Human Hair Lipid

Lipid extracted from human hair is similar in composition to scalp lipid [134]. Thus, the bulk of the extractable lipid in hair is free lipid however, cell membrane complex lipid is also partially removed by extraction of hair with lipid solvents or surfactants. In a sense, the scalp serves as a lipid supply system for the hair, with sebum being produced continuously by the sebaceous glands [135]. Sebum production is controlled hormonally by androgens that increase cell proliferation in the sebaceous glands, and this in turn increases sebum production [135,136], although seasonal and even daily variations in the rate of sebum production do occur [137]. 

Extraction of human hair with fat solvents removes approximately 1 to 9% matter. Ethanol, a solvent that swells hair, removes more lipid from hair than nonswelling solvents like benzene, ether, or chloroform. Hair consists of surface (external) lipid and internal lipid. In addition, part of the internal lipid is free lipid, and part is structural lipid of the cell membrane 

The data (1 to 9% extracted hair lipid) represent total matter extracted from hair clippings of individual men and women. Although the conditions for extraction can influence the amount of matter extracted from hair [138], the values here represent approximate maxima and serve to indicate the variation in the amount of extractable material from hair among individuals. Presumably, the principal material in these extracts is derived from sebum and consists primarily of free fatty acids and neutral fat (esters, waxes, hydrocarbons, and alcohols). Gloor [137] classifies the different components of sebum into six convenient groups free fatty acids (FFA), triglycerides (TG), free cholesterol (C), cholesterol and wax esters (C WE), paraffins (P), and squalene (S). 

Analysis of the FFA extracted from pooled hair clippings of adult males has been carried out by Weitkamp et al. [140]. Their study did not contain data concerning the effect of lipolysis on the structures of FFA found in hair fat. Saturated and unsaturated fatty acids ranging in chain length from 5 to 22 carbon atoms have been found in human hair fat [140,141]. Location of the double bond in the unsaturated acids is suggested to occur at the 6,7 position, with some 8,9 and other isomers. Data from the study by 

Chain length % total FFA % unsaturated FFA of this chain length 

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Composition of Human Hair Lipid 93 Table 2-13. Composition of FFA in human hair lipid. 

With the exception of the Cie and C20 acids, the data in the columns of relative ratio to Cm acids of Table 2-14 are very similar for each corresponding acid. Equivalence suggests that the relative amounts of each acid in ester form would be the same as the relative ratios of the free acids and that hydrolysis may occur on standing (or other conditions) to increase the ratio of FFA to esters [134], The noteworthy exceptions are the saturated acid, which must exist in the ester form to a greater extent than is suggested by the relative ratios of free acids, and the C20 unsaturated acid, which was found only in trace quantities by Gershbein and Metcalf [141], A further conclusion from these studies is that the principal acyl groups present in human hair lipids are from the fatty acids. 

Analysis of some of the neutral material from human hair lipid (e.g., triglycerides, cholesterol or wax esters, and paraffins) provides a mixture as complex as that of the fatty acids [134,139,140,142]. Although not all the compounds of these different components of sebum have been fully analyzed, it is obvious from the discussion on fatty acids and the literature on wax alcohols in human hair lipid [142-145] that the variation in chain length and isomer distribution of all these esters must be extremely complex. 

Nicolaides and Rothman [134] have shown with small sample sizes that hair from blacks contains more lipid than hair from Caucasians. Gershbein and O Neill [142] examined the distribution of fatty alcohols of human hair lipid to determine the relative amounts of fatty alcohols and sterols with regard to sex, race, and scalp condition. Samples originated from Caucasians and blacks, both full head and balding, and from Caucasian women. The data indicated essentially no differences among these parameters between the two racial groups or between the sexes. 

Wertz, P. W. (1997). Integral lipids of hair and stratum corneum, in Formation and Structure of Human Hair (P. Jolles, H. Zahn, and H. Hocker, Eds.). Basel Birkhauser Verlag, 227-237. 

Chemically, human hair contains approximately 85 percent protein, 7 percent water, 3 percent lipid, 4.7 percent protein-bound sulfur (as cystine), and low concentrations of trace minerals (e.g., iron, zinc, copper). The phosphorus content is approximately 80 milligrams per 100 grams (0.003 ounces per 3.5 ounces) of hair. Hair is normally associated with sebum and exocrine secretions from skin glands that confer greasiness but influence its water content and mechanical and physical properties. 

Methylmercury is rapidly and nearly completely absorbed from the gastrointestinal tract 90-100% absorption is estimated. Methylmercury is somewhat lipophilic, allowing it to pass through lipid membranes of cells and facilitating its distribution to all tissues, and it binds readily to proteins. Methylmercury binds to amino acids in fish muscle tissue. The highest methylmercury levels in humans generally are found in the kidneys. Methylmercury in the body is considered to be relatively stable and is only slowly transformed to other forms of mercury. Methylmercury readily crosses the placental and blood/brain barriers. Its estimated half-life in the human body ranges from 44 to 80 days. Excretion of methylmercury is via the feces, urine, and breast milk. Methylmercury is also distributed to human hair and to the fur and feathers of wildlife measurement of mercury in hair and these other tissues has served as a useful biomonitor of contamination levels. 

