Localization of aminopeptidase activity in freshly excised human skin: direct visualization by confocal laser scanning microscopy.

The aim of this study was to localize and visualize aminopeptidase activity within freshly excised, dermatomed human skin without perturbation of its histologic integrity. The use of confocal laser scanning microscopy (CLSM) is introduced as a novel approach by which to monitor the degradation of suitable substrates in the skin. The fluorescence of the metabolites originating from the cleavage of the aminopeptidase probe bis-Leu-rhodamine 110 (Leu2-R11O) was interpreted to reflect the local aminopeptidase activity in the tissue. To separate the kinetics of diffusion and degradation of Leu2-R110, a lateral application mode was introduced: the probe was applied at the cutting plane of a mechanical cross-section of the sample, and optical cross-sections were made parallel to the cutting plane of the mechanical section. By this means, simultaneous and equal access of the substrate to the various strata and domains of the skin was achieved. The observations revealed that the fluorescence, i.e., aminopeptidase activity, was evenly distributed throughout the viable part of the epidermis, with enhanced fluorescence ("hot spots") in the upper layers of the stratum granulosum, while dermis and stratum corneum showed considerably less aminopeptidase activity. Independent studies with hair follicles (obtained from trypsin-separated stratum corneum) also showed aminopeptidase activity, mostly at the root sheath. Because of the advantage of direct visualization and localization of enzymatic activity in intact tissue, the lateral application mode of substrate administration in combination with CLSM may be beneficial to further elucidate the location and intensity of metabolic activity in other living tissues as well.