Activation discrepancies with membrane

Early discovery also makes extensive use of in vitro and cell-culture activity models. Activity measurements can be inconsistent when the compound has low aqueous solubility. Thus, the solubility assay, can assist with the interpretation of biological activity tests. Another cause of poor bioactivity in vitro is poor stability in the assay medium. Also, there can be discrepancies with activity when compounds are moved from biological models in 96-384-well format to living-cell models. Differences in activity can often be traced to permeability of the compound through the cellular lipid membrane and correlate with results of the permeability assay. 

An anomaly associated with citrulline that became evident when detailed kinetic studies were made in the 1950s (R.B. Fisher and J.R. Bronk) was the irreproducibility of its catalytic activity in liver slices on the formation of urea, despite the clear evidence from Ratner and Petrack of its importance in arginine synthesis. Initially the discrepancy in catalytic activity between ornithine and citrulline was ascribed to the possible impermeability of the liver cell plasma membrane to the latter intermediate, a hypothesis which was rapidly disproved experimentally. Only recently has it been shown that ornithine transcarbamylase is clearly associated with the ornithine/... 

Despite very good inhibitory potency on the enzyme level, AG1284 has rather modest antiviral activity in vitro (Table 6). The reason for this discrepancy is unclear but could be related to the low water solubility and higher affinity for membranes, which may effect cell partitioning. A similar lack of correlation between the potency of enzyme inhibition and antiviral activity has been previously observed with other HIV PR inhibitors [11]. 

As shown in Fig. 18, the simulation conductivity data were generally consistent with the experimental results. However, there are appreciable differences between the simulation and experimental results. At some points, the differences can be 100%. The other observation is that the simulation must underestimate the activation energies of the conduction. The primary reason for this discrepancy is that these simulation models do not take into account interaction between the membrane itself and its environment. In reality, the water uptake at elevated temperatures may be greater than that at room temperature. In the simulations, it was assumed that both... 

The regions of the presynaptic membrane where fusion of vesicles and the plasmalemma occur are limited to what has been termed the active zones. Closely associated with these active zones are what electron microscopists believe may be clusters of calcium ionophores necessarv for the entry of Ca + for the initiation of exocytotic release. Other morphological entities at the active zone have also been identified, but their physiological role in transmitter release has not been elucidated. It should be noted that numerous freeze-fracture micrographs taken of active synapses reveal many more vesicle fusions than would be predicted for one or two release events. These observations are not consistent with the one vesicle-one quantum hypothesis that was briefly discussed earlier. To date, no explanation for the discrepancy between the number of vesicles and the number of quanta released has been proposed except to suggest that the vesicle, in fact, only releases a fraction of the quantum, which has been termed a microquantum. 

Whether these discrepancies are due to a lower activity coefficient of potassium inside of the cell as compared with the activity coefficient outside of the cell, or whether they are due to leakage of potassium outside and penetration of sodium inside the cell is not certain. Better agreement between theoretical and actual values can sometimes be obtained by calculating the potential by the Goldman equation rather than the Nernst equation. The Goldman equation does not express the membrane potential in function of the intracellular movements of potassium, but it takes into consideration the participation of other cations, particularly sodium. Thus, in Goldman s equation the potential is expressed as a function of the relative permeabilities and the electrochemical gradients of each ion. 

Although as a class the chelates are definitely bacteriostatic, the relative contributions of uptake and dissociation for both inert and labile species need to be more clearly defined before a full picture is obtained. The results suggest that kinetic reactivity may be more important than thermodynamic stability [30]. In the case of copper(II) complexes with 1,10-phenanthroline and 2,9-dimethyl-1,10-phenanthrolines, more recent studies showed that the major mode of action on P. denitrificans in vitro was in fact inhibition of respiratory electron transport in the cytoplasmic membrane, no correlation with inhibition of macromolecular synthesis being apparent [31]. These results are relevant in view of the discrepancy in the reported antitumour activity of these species (see Section 6.2). 

---