At the other end of the spectrum, combined X-ray (fluorescence, absorption and diffraction) and IR microscopic analyses on the same sample represent an activity of growing interest among the biomedical community. For example, protein mis-folding has been correlated with trace metal uptake in Alzheimer s disease [32], Parkinson s disease [91], scrapie [36] and amyotrophic lateral sclerosis [91], Likewise, in human hair the protein and lipid composition of the cuticle, cortex and medulla have been correlated with trace metal content associated with mummification techniques [74], and with environmental metal exposure [92]. 

Recently, a unique anteiso methyl-branched saturated fatty acid of 21 carbons, 18-methyl eicosanoic acid or 18-MEA, was identified in the outermost portion of the epicuticle, which is part of the CMC [104,106-110], 18-MEA is the predominant fatty acid in the epicuticle. It makes up approximately 40% of the surface lipid layer of wool and human hair [106,107,109], In addition to 18-MEA, other fatty acids have been isolated in smaller amounts from the epicuticle including... 

The most common soil found on human hair is sebum, a natural oily substance secreted onto the scalp by the sebaceous glands [146,147]. This material, which is a mixture of lipid components (Table 10.15), is distributed more or less uniformly over the hair surface as a result of contact with sebum-filled follicles by... 

Holmes [74] has isolated a fatty-acid protein complex from human hair that appears to protect the hair during papain digestion (cell membrane complex). Analysis of this complex indicates 20 to 30% fatty acid (lipid material) and 60 to 70% protein, rich in the amino acid lysine. Holmes suggests that this substance is either epicuticle or a fraction of the epicuticle. To avoid confusion, this material should be called either cell membrane complex or a portion of the cell membrane complex, which consists of protein and lipid components. 

The inert beta layers of the cell membrane complex are Upid-protein-type structures [78]. Sakamoto et al. [83] claim that fatty acids and wax esters are the main components of the internal lipids of human hair. Hilter-haus-Bong and Zahn [84] also find fatty acids, cholesteryl esters, and wax esters as main components however, they find polar lipids as major components. The fatty acids of this important component of human hair are predominately palmitic, stearic, and oleic acids. 

Leeder and Rippon [85] have analyzed the lipid composition of wool fibers after removing surface grease. Continued extraction with solvent removed the beta layers evidenced by electron microscopy however, the extract contained free cholesterol and free fatty acid and triglycerides but negligible quantities of phospholipid normally associated with biological membrane lipids. Koch [86], in his work with internal lipid of human hair, did not report significant quantities of phospholipid. These lipid-protein layers of hair are most likely related structurally to those of the epicuticle. 

In fine wools, such as those obtained from Merino sheep, the cuticle is normally one eell thick and usually constitutes about 10% by weight of the total fiber. By contrast, human hair cuticle may contain up to 10 layers of cells and pig bristle cuticle, about 35 layers. Sections of cuticle cells show an internal series of laminations comprising outer sulfur-rich bands known as the exocuticle and inner regions of lower sulfur content called the endocuticle. On the exposed surface of cuticle cells, a membranelike proteinaceous band (epicuticle) and a unique lipid component form a resistant barrier. These moieties are the functional components of the fiber surface and are significant in fiber protection and textile processing. 

Hair is complex multicomponent fibre with both hydrophilic and hydrophobic properties. It consists of 65-95% by weight of protein and up to 32% water, lipids, pigments and trace elements. The proteins are made of structured hard a-keratin embedded in an amorphous, proteinaceous matrix. Human hair is a modified epidermal structure, originating from small sacs called follicles that are located at the... 

More is known about nitrogen. In a study of modern humans where diet components (protein, lipid and carbohydrate) were measured against the corresponding body components, a shift of between 4.2 and 4.4%o was observed for nitrogen in both plasma protein and hair (Schoeller et al. 1986). This is just outside the usual 3-4%o range. Salmon fishers from coastal British Columbia are enriched by 3%o compared to their diet (Chisholm et al. 1983). Ancient Mexicans have constant 8 N values, as reported by DeNiro and Epstein (1981) 8-10%o, and While and Schwarcz (1989) 9.8 0.8%o. In the latter... 

As with urine, saliva (spumm) is easy to collect. The levels of protein and lipids in saliva or spumm are low (compared to blood samples). These matrices are viscous, which is why extraction efficiency of xenobioties amoimts to only 5 to 9%. By acidifying the samples, extraction efficiencies are improved as the samples are clarified, and proteinaceous material and cellular debris are precipitated and removed. Some xenobioties and their metabohtes are expressed in hair. Hair is an ideal matrix for extraction of analytes to nonpolar phases, especially when the parent xenobioties are extensively metabolized and often nondetectable in other tissues (parent molecules of xenobioties are usually less polar than metabolites). Hair is a popular target for forensic purposes and to monitor drug compliance and abuse. Human milk may be an indicator of exposure of a newborn to compounds to which the mother has been previously exposed. The main components of human milk are water (88%), proteins (3%), lipids (3%), and carbohydrates in the form of lactose (6%). At present, increasing attention is devoted to the determination of xenobioties in breath. This matrix, however, contains only volatile substances, whose analysis is not related to PLC applications. 

Wertz, P. and Norlen, L., Confidence intervals for the true lipid composition of the human skin barrier in Skin, Hair, and Nails. Forslind, B. and Lindberg, M., Eds., Marcel Dekker Inc., New York, Basel, 2004, p. 85. 

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