Inactivation of pyridoxal phosphate dependent enzymes by mono- and polyhaloalanines

beta,beta-Dichloro- and beta,beta,beta-trifluoroalanine irreversibly inactivate a number of pyridoxal phosphate dependent enzymes which catalyze beta- or gamma-elimination reactions. The inactivation is time dependent and the rate of inactivation is first order in enzyme concentration. This suggests that inactivation is due to covalent modification of the enzyme by a species generated at the active site from the polyhaloalanine (i.e., suicide inactivation). Monohaloalanines are substrates and do not inactivate. For gamma-cystathionase, covalent and stoichiometric attachment of [1-14C]beta,beta,beta-trifluoroalanine was shown. It is proposed that the mechanism of inactivation involves Schiff base formation between inactivator and enzyme-bound pyridoxal and subsequent elimination of HC1 from dichloroalanine or HF from trifluoroalanine. This results in the formation of a beta-halo-alpha,beta unsaturated imine, an activated Michael acceptor. Michael addition of a nucleophile at the active site leads to covalent labeling of the enzyme and inactivation. Alanine racemase is also inactivated by the two polyhaloalanines. Glutamate-pyruvate and gultamate-oxaloacetate transaminase are inactivated by monohaloalanines but not by polyhaloalanines.     

How long does chlorpromazine last?

An attempt has been made to relate the retarded adsoprtion to red cells of the slow reacting hemolytic phosphatide Rac. 1-octadecyl-2-benzyl-glycero-3-phosphorylcholine (benzyl-lysolecithin) to its aggregation status in aqueous solution. Light scattering measurements indicate a critical micelle concentration at 37 degrees of less than 2 X 10(-6) M. The micellar weight as determined by angle dependent light scattering is 6 X 10(7) with about 97 000 molecules per micells. The aggregates, which according to electron-microscopic observations are more similar to lecithin-liposomes than to usual lysolecithin-micelles, undergo a phase transition at 14 degrees from a tightly packed liquid-crystalline state to the more loose structure of a gel phase with increased mobility of the aliphatic chains. The enthalpy of transition is 4.2 kcal/mole. These changes of the micellar structure are reflected in the binding kinetics of benzyl-lysolecithin to red cells in that the binding rate is rather constant below, but strongly increasing above the transition temperature. It is concluded that the unusual micellar structure is responsible for the retarded adsorption of this lysolecithin analog to red cells and that the rate of adsorption is probably determined by the rate of escape of single lysophosphatide molecules from the micelles.     

Standardization of the chromium-51 release, cell-mediated cytotoxicity assay: cryopreservation of mouse effector and target cells

Utilizing controlled cryopreservation techniques, we were able to standardize the 51Cr release cytotoxicity assay and thereby ensured reliable comparisons between results obtained on different days. Optimal conditions for freezing of both effector and target cells were quite similar. Dimethyl sulfoxide (DMSO) at a concentration of 7.5-10.0% was employed as the cryoprotective agent and cells were frozen at the rate of -1 degrees C/minute. The handling procedures for the cells before and after freezing were important. Factors affecting recovery of functional reactivity were related to toxicity of DMSO for the cells, the osmotic stress placed upon the cells as the DMSO was being removed after thawing, the handling temperature of the freshly thawed cells, and the susceptibility of cells to mechanical damage immediately after thawing. The recovery of lymphocytes after freezing was about 70%; the recovery of cytotoxicity was around 85%. Syngeneic cytotoxic reactivity induced by inoculation with the Moloney strain of murine sarcoma virus was cryopreserved, as were allogeneic cytotoxicity and natural cytotoxic reactivity. Multiple tests employing effector cells from the same frozen pool gave reproducible results; the standard error of the mean percent cytotoxicity was less than 1.5%. Cryopreserved target cells gave decreased day-to-day variability in susceptibility to lysis, since the same population of cells could be employed in each assay. These results demonstrated conclusively that we can now have a constant source of effector cells and target cells, which can be used from assay to assay as an internal standard.     

Sudden incapacitation: USAF experience, 1970-80

During the period 1970-80, there were reported 146 cases of in-flight sudden incapacitation in the USAF. Of these, 62 involved pilots, 14 were navigators, and 70 were student pilots. The etiologies of sudden incapacitation included illness without loss of consciousness, loss of consciousness, spatial disorientation, and improper M-1 maneuver. Each of these categories is analyzed with emphasis upon prevention, for example, not flying with symptomatic preexisting disease, continued emphasis upon spatial disorientation training, and correct performance of the M-1 maneuver. Based upon the data, conclusions and recommendations are suggested to minimize the risk of episodes of in-flight sudden incapacitation.     

Adaptive resynthesis of O6-methylguanine-accepting protein can explain the differences between mammalian cells proficient and deficient in methyl excision repair

Mammalian cells have been classified as proficient (Mer(+)) or deficient (Mer(-)) in methyl excision repair in terms of their cytotoxic reactions to agents that form O(6)-alkylguanine and their abilities to reactivate alkylated adenoviruses. O(6)-Methylguanine (O(6)MeGua) is considered to be a lethal, mutagenic, and carcinogenic lesion. We measured the abilities of cell extracts to transfer the methyl group from an exogenous DNA containing O(6)MeGua to acceptor protein. The constitutive level of acceptor activity was independent of the Mer phenotype and was approximately 100,000 acceptor sites per cell. Treatment of cells with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) results in a dose-dependent decrease in the acceptor activity in extracts because the rapid reaction between endogenous O(6)MeGua and acceptor protein makes the latter unavailable for further reaction. Treatment of cells with 1 muM MNNG for 15 min or 2 muM for approximately 2 min uses up >95% of the constitutive activity. However, Mer(+) cells, which are resistant to MNNG, rapidly resynthesize new acceptor protein, and the activity returns to the basal level in approximately 90 min. In Mer(-) tumor cells and Chinese hamster cells, which are sensitive to MNNG, resynthesis is not detectable in 90 min. Mer(-) simian virus 40-transformed fibroblasts, known to have an intermediate sensitivity to MNNG, have an intermediate resynthesis rate. Treatment of cells with multiple low doses of MNNG results in the enhanced production of O(6)MeGua-accepting protein in levels 2.5-fold above the constitutive values for Mer(+) tumor cells and to approximately 1.5-fold for Mer(+) fibroblasts or Mer(-) simian virus 40-transformed cells. Such treatments reduce the activities in Mer(-) tumor cells and Chinese hamster cells. We conclude: (i) estimates of O(6)MeGua in cellular DNA shortly after treatment may be seriously in error because of the rapid repair of this lesion, and (ii) the adaptive resynthesis of acceptor protein, not its constitutive level, is the important correlate of cell resistance to methylating agents.     

Effects of phentermine on responding maintained under multiple fixed-ratio schedules of food and cocaine presentation in the rhesus monkey

Drugs that decrease drug-maintained responding at doses that do not decrease other behaviors in animals may be suitable candidates for development as medications to treat drug abuse in humans. The present study examined whether this effect could be obtained with phentermine, a drug that has been reported to decrease cocaine intake in humans. Rhesus monkeys were trained under multiple fixed-ratio 30-response schedules of food and i.v. cocaine delivery. Phentermine was always given as a slow, i.v. infusion. Acute treatment with phentermine (0.3-10 mg/kg) decreased cocaine-maintained responding at doses that did not decrease, or decreased less, food-maintained responding for each of three unit doses of cocaine (10-100 microg/kg/injection). Subacute treatment with phentermine (3 or 5.6 mg/kg, daily) also decreased cocaine-maintained responding more than food-maintained responding. After subacute treatment was terminated, rates of cocaine-maintained responding generally recovered to levels comparable to those seen during untreated control sessions. Phentermine (0.3-3 mg/kg) did not generally increase responding associated with a very low (1 microg/kg/injection) unit dose of cocaine, suggesting that the decrease in cocaine-maintained responding at higher unit doses was not the result of a leftward shift in the cocaine unit dose-effect function. Phentermine (0.1-3 mg/kg) decreased responding maintained by 1-[2-[bis(4-fluorophenyl) methoxy]ethyl]-4-[3-phenylpropyl] piperazine (GBR 12909) (30 microg/kg/injection) at doses similar to those that decreased food-maintained responding. These results show that phentermine is effective in decreasing cocaine self-administration and suggest that it may be an effective medication for cocaine abuse.