Fatty acids, C16-18, sodium salts

Endpoint summary

Additional information

In the study of Howes et al., the percutaneous absorption of some [14C] labelled anionic surfactants has been measured in vivo in rats, after both consumer-type applications and applications of longer duration, and the results have been compared with those from in vitro studies using isolated rat skin and human epidermis.

In vitro penetration through rat skin:

Rat skin was excised and mounted in 2.5 cm diameter penetration cells. 0.25 ml of the [14C] surfactant solution was pipetted onto the epidermal surface of the skin and 10.0 ml of saline was added to the sampling compartment against the dermis. The cells were kept in a warm room at 37°C throughout the experiment for 24 hours.

In vitro penetration through human epidermis:

The human epidermal samples were mounted in 1 cm diameter penetration cells. 0.1 ml of the [14C] surfactant solution was placed on the corneum. 1.0 ml aliquots of the saline in the sampling compartment (8.0ml) were monitored for 14C at 0.5, 1, 2, 3, 4, 6, 7, 8, 24 and 48 h, each time 1.0 ml fresh saline was added to maintained the volume at 8.0ml.

Turnover of surfactants:

The turnover of each [14C] labeled surfactant was measured by injecting 3 animals intraperitoneally and 3 animals subcutaneously with 0.1 or 0.5 ml of surfactant solution. The animals were then placed in sealed metabolism cages for 6 or 24 h, where urine, faces and expired air were collected and monitored for 14C.

Percutaneous absorption:

Topical application of 0.1 or 0.5 ml of the [14C] test solution was made from a microlitre syringe on to an area of skin (7.5 or 10 cm²) previously marked out on the animal’s back with a felt-tipped pen. After 15 min contact with the skin the animal was inverted over a 6-inch diameter funnel and the excess of the test solution was rinsed off with distilled water at 37°C. The animals were then fitted with either restraining collars or non-occlusive protection patches and placed in metabolism cages for collection of excreta as described above.

Results:

The results from the excised rat skin experiments showed penetration of the shorter chain length soaps, where the permeability constants were 2.5-3.9 gcm min -1 for the C10: 0, C12:0 and C14: 0 soaps at 24 h after application, but the penetration of the other surfactants was not measurable. No autoradiographic studies on these skin samples were performed and little can be deduced from these results as to the distribution of the [14C] surfactants in the skin. The observed rate of penetration will depend upon the time required for equilibration of the skin samples in the cell and the interaction between the skin and the surfactant. It is likely that some

penetration occurred through the stratum corneum in most of the samples but, whereas in the in vivo state it would be removed in the peripheral blood supply, in the in vitro state the dermis has to be traversed.

From the rat skin data some deposition of surfactant on the skin surface could be predicted but the amounts of SDS, SDI, DOBS, C18: 0 and C16: 0 soaps penetrating from a 15 min wash and rinse would be very small. The C10: 0, C12:0 and C14: 0 soaps had permeability constants of 3 µ cm/min in vitro so that from a 15 min wash and rinse with a 6 mM solution a penetration of between 0.05 and 0.1 µg/cm would be predicted.

From the human epidermis studies in vitro only small amounts of the C10: 0, C12: 0, C14:0 soaps and the SDI would be likely to penetrate from a 15 min wash and rinse in vivo. The low penetration rates of the C16: 0 and C18: 0 soaps and DOBS and the very long lag time before SDS penetrates suggests that little or none of these would penetrate from a 15 min wash and rinse in vivo. The in vitro penetration through human of the C16:0 soap skin was 0.3 µg/cm²after 24 hours and of the C18:0 soap 0.1 µg/cm². The turnover of the [14C] surfactants in the rat showed that there was no significant difference in the rate or route of excretion of 14C given by intraperitoneal or subcutaneous administration. It was thus thought valid to assume that [14C] surfactant penetrating the skin and entering the blood stream would be excreted at a similar rate. The turnover of the C14: 0, C16: 0 and C18: 0 soaps was slow but for the other [14C] surfactants levels of 14C in the excreta could be used as good indications of percutaneously absorbed material.

Penetration of the [14C] soaps in vivo followed the same order as those obtained with excised human epidermis, i.e. C12: 0>C10:0>C14: 0> C16:0 = C18:0. The actual amounts of soap which penetrated from the 15 rain wash and rinse applications to untreated skins with the 6 mM soap solutions ranged from 0.674 +/- 0.34 µg/cm² for the C12:0 to 0.7 +/- 0.02 µg/cm² for the C18:0. These amounts are considerably higher than those predicted from the in vitro study with excised rat skin. Prewashing the skin with 300 mM model soap solution - approximately 7.5% w/v solution which is similar to that found during consumer use, increased the permeability of the skin, especially for the C10:0 and the C12:0 soaps.

The SDI penetration in vivo was below our limits of accurate measure- ment in this study, i.e. < 0.3 µg/cm² penetrated from a 15 min waash and rinse.

The penetration of the DOBS isomer was below our limits of detection (0.1 µg/cm²) for all experiments.

Thus the in vivo studies show that all of these [14C] surfactants penetrate rat skin with the exception of the [14C] DOBS, the solubility of which was very low. From the in vivo penetration data presented, it can be seen that there is an order of magnitude difference between the most penetrating of the soaps (C12:0 -0.6 µg/cm²) and the least penetrating (C18:0 -0.07 µg/cm²) when applied as 6 mM solutions. The penetration of the synthetic surfactants from 25 mM solutions showed that some 0.25 µg of SDS and 0.15 µg of SDI penetrated per cm² of skin. Thus, provided a linear relationship between the amount penetrating and concentration of these surfactants in the applied solution exists, then the C12:0 soap is about ten times as penetrating as SDS or SDI which penetrate at similar rates to the C18:0 soap.

Autoradiography of the treated skins from the 15 min wash and rinse applications showed deposition of surfactant on the skin surface and in the hair follicles especially at their entrances. This deposition suggests that penetration occurs both transepidermally and via the hair follicles which have been regarded as the main source of penetration for applications of short duration. The presence of 14C in the epidermis and upper dermis at 6 h after application of the C10:0 and C12:0 soaps shows the penetration of these soaps but gives no indication when they penetrated. Penetration may have occurred only during the 15 min washing time but penetration may also have taken place from the labelled soap deposited on the skin surface. The fact that the rate at which 14CO2 was recovered from the animals washed with C12:0 soap was slightly slower than from animals injected with C12:0 soap may be a reflection of the route of administration but is probably due to the fact that penetration occurs from the [14C] soap deposited on the skin.

Because of the short test time of the turnover of [¹⁴C] surfactants in rats of only six hours, a bioaccumulation potential cannot be judged based on this study results. The turnover of the C16:0 and C18:0 soaps were slow. But for a statement of the bioaccumulation potential, the test would have to run longer than 6 hours.

The review of P. Harris et al. summarised the work which they have carried out on the subject "Transport of fatty acids in the heart". Fatty acids are taken up by the myocardium in the unesterified form, either directly from the unbound pool of free fatty acids in the plasma or following release from plasma triglycerides by lipoprotein lipase. They pass extremely rapidly into the cardiac cells, where their distribution is determined by a number of physical and biochemical factors. For the intracellular spaces these factors are essentially physical and depend on gradients of concentration of unbound fatty acids and on the concentration and affinity of protein binding sites. In the cytoplasm of the myocardial cell, myoglobin seems to play a specific carrier role. Passage of fatty acids across cell membranes depends on their chemical forms and, largely, on the degree of solubility of these forms in the lipid layer of the membranes. In addition, there may be specific transport systems, such as is mediated by carnitine in the inner mitochondrial membrane. Finally, the mass movement on fatty acids through the cells depends on the rate at which they are removed by oxidation or esterification. Under normal conditions, these various factors lead to a rapid movement of fatty acids into the mitochondria where they are oxidised and a more gradual esterification into complex lipids.

In the a study of Abumradt et al., fatty acid (FA) transport is characterized with regard to its specificity and susceptibility to inhibition by protein modifiers. The kinetics of competitive inhibition for transport of oleate and stearate are shown under conditions where complications due to competition for binding of FAs to the albumin in the medium are minimized. Stearate inhibits influx of tracer oleate with a K_ithat closely approximates its K_mand, conversely, oleate inhibits similarly the influx of tracer stearate. Specificity of the FA transport system is shown in studies using a variety of natural FAs of different chain length, or FA analogues. Oleate (K_m= 0.06 µM), stearate (K_m= 0.16 µM), linoleate (K_m= 0.22 µM), palmitate, (K_m= 0.2 µM), and laurate (K_m= 1.5 microM) are good substrates, but octanoate is not transported. An oxazolidine ring on C-5 but not on C-16 of stearate blocks binding to the transporter. Methylation of the carboxyl function but not alpha-bromination inhibits transport. These studies suggest that a FA must have a hydrocarbon chain of at least nine carbons and a free carboxyl function to be recognized by the transporter. FA transport does not require Na or ATP. Pronase but not trypsin treatment of intact cells reduces fatty acid influx. Transport is insensitive to maleimides. It is strongly and irreversibly blocked by pretreatment of the cells with the stilbene compounds, 4,4'-diisothiocyanostilbene-2,2'-disulfonate and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, but only slightly inhibited by dipyridamole. Polyacrylamide gel electrophoresis of plasma membrane proteins from cells treated with [3H] 4,4'-diisothiocyanostilbene-2,2'-disulfonate shows a peak of radioactivity at about Mr = 85,000. When cells are incubated in various concentrations of this agent, the counts recovered in the peak reach a maximum coincident with maximum inhibition of transport. They conclude that permeation of the plasma membrane of the adipocyte by long-chain FAs at physiological concentrations is mediated by a protein transporter with distinct specificity requirements.