Subcellular localisation of leucine aminopeptidase in human polymorphonuclear leukocytes

Human polymorphonuclear leukocytes were homogenised in isotonic sucrose and subjected to analytical subcellular fractionation by sucrose density gradient centrifugation. The gradient fractions were assayed for leucine aminopeptidase and for principal organelle marker enzymes. Leucine aminopeptidase, when assayed with both L-leucine-7-amido-4-methyl-coumarin and leucyl-2-naphthylamide as substrate, showed a unimodal distribution with an equilibrium density of 1.18 g X cm-3. This distribution was quite distinct from that exhibited by marker enzymes for all the recognized subcellular organelles: there was no leucine aminopeptidase associated with the plasma membrane. Fractionation experiments with neutrophils treated with isotonic sucrose containing a low concentration of digitonin, and studies with the non-permeant ectoenzyme inhibitor, diazotised sulphanilic acid, confirmed that leucine aminopeptidase had a purely intracellular localisation. Fractionation experiments with neutrophils homogenised in sucrose medium containing digitonin, showed leucine aminopeptidase associated with a membrane fraction. It is suggested that leucine aminopeptidase is located to the membrane of a previously unrecognised population of cytoplasmic granules of the human neutrophil.