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 × 10⁶/ml and increased with higher concentrations. Maximal LTD₄-dipeptidase activity within the extracellular environment was detected after 15 min of incubation (2x10⁷/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 exogenoas 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 K_m of LTD₄-dipeptidase for LTD₄ was 0.95±0.2 γM and v_max was 737.5±112.5 pmol/min×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). [ABSTRACT FROM AUTHOR]