THE NUMBER OF ACETYLCHOLINESTERASE MOLECULES IN THE RAT MEGAKARYOCYTE

A megakaryocyte cell series from rat bone marrow has been examined by the isotopic di-isopropyl fluorophosphate (DFP) method for esterases. After complete reaction with 32 P-DFP, the numbers of DFP-reacted molecules in individual cells have been determined by β track auto-radiography. Previous work has shown the percentage of organophosphate-sensitive sites in these cells which can be taken as active centers of acetylcholinesterase (AChase). Combining these data, the absolute numbers of organophosphate-sensitive esterase molecules and AChase molecules per cell were determined. Histograms show a narrow spread of values within each of four size classes from megakaryoblast to fully mature megakaryocyte, but, with means increasing 4-fold through this series, approximately in proportion to cell volume. A rat megakaryoblast has 2 x 10 6 AChase molecules, and a megakaryocyte (of 48-µ diameter) has 7.6 x 10 6 molecules. The apparent turnover number of the enzyme for intracellular reaction with substrate is calculated and compared with turnover numbers available for other AChases.