Subcellular localization and properties of lipase activities in human polymorphonuclear leukocytes

Abstract

A fluorimetric assay for lipase activity has been optimized for measurement of the enzyme in human neutrophils. Activity was maximal at acid (4.5) and alkaline (9.5) pH, although there was also a neutral peak of activity at pH 6.5. Neutrophils were homogenised in isotonic sucrose and subjected to analytical subcellular fractionation by sucrose density gradient centrifugation. The gradient fractions were assayed for acid, neutral and alkaline lipase activity and for the principal organelle marker enzymes. Neutral lipase showed a unimodal distribution with an equilibrium density of 1.19 g δ cm −3, corresponding to the distribution of particulate leucine aminopeptidase. Acid and alkaline lipase activities showed very similar distribution profiles to each other with both soluble components and a broad peak of particulate activity. The broad modal density of 1.19–1.22 g · cm −3 suggests that acid and alkaline lipase activities could be localised to more than one population of cytoplasmic granule. Fractionation experiments with neutrophils homogenised in sucrose medium containing digitonin confirmed the localisation of neutral lipase and leucine aminopeptidase to the same cytoplasmic granule, and suggested that at least part of the acid lipase activity was localised to the specific granule. No lipase activity could be attributed to the alkaline phosphatase-containing granule. Neutrophils were isolated from control subjects, patients with chronic granulocytic leukaemia and women in the third trimester of pregnancy. The specific activity of acid, neutral and alkaline lipase, and leucine aminopeptidase, in contrast to that of alkaline phosphatase, were similar in the three patient groups.