Separation of enantiomeric amines and acids using chiral ion-pair chromatography on porous graphitic carbon

The application of porous graphitic carbon as adsorbing phase for direct separation of enantiomeric acids and amines using chiral ion-pair chromatography is described. The enantiomeric amines were separated as diastereomeric ion pairs with N-benzyloxycarbonylglycyl-L -proline, N-benzyloxycarbonylglycylglycyl-L -proline, or captopril as the chiral counterion. High enantioselectivities were obtained for amines having a hydrogen bonding function in the vicinity of the asymmetrical carbon atom. Quinine was the chiral counterion used to separate the enantiomeric acids. The strongly UV-absorbing quinine improved detection of solutes having low UV-absorbing properties, e.g., (R,S)-2-chloropropionic acid, by “indirect detection.” Retention and stereoselectivity of enanticmeric acids were regulated by the quinine concentration and by the addition of carboxylic acids as well as polar modifiers, e.g., methanol and 2-propanol, to the mobile phase. © 1992 Wiley-Liss, Inc.     

Liver alcohol dehydrogenase

The article deals with the structure and function of liver alcohol dehydrogenase and reviews mainly literature published after 1979, i.e., summarizes progress made in the field since Klinman presented her review on alcohol dehydrogenases. The emphasis will be on high-resolution crystallographic data, results obtained with metal-substituted enzyme derivatives, and on the mechanism and pH dependence of the catalytic reaction.     

Promoters and processