REGIONAL VARIATION IN FAAs IN STRATUM CORNEUM 307 not measure PCA, urocanic acid, lactates, urea, sugars, or ions. Other NMF components such as lactates are positively correlated to SC hydration (24) and omission of lactate lev- els could account in part for the lack of correlation between putative NMF and MAT. Another potential source of protein that could account for higher citrulline levels in the samples is keratin 1. Peptidylarginine deiminase isoform 1 (PAD1) is responsible for deimination of K1 in the SC (25). Keratin 1 is expected to be present at higher levels in dry Figure 3. MAT was signifi cantly lower for the jaw than for either the torso or back, but the jaw and calf were not different. The total FAA levels were not correlated with skin hydration, measured as the rate of moisture accumulation (MAT), for the body sites as indicated here for Study 2. Figure 4. Cumulative citrulline (normalized to cumulative protein) for tape 1, sum 1+3, sum 1+3+5, sum 1+3+5+10, and the sum of all tapes is shown versus the respective cumulative protein amounts for the four sites. Similar trends were observed for total FAA and individual amino acids. With SC depth, FAA fi rst increased and then leveled off for the back and torso. The increase continued for the jaw but remained rela- tively constant for the calf.
JOURNAL OF COSMETIC SCIENCE 308 skin (26). If conditions were favorable for proteolysis to individual amino acids, keratin 1 may have been an additional source of citrulline for the sites with lower SC hydration. SC cohesiveness, measured as protein removed by the tape strip, showed signifi cant dif- ferences between body sites, and these differences persist through the SC (Figure 4). This confi rms recent work by Breternitz et al. (27), showing signifi cant differences in protein amounts removed from varying body sites. Koyama et al. (6) found lower levels of citrulline and serine in the cheek versus the back and higher arginine and histidine. Similarly, Egawa and Tagami (5) reported lower NMF for the cheek versus the forearm. These accounts are consistent with our result of lower citrulline in “face” versus back, although we evaluated the jaw rather than the cheek. The outcomes are not consistent with those of Horii et al. (2), who found signifi cantly lower FAA in the lower leg along with reduced skin hydration and increased xerosis. The higher FAA in the calf is somewhat consistent with the fi ndings of Takahashi and Tezuka (4), who reported higher FAA in the lower leg of older subjects with xerosis versus younger normal controls, but some of the increase was attributed to age (4). Proteolysis of fi laggrin to free amino acids and to further conversion products takes place when fi laggrin-containing coenocytes move up into the drier regions of the stratum cor- neum. Thus, proteolysis may not occur until the outermost layers need moisture for SC plasticization of the outermost layers (28). While we did not quantify dryness, calf skin is often visibly scaly, suggestive of aberrant desquamation. Greater amounts of FAA in the calf may indicate an up-regulation in order to subsequently increase hydration. Reduced SC turnover may also explain higher levels of FAA (29). Assessments of SC thickness and turn- over rates are needed to evaluate this explanation. Additional experiments including the analysis of PCA, urocanic acid, and lactates by other analytical methods are warranted. ACKNOWLEDGMENTS This work was supported by an SCC Graduate Fellowship (M. Robinson). REFERENCES (1) I. R. Scott and C. R. Harding, Filaggrin breakdown to water binding compounds during development of the rat stratum corneum is controlled by the water activity of the environment, Dev. Biol., 115(1), 84–92 (1986). (2) I. Horii, Y. Nakayama, M. Obata, and H. Tagami, Stratum corneum hydration and amino acid content in xerotic skin, Br. J. Dermatol., 121(5), 587–592 (1989). (3) J. Nikolovski, G. N. Stamatas, N. Kollias, and B. C. Wiegand, Barrier function and water-holding and transport properties of infant stratum corneum are different from adult and continue to develop through the fi rst year of life, J. Invest. Dermatol., 128(7), 1728–1736 (2008). (4) M. Takahashi and T. Tezuka, The content of free amino acids in the stratum corneum is increased in senile xerosis, Arch. Dermatol. Res., 295(10), 448–452 (2004). (5) M. Egawa and H. Tagami, Comparison of the depth profi les of water and water-binding substances in the stratum corneum determined in vivo by Raman spectroscopy between the cheek and volar forearm skin: Effects of age, seasonal changes and artifi cial forced hydration, Br. J. Dermatol., 158(2), 251–260 (2008). (6) J. Koyama, I. Horii, K. Kawasaki, Y. Nakayama, Y. Morikawa, and T. Mitsui, Free amino acids of stratum corneum as a biochemical marker to evaluate dry skin, J. Soc. Cosmet. Chem., 35, 183–195 (1984). (7) M. Robinson, M. Visscher, A. LaRuffa, and R. Wickett, Natural moisturizing factors (NMF) in the stratum corneum (SC). I. Effects of lipid extraction and soaking, J. Cosmet. Sci., 62, 13–22 (2010).
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