Release and functional characterization of the leukotriene D4-metabolizing enzyme (dipeptidase) from human polymorphonuclear leucocytes.

Polymorphonuclear leucocytes released LTD4-dipeptidase activity in a time-, calcium-, and cell number-dependent fashion. The LTD4-dipeptidase released from polymorphonuclear leucocytes (PMN) by incubation with calcium (0.91 mM) was detectable up to a cell concentration of 1 X 10(6)/ml and increased with higher concentrations. Maximal LTD4-dipeptidase activity within the extracellular environment was detected after 15 min of incubation (2 X 10(7)/ml) in the presence of 2-4.5 mM calcium or after 30 min, when stimulation was carried out with 0.91 mM calcium. The activity of the released LTD4-dipeptidase was modulated by various metal ions and other compounds. The addition of Mn2+, Co2+, and Zn2+ (final concentration 1 mM) enhanced the LTD4-dipeptidase activity, while Cu2+ led to a complete inhibition. In the absence of exogenous calcium EDTA inhibited LTD4-dipeptidase. Calcium up to a concentration of 5 and 10 mM decreased the dipeptidase activity. The LTD4-dipeptidase is not affected by bestatin, leupeptin, or N-ethyl-maleinimide (NEM). The Km of LTD4-dipeptidase for LTD4 was 0.95 +/- 0.2 microM and Vmax was 737.5 +/- 112.5 pmol/min X mg protein (n = 3 +/- SEM). The highest LTD4-dipeptidase activity was obtained at physiological pH values. LTD4-dipeptidase activity can also be released from other cell types, but the enzyme activity from human PMN exceeded that of other cells (e.g. human lymphocytes/monocytes and basophils (LMB) and human lung cell suspension).