Purification and characterization of leukotriene A₄ hydrolase from human epidermis

The leukotriene A₄ hydrolase is a central enzyme in leukotriene B₄, formation. Unlike 5-lipoxygenase, leukotriene A₄ hydrolase activity is present in normal human epidermis, where it is likely to be involved in transcellular leukotriene formation. In this study the leukotriene A₄ hydrolase was purified from human epidermis and human cultured keratinocytes and compared with leukotriene A₄ hydrolase from human neutrophils. To purify leukotriene A₄ hydrolase from human epidermis a new non-specific affinity chromatography column, with the leukotriene A₄ hydrolase inhibitor bestatin coupled to AH-Sepharose, was introduced. The epidermal leukotriene A₄ hydrolase was purified to apparent homogeneity and the molecular weight was determined to be approximately 70,000 Da by SDS-PAGE. The pI was 5.1-5.4 for the epidermal as well as the keratinocyte and neutrophil leukotriene A₄ hydrolase, as determined by chromatofocusing. Only minor differences in the amino acid composition were seen between the three enzyme sources. The optimal pH for the hydrolase activity was 7.5-8.5 for the epidermal and neutrophil leukotriene A₄ hydrolases. Finally, it was also shown that the epidermal leukotriene A₄ hydrolase undergoes suicide inactivation when transforming leukotriene A₄ into leukotriene B₄. It was concluded that there is a close resemblance between the epidermal leukotriene A₄, hydrolase and the hydrolase found in other cell types. Therefore, the human epidermis may be a good model for the in vivo study of transcellular leukotriene formation.