he development of peroxidase (PO) reaction in the nuclear envelope (NE) and endoplasmic reticulum (ER) of monocytes differentiating in vitro and its relationship with arachidonic acid metabolism were studied. The PO, as visualized by the diaminobenzidine (DAB) technique, appeared in the NE and ER of the majority of monocytes within 24 hours of culture, with a substantial decrease thereafter. The influence of three major groups of agents--inhibitors of PO, of prostanoids, and of protein biosynthesis--upon the development of the PO reaction was examined. When aminotriazole, a PO inhibitor, was added to the culture medium, the appearance of PO was suppressed in the monocytes. The cyclooxygenase blocker, indomethacin, however, did not influence the development of PO. Also the blockers of protein synthesis, puromycin, cycloheximide, and actinomycin D, did not affect the appearance of PO. The prostanoids released from the monocytes, ie, prostaglandin E and thromboxane B2, were determined by radioimmunoassay and showed a time sequence of secretion that corresponded to the appearance of PO in the cells: a marked increase within the first 24 hours with a substantial decrease thereafter. The presence of the PO inhibitors aminotriazole and sodium azide in the culture medium produced a suppression of prostanoid release from the monocytes comparable with that of indomethacin. The data suggest that the PO in the NE and ER of differentiating monocytes in vitro (1) is associated with arachidonic acid metabolism, and (2) is not formed by de novo protein synthesis but rather by an activation process.

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