Differentiation of HL‐60 cells by dimethylsulfoxide induces 5‐lipoxygenase protein expression, but only low cellular 5‐lipoxygenase activity. Similarly, B‐lymphocytes express 5‐lipoxygenase protein and show activity in cell homogenates but not in intact cells. Here, we demonstrate that suppression of cellular 5‐lipoxygenase activity in these cell lines is serum dependent and that the serum effect can be mimicked by selenium. Selenium‐dependent inhibition of 5‐lipoxygenase activity was also observed in the corresponding cell homogenates or 100000×g supernatants when dithiothreitol or glutathione (GSH) was added. The properties of the endogenous selenium‐dependent inhibitor, i.e. molecular mass, utilization of GSH and dithiothreitol as substrates, sensitivity to iodacetate, inhibition of 5‐lipoxygenase activity in the presence of the GPx‐1 inhibitor mercaptosuccinate, suggest that a selenoenzyme with properties of the phospholipid hydroperoxide glutathione peroxidase (GPx‐4) is responsible for the 5‐lipoxygenase inhibition in BL41‐E95‐A and immature HL‐60 cells.

Differentiation of HL‐60 cells in the presence of 1,25‐dihydroxyvitamin D₃ and transforming growth factor‐β(TGFβ) upregulated cellular 5‐lipoxygenase activity regardless of whether the cells were grown with or without serum or selenium. Also, 5‐lipoxygenase activity in homogenates or 100000×g supernatants of 1,25‐dihydroxyvitamin D₃/TGFβ differentiated HL‐60 cells and of human granulocytes was not inhibited by dithiothreitol or GSH. Thus, after 1,25‐dihydroxyvitamin D₃/TGFβ differentiation, HL‐60 cells resemble normal granulocytes with respect to the high 5‐lipoxygenase activity in intact cells and to the dithiothreitol effects in broken cell preparations. Combination experiments with 100000×g supernatants of BL41‐E95‐A cells and neutrophils revealed that the high 5‐lipoxygenase activity of granulocytes is due to stability of the 5‐lipoxygenase catalytic activity against selenium‐dependent peroxidases, but not to low peroxidase activity. Our data suggest that the capability of mature myeloid cells to release large amounts of leukotrienes after stimulation is due to a peroxidase‐insensitive 5‐lipoxygenase catalytic activity.