An investigation of basic assumption in enzyme kinetics using results of the geometric theory of differential equations

Up to the present time, the following property of the product component in the reversible one substrate-one intermediate-one product enzymic mechanism has been taken only as anassumption, viz., during the course of the reaction, the time-rate of change of product concentration is never negative and the product concentration never exceeds its equilibrium value. Applying the methods of the geometric theory of ordinary differential equations it is shown that this result follows as a direct deduction from the differential equations governing the mechanism together with the initial conditions. Further, the nature of the equilibrium point as a stable node is established.     

Coat proteins of strains of two RNA viruses: comparison of their amino acid sequences

Summary The amino acid sequences of four strains of tobacco mosaic virus isolated in different parts of the world are compared. The differences between the strains are discussed with respect to special proteinchemical features (such as beginning of the chain, deletion of amino acids, number of different amino acids, sizes and distribution of regions with invariable amino acids) and with respect to the possibility of deducing the most probable nucleotide sequence for the coat protein cistron of tobacco mosaic virus.The complete amino acid sequences of the two RNA bacteriophage strains fr and f₂ are compared. According to their coat proteins three groups of phages can be formed: 1) MS 2, f₂ M 12 and R 17, 2) fr and 3) Q.     

Kinetic relations of the Na-amino acid interaction at the mucosal border of intestine

The relation between unidirectional influxes of Na and amino acids across the mucosal border of rabbit ileum was studied under a variety of conditions. At constant Na concentration in the mucosal bathing solution, amino acid influx followed Michaelis-Menten kinetics permitting determination of maximal influx and the apparent Michaelis constant, K_t. Reduction in Na concentration, using choline as substitute cation, caused an increase in K_t for alanine but had no effect on maximal alanine influx. The reciprocal of K_t was a linear function of Na concentration. Similar results were obtained for valine and leucine and these amino acids competitively inhibited alanine influx both in the presence and in the absence of Na. These results lead to a model for the transport system which involves combination of Na and amino acid with a single carrier or site leading to penetration of both solutes. The model predicts that alanine should cause an increase in Na influx and the ratio of this extra Na flux to alanine flux should vary with Na concentration. The observed relation agreed closely with predicted values for Na concentrations from 5 to 140 mM. These results support the hypothesis that interactions between Na and amino acid transport depend in part on a common entry mechanism at the mucosal border of the intestine.     

Properties of substituted 2-trifluoromethylbenzimidazoles as uncouplers of oxidative phosphorylation

1. The activity of 25 substituted 2-trifluoromethylbenzimidazoles in uncoupling oxidative phosphorylation by rat-liver mitochondria has been compared. 2. For halogen- or mixed-halogen- and alkyl-substituted analogues, uncoupling activity was proportional to the acidity of the imidazole -NH group. Tetrachloro-2-trifluoromethylbenzimidazole was the most active (50% uncoupling of oxidative phosphorylation at 7.9x10(-8)m, pK5.04). Nitro-substituted analogues were less active than predicted from pK considerations or from partition-coefficient measurements. 3. Introduction of an -NH(2) or -CO(2)H substitutent caused a loss of uncoupling activity, as did alkylation at position 1 of the imidazole ring. 4. Benzimidazoles active as uncouplers stimulated mitochondrial adenosine triphosphatase but not all stimulated the oxidation of succinate in the absence of a phosphate acceptor. 5. 4,5-Dichloro-2-trifluoromethylbenzimidazole inhibited the succinate-oxidase system at about the same concentration required for uncoupling (0.52mum for 50% inhibition of both activities) and the site of this inhibition appears to lie between succinate dehydrogenase and cytochrome b.