Phosphatidylcholine

Looking at the horny layer, which is the barrier against external materials, phospholipids and phosphatidylcholine in particular play a minor role. The lipid bilayers contain only traces of phospholipids, and the main components are free fatty acids, cholesterol, triglycerides, hydrocarbons, and ceramides. But looking deeper into the living part of the epidermis, phosphatidylcholine is usually found as the most important constituent of all biological membranes, especially of plasma cell membranes. Over and above that phos-phatidylcholine is the source of phosphocholine to transform ceramides to sphingomyelins. In this context, phosphatidylcholine stands for living tissues whereas the increase of ceramides in the cells means that their death by apoptosis is soon ahead (Fig. 1).

Human phosphatidylcholine and phosphatidylcholine of vegetable origin show a fatty acid composition, which is dominated by unsaturated fatty acids. The fatty acid content of soy phosphatidylcholine, which is readily available and mostly used in cosmetic formulas, is characterized by a ratio of linoleic acid up to 70% of the total fatty acids. Consequently, soy phosphatidylcholine has a very low phase-transition temperature of below 0°C in water-containing systems. This may be the reason for its ability to fluidize the lipid bilayers of the horny layer, which can be measured by an increase of the transepi-dermal water loss (TEWL) after application for a short while. The slight increase of TEWL

Homoeostasis

Cera mid es - "dealh" Sphingomyelins - "life"

Figure 1 Homoeostasis of epidermal cells.

Cera mid es - "dealh" Sphingomyelins - "life"

Figure 1 Homoeostasis of epidermal cells.

coincides with the penetration of phosphatidylcholine and active agents, which are cofor-mulated with phosphatidylcholine. Because of its high content of linoleic acid and penetration capability, soy phosphatidylcholine delivers linoleic acid very effectively into the skin, and antiacne properties have been shown as a result [5].

By adhering very strongly to surfaces containing proteins like keratin, phosphatidyl-choline shows conditioning and softening effects, which are known from the beginning of skincare products' development. So, e.g., shampoos were formulated in the past very often with egg yolk to soften hair and prevent it from becoming charged with static electricity. Egg yolk is very rich in lecithin. The main compound of egg lecithin is phosphati-dylcholine.

In a given mixture it is not relevant in which form the phosphatidylcholine is incorporated. However, when phosphatidylcholine is formulated, it is practically inevitable that bilayer-containing systems like liposomes will occur, because this is the most natural form of the material. For example, phosphatidylcholine swollen by water transforms spontaneously to liposomes when ''disturbed'' by little amounts of salts or watersoluble organic compounds, like urea. On the other hand, it has been known for a long time that horny layer pretreated by phosphatidylcholine can be penetrated much more easily by nonencapsulated materials. So liposomes are not really needed to turn out the functionalities of phosphati-dylcholine, but they are very convenient because the handling of pure phosphatidylcholine requires a lot of experience and sometimes patience as well.

Because phosphatidylcholine is known as a penetration enhancer, this property is usually associated with liposomes. Liposomes are the vesicles said to transport cosmetic agents better into the horny layer. That is true and, moreover, the conditioning effect causes the horny layer to become a depot for these agents. Measurements of systemically active pharmaceuticals revealed that an increase of penetration is not synonymous with an increase of permeation. Actually, permeation of active agents is often slowed by phos-phatidylcholine in such a way that a high permeation peak in the beginning of the application is prevented. Instead, a more continuous permeation takes place out of the horny layer depot into the living part of the body over a longer period of time. This property makes phosphatidylcholine and liposomes very attractive for the application of vitamins, provitamins, and other substances influencing the regenerating ability of the living epidermis.

CHrO-CCHCHJn-CHs CH-0-C0-(CH2),-CHs

GH2-0-P02-0-CHrQH2-N(CH3)3

Figure 2 Hydrogenated phosphatidylcholine (n = 14,16).

On the other hand, liposomes consisting of unsaturated phosphatidylcholine have to be used with caution in barrier creams because they do not strengthen the natural barrier function of the skin with the exception of its indirect effect of supporting the formation of ceramide I. Ceramide I is known for containing linoleic acid and for being one of the most important barrier-activating substances. Instead of unsaturated phosphatidylcholine, a fully hydrogenated phosphatidylcholine (Fig. 2) should be selected for products designed for skin protection.

Hydrogenated phosphatidylcholine stabilizes the normal TEWL similarly to cera-mides when the horny layer is attacked by hydrophilic or lipophilic chemicals [6]. Table 1 shows a summary of the properties of unsaturated and hydrogenated phosphatidylcholine. Hydrogenated phosphatidylcholine is synonymous with hydrogenated soy phosphatidyl-choline, which contains mainly stearic and palmitic acid, and semisynthetic compounds like dipalmitoylphosphatidylcholine (DPPC) and distearoylphosphatidylcholine (DSPC). Because of their special properties it can make sense to combine unsaturated with saturated phosphatidylcholine in one and the same cosmetic or dermatological product.

Table 1 Properties of Phosphatidylcholines

Hydrogenated soy

Parameter

Soy phosphatidylcholine

phosphatidylcholine

Skin barrier function

Penetration enhancement;

Stabilizing the barrier func

conditioning the horny

tion; conditioning the horny

layer

layer

Barrier compatibility

Yes, slightly enhancing

Yes, stabilizing normal

TEWL

TEWL

Phase transition temperature

Below 0°C

50-60°C

(aqueous system)

Fatty acid composition

Unsaturated fatty acids: pre-

Saturated fatty acids: predomi

dominantly linoleic acid,

nantly stearic and palmitic

oleic acid

acid

Solubility

Soluble in triglycerides, alco

Insoluble in triglycerides, alco-

hols, water (lamellar)

hols, and water

Toxicity

CIR-report [7]; anticome-

CIR-report [7]

dogen

Dispersing ability

Hydrophilic and lipophilic

Hydrophilic and lipophilic

compounds

compounds

Abbreviations: TEWL, transepidermal water loss; CIR, Cosmetic Ingredient Review. S

Abbreviations: TEWL, transepidermal water loss; CIR, Cosmetic Ingredient Review. S

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