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Background and purpose: Carboxylesterases (CEs) metabolize a wide range of xenobiotic substrates including heroin, cocaine, meperidine and the anticancer agent CPT-11. In this study, we have purified to homogeneity human liver and intestinal CEs and compared their ability with hydrolyse heroin, cocaine and CPT-11. Experimental approach: The hydrolysis of heroin and cocaine by recombinant human CEs was evaluated and the kinetic parameters determined. In addition, microsomal samples prepared from these tissues were subjected to chromatographic separation, and substrate hydrolysis and amounts of different CEs were determined. Key results: In contrast to previous reports, cocaine was not hydrolysed by the human liver CE, hCE1 (CES1), either as highly active recombinant protein or as CEs isolated from human liver or intestinal extracts. These results correlated well with computer-assisted molecular modelling studies that suggested that hydrolysis of cocaine by hCE1 (CES1), would be unlikely to occur. However, cocaine, heroin and CPT-11 were all substrates for the intestinal CE, hiCE (CES2), as determined using both the recombinant protein and the tissue fractions. Again, these data were in agreement with the modelling results. Conclusions and implications: These results indicate that the human liver CE is unlikely to play a role in the metabolism of cocaine and that hydrolysis of this substrate by this class of enzymes is via the human intestinal protein hiCE (CES2). In addition, because no enzyme inhibition is observed at high cocaine concentrations, potentially this route of hydrolysis is important in individuals who overdose on this agent.

Platelet producing capacity was examined at intervals after acute platelet depletion by reinduction of acute thrombocytopenia and assessment of the subsequent platelet recovery rates of 35S-sulfate incorporation into platelets. Platelet producing capacity was increased on days 1 and 2, somewhat decreased on days 3 and 4 and had returned to baseline by day 5. These studies suggest that there is an initial increase followed by a decrease on days 3 and 4 in cells which can respond to thrombopoietic stimulating factor after induction of acute thrombocytopenia.

Appropriate low dosages of vincristine stimulate megakaryocytopoiesis and produce thrombocytosis. In this study of the thrombocytotic action of vincristine, administration of a single dose of 0.1 mg/kg to rats produced an increase in megakaryocyte concentration, diameter and 24 h [3H]thymidine labelling index. Transfusion of one body equivalent of platelets from normal donors prevented stimulation of megakaryocytopoiesis by vincristine whereas platelets from vincristine-treated donors did not. These results suggest that vincristine stimulates megakaryocytopoiesis by altering the functional role of circulating platelets in the regulation of thrombopoiesis.

Thrombopoiesis has been studied during and after an 11 d exposure to discontinuous hypobaric hypoxia. Exposure of rats to 0.4 atmospheres for 16--17 h daily initially caused an increase in platelet count which reached a peak of 1.5 times baseline on days 4 and 5. This thrombocytosis was followed by a decrease in platelets to a nadir of 50--60% of baseline on days 12 and 13. That thrombocytosis results from increased platelet production is supported by increased [35S]sulphate incorporation into platelets and increased megakaryocyte size and turnover. The thrombocytopenia with continued hypoxia seems to result from decreased platelet production since 51Cr-platelet survival was normal while megakaryocyte concentration was decreased to one-half that of untreated controls. These observations suggest that differentiation of precursors into megakaryocytes was decreased during the thrombocytopenic period, although the fewer remaining megakaryocytes appeared stimulated because of their larger size and increased [3H]thymidine labelling. Thus, hypobaric hypoxia had a biphasic effect on thrombopoiesis with increased platelet production in the first few days of exposure followed by subnormal production.