the difference between rate of appearance of lactic acid and protons (as a consequence of the buffer present in the nasal epithelium homogenate), since the slope of the two curves after this lag phase still differed considerably.

Type:

metabolism

Results:

The following kinetic parameters of the enzymatic hydrolysis of ethylhexyl-S-lactate by rat olfactory epithelium homogenate: KM: 0.17 mM; Vmax: 420 nmol/min/mg protein.

Metabolites identified:

yes

Details on metabolites:

L-lactic acid is formed by enzymatic hydrolysis of ethylhexyl-S-lactate by carboxylesterase present in rat nasal olfactory tissue.

Kinetic parameters of the enzymatic hydrolysis of ethylhexyl-S-lactate by rat olfactory epithelium homogenate were: Km = 0.17 mM and Vmax = 420 nmol/min/mg protein.

Conclusions:

In an in vitro study to assess the hydrolysis of L-lactate esters by rat nasal olfactory epitheliun homogenate the following observations were made:
Lactic acid is formed by enzymatic hydrolysis of ethylhexyl-S-lactate by carboxylesterase present in rat nasal olfactory tissue. Kinetic parameters of the enzymatic hydrolysis of ethylhexyl-S-lactate by rat olfactory epithelium homogenate were: Km = 0.17 mM and Vmax = 420 nmol/min/mg protein. In general, the olfactory epithelium carboxylesterase showed increasing capacity (increasing Vmax) and affinity (decreasing Km) towards L-lactate esters with increasing molecular weight of the alkyl group. From a large discrepancy between the amount of lactic acid formed and the increase in proton concentration even in very poorly buffered systems it is suggested that a certain defense against acidification exists.

Executive summary:

The hydrolysis of ethylhexyl-S-lactate by rat nasal olfactory epithelium homogenate was investigated in vitro. The ester was incubated with male Wistar rat olfactory epithelium at pH 7.0 and 37 °C. The amount of liberated L-lactic acid (buffered) was quantified at a series of time points, from which the initial rate of hydrolysis was estimated. Using a concentration range of 0.05–0.8 mM, the enzyme kinetic parameters were calculated to be Km = 0.17 mM and Vmax = 420 nmol/min/mg protein. Based on these values, the calculated half-life of the enzymatic hydrolysis of ethylhexyl-S-lactate is 0.0004 min or 0.024 sec.

Seven other lactate esters were also tested. In general, the olfactory epithelium carboxylesterase showed increasing capacity (increasing Vmax) and affinity (decreasing Km) towards L-lactate esters with increasing molecular weight of the alkyl group.

Since the pKa value of lactic acid is 3.80, the formation of lactic acid will (in non-buffered systems) directly result in acidification of the solution. However, even in poorly buffered systems (non-buffered incubation mix) a large discrepancy between the amount of lactic acid formed and the increase in proton concentration is observed. This suggest that a certain defense against acidification exists, and that in vivo, only high doses and/or prolonged exposure will result in acidification of tissues.

Description of key information

In body fluids, dissociation of potassium-S-lactate takes place immediately, resulting in formation of potassium (K+) and L(+)-lactic acid. Lactic acid is a ubiquitous and essential biological molecule in humans and other mammals, but also in most if not all vertebrate and invertebrate animals, as well as in many micro-organisms. Therefore, the biokinetics, metabolism and distribution of lactic acid have to be considered in the context of its normal biochemistry; lactic acid is of minor toxicological concern given its ubiquitousness and function as a common metabolite.

Potassium is an essential constituent and one of the most abundant ions in all animal species. In adult humans, the total body potassium is approx. 3.5 mol (135 g). 98 % of this is located intracellularly (150 mmol/L), the extracellular potassium concentration is approx. 4 mmol/L.

Ethylhexyl-(S)-lactate is a suitable read-across partner to the target substance potassium-S-lactate due the common hydrolysis/dissociation product lactic acid/lactate (Bogaards & van Ommen, 2000). In this study, ethylhexyl-(S)-lactate was incubated with male Wistar rat olfactory epithelium at pH 7.0 and 37 °C. The amount of liberated L-lactic acid (buffered) was quantified at a series of time points, from which the initial rate of hydrolysis was estimated. Using a concentration range of 0.05–0.8 mM, the enzyme kinetic parameters were calculated to be Km = 0.17 mM and Vmax = 420 nmol/min/mg protein. Based on these values, the calculated half-life of the enzymatic hydrolysis of ethylhexyl-S-lactate is 0.0004 min or 0.024 sec. Accordingly, toxicity studies using ethylhexyl-(S)-lactate as test substance actually measure the effects of the hydrolysis products lactic acid and ethylhexanol. Effect levels (e.g. NOAELs) from such studies, extrapolated to lactate, are therefore protective with respect to any effects potentially exerted by lactate/lactic acid.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential