Linoleic acid is an essential fatty acid that plays a vital role in skin health. It is a precursor to ceramides, which are lipids that are essential for the formation of the stratum corneum, the outermost layer of the skin. The stratum corneum acts as a barrier to protect the skin from environmental stressors and prevent trans-epidermal water loss (TEWL). The lipids that constitute the extracellular matrix of the stratum corneum have a unique composition and are 50% ceramides, 25% cholesterol, and 15% free fatty acids(1). A deficiency of Linoleic acid can lead to not only insufficient Ceramide production(2), but also to all the other symptoms that are associated with essential fatty acid (EFA) deficiency which include reduced barrier function and other skin ailments(3–7). The following text outlines the mechanism through which linoleic acid when applied to the skin in the source of a plant oil is metabolized through the actions of enzymes present in the epidermis to the form of a free fatty acid. Being in the form of a free fatty acid it is then available to be further processed to longer chain fatty acids that have biological functions in themselves or can be used as building blocks for ceramides.
Fatty Acids and Triglycerides
A fatty acid consists of a chain of carbon atoms, to which hydrogen atoms are bonded along the length of the chain and at one end of the chain a carboxyl group is found. It is that carboxyl group that makes it an acid. It is from the carboxyl group that this class of acids gets it name… carboxylic acid.
Below in Fig. 2 is a picture of three fatty acids, linoleic acid, alpha linolenic acid and gamma linolenic acid. In this picture the 18 carbon atom chain can be seen with double bonds along its length at various positions and a carboxyl group to the right end of the chain. (8)
In plant oils the fatty acids are bonded with a glycerin molecule to form a Triglyceride. A triglyceride is three fatty acids that are bonded to a glycerin molecule.
It is in the form of a Triglyceride that the fatty acids are introduced to the epidermis in the topical application of a cream that has plant oils as ingredients.
Metabolism of plant oils in the stratum corneum
In order for the cells of the epidermis to be able to use Linoleic acid and other fatty acids as building blocks for their metabolites the triglyceride must be broken down into its basic components of fatty acids and glycerin. The process by which a triglyceride is broken downinto its component fatty acids is called lipolysis. Lipolysis is a multi-step process that is catalyzed by a number of enzymes. The lipolytic cleavage of TGs occurs in essentially all cells and tissues of the body. In all of them, the resulting fatty acids are utilized endogenously for energy production or biosynthetic pathways(10).
The first step in lipolysis is the removal of one fatty acid from the triglyceride molecule. This is done by the enzyme Adipose Triglyceride Lipase (ATGL). The results of this action is a diacylglyceride molecule and a free fatty acid.
The second step in lipolysis is the removal of another fatty acid from the diacylglycerol molecule. This is done by the enzyme Hormone-Sensitive Lipase (HSL). The results of this reaction is a monoacylglycerol molecule and a second free fatty acid.
The third and final step in lipolysis is the removal of the last fatty acid from the monoglyceride molecule. This is done by the enzyme Monoglyceride Lipase (MGL). The results of this reaction is a glycerol molecule and a third free fatty acid.
All three of the enzymes that catalyze lipolysis are present in the epidermis. Lipolysis is therefore able to occur in the epidermis, which allows the fatty acids that are released from triglycerides to be used by the epidermal cells.
Free fatty acids from triglycerides as building blocks for further Metabolites
Both linoleic acid and alpha linolenic acid are essential fatty acids meaning that the body can not make them itself and they need to be taken in from outside sources (11,12). The free fatty acids can be metabolized in the endoplasmic reticulum of cells into longer chain fatty acids by the enzymes present there (13–15). The metabolites that result either have a biological function in themselves or can be used as building blocks for even more complex molecules such as Sphingolipids and ceramides (16–19).
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