Studies of D-amino acid oxidase activity in human epidermis and cultured human epidermal cells.

The occurrence of the enzyme D-amino acid oxidase in human epidermis and cultured epidermal cells was investigated. When explant cultures of human epidermis were cultured in a medium containing D-valine instead of L-valine and supplemented with undialyzed serum, good growth of both epithelial cells and fibroblasts was observed. However, when the serum was dialyzed neither cell type could be cultured in D-valine medium indicating the absence of D-amino acid oxidase in both cell types. When epithelial cultures initiated in L-valine medium were changed to D-valine medium after 1-2 weeks, growth stopped immediately, and the epithelial cells showed signs of extensive degeneration, indicating that skin epithelial cells have a very low endogenous pool of L-valine. When these cultures were re-fed L-valine medium after 2 weeks in D-valine medium, this resulted within a few days in the reappearance of epithelial outgrowth. The activity of D-amino acid oxidase in human epidermal cells was further studied by histochemistry and in homogenates of epidermis and cultured epidermal cells. Whereas high activity of histidase was observed in the epidermal cells, no activity of D-amino acid oxidase could be detected. This report shows that D-amino acid oxidase does not serve as a market enzyme for epithelial cells in general as previous tissue culture studies might indicate. Human skin epithelial cells do not contain detectable amounts of D-amino acid oxidase activity, and therefore, D-valine medium is not suitable for selection of growth of skin epithelial cells in culture.