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At least two types of animal plasminogen activating enzymes exist, differing in amino acid sequence, molecular mass and immunological reactivity: the urokinase‐type and the tissue‐type plasminogen activators. By affinity chromatography with monoclonal antibodies, we have purified the human activators of both types to homogeneity. Using immunocytochemistry with rabbit antibodies raised against these preparations, we now demonstrate that the plasminogen activator present in endothelium of veins and other blood vessels is of the tissue‐type. No urokinase‐type plasminogen activator immunoreactivity was detected in endothelial cells in the intact organism. These findings support the assumption that mobilization of plasmin for different purposes may involve different types of plasminogen activators, and that the plasminogen activator involved in thrombolysis is of the tissue‐type.

The leukotriene A 4hydrolase is a central enzyme in leukotriene B 4, formation. Unlike 5-lipoxygenase, leukotriene A 4hydrolase activity is present in normal human epidermis, where it is likely to be involved in transcellular leukotriene formation. In this study the leukotriene A 4hydrolase was purified from human epidermis and human cultured keratinocytes and compared with leukotriene A 4hydrolase from human neutrophils. To purify leukotriene A 4hydrolase from human epidermis a new non-specific affinity chromatography column, with the leukotriene A 4hydrolase inhibitor bestatin coupled to AH-Sepharose, was introduced. The epidermal leukotriene A 4hydrolase 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 A 4hydrolase, 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 A 4hydrolases. Finally, it was also shown that the epidermal leukotriene A 4hydrolase undergoes suicide inactivation when transforming leukotriene A 4into leukotriene B 4. It was concluded that there is a close resemblance between the epidermal leukotriene A 4, 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.