Extraction and quantification of amino acids in human stratum corneum in vivo

Background: Amino acid (AA) levels in stratum corneum (SC) are potential biomarkers of skin health while their systemic levels may be used to diagnose inherited metabolic diseases.
Objectives: To examine reverse iontophoresis, in human volunteers, as a minimally invasive tool to analyse AAs within the skin and subdermally.
Methods: In four volunteers, the amounts of iontophoretically extracted AAs were compared with those determined in the SC following repetitive tape stripping and with the plasma concentrations. Glucose levels, evaluated in the different compartments, were used as a control.
Results: SC concentrations of 13 essentially zwitterionic AAs were ∼100-fold higher than the respective plasma levels. Passive and reverse iontophoretic extraction for 4 h did not deplete the SC depot of AAs, a fact reinforced by postextraction tape stripping, which revealed that AAs remained in the SC at this time. In contrast, glucose was much less abundant in the SC and was fully and relatively quickly extracted by reverse iontophoresis.
Conclusions: It follows that reverse iontophoresis is useful for quantifying AAs in the SC and these data are highly correlated with levels obtained by tape stripping. However, reverse iontophoresis is impractical for the routine monitoring of AA plasma concentrations (unlike the situation for glucose, the skin reservoir of which is much smaller).