SKIN IRRITATION BY ANIONIC SURFACTANTS 63 =15 .. / ptt 8.5 /' //6 //•H 5.0 ß .-' C12 MAP monoTEA (5mM) o -"•" L-cell 0.146 mg/ml / i 6 0 120 180 Incubation Time fmin) Figure 25. Enzymic degradation of C•2MAP monoTEA salt in L-cell homogenate. for 10 min and at 12000 g for 10 min, was incubated at 37øC for 60 min with ! ml of each surfactant solution or ion-exchanged water as control. In order to evaluate the labilizing effect, the activity of lysosomal enzymes released after incubation was assayed according to the method of Shibko (21) for acid phosphatase in the supernatant obtained after centrifugation at 12000 g for 10 min. Figure 24 shows the labilizing effect of MAP on the lysosome fraction compared to C•2AS and C•2-14EO (16). With C•2AS , a release of acid phosphatase beyond control level was first found at 5 x 10-Sg/ml concentration. However, when a concentration of 5 x 10-4g/ml was employed, less acid phosphatase activity than that of control was observed, probably due to its inhibitory action. In the case of C•2-14EO , while the level of acid phosphatase activity released in the supernatant fraction was the same as control at the concentration of 5 x 10-Sg/ml, at 5 x 10-4g/ml the level rose beyond that of control. The labilizing data for various MAP's indicate that the induction of labilization requires a surfactant concentration of more than 5 x 10-4g/mi. These 20 Incubation Hours 5mM Ci2MAPmonoTEA Acetate Buffer pH5.0 Guinea Pig Epidermis Homogenate Figure 26. Enzymic degradation of C•2MAP monoTEA salt in epidermal homogenate of guinea pig skin.

64 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS findings indicate that MAP has a labilizing feature in common with non-ionic surfactants such as C12-14EO , and is different from ionic surfactants of C12AS type. It is possible that when surfactants penetrate epidermis, actual contact concentration of surfactants with native epidermal cells is relatively low (18). At low concentrations less than cmc, there is a significant difference in the biological membrane disrupting effects between MAP and other common anionic surfactants. This fact seems to account for the low irritating effect by MAP. DECOMPOSITION OF MAP BY L-CELL AND GUINEA PIG EPIDERMIS Decomposition characteristics of MAP by L-cell homogenate as revealed by released Pi is shown in Figure 25 (16). Decomposition curves demonstrate that this compound has a tendency to be easily degraded, especially at pH's 8.5 and 5.0, suggesting that alkali or acid phosphatase-like enzymes can act on MAP molecules. Figure 26 also shows enzymic degradation of MAP by guinea pig epidermis homogenates. These results indicate that only 2.8-8.6% of MAP is decomposed during I hr, suggesting that enzymic decomposition is an insignificant factor for the labilizing study, but probably is of considerable importance in skin application where a long time is required for initiation of irritation. The fact that in epidermis acid phosphatase activity is 10 times C12MAP C12AS Concentration Skin-Roughness by Circulation Method 1.0% Closed Patch Test ( Human Skin ) • 1.0-4.0% Irrita- Closed Patch Test • tion ( Guinea Pig Skin ) 0.5-30 % Cumulative Open Patch Test ( Guinea Pig Skin ) • 2.0-4.0% Surface Tension •- 10-3M e Pysico-chemical C M C •- 10-3M Properties Defatting Ability -v- 0.2-1.0% Protein Denaturation BSA Keratin •-- 0.5-1.5% • 0.5-1.5% Enzyme • 0.05-0.1% Membrane Damage( Lysosome Labilization ) *• 0.005-0.05% 5 mM Degradation by naturally occuring .• Enzymes in Epidermis BSA Optical Rotation Test • 0.5-1.5% Adsorption Indigo Carmine Technique • 1.0% Figure 27. Summary of cutaneous, surface active and biological properties in comparison of monolauryl phosphate mono sodium or mono TEA salt and sodium lauryl sulfate. means that the right surfactant (C•2AS) has a stronger effect in each studied property than the left one (C•2MAP). .- means similar effects between C•2AS and C•2MAP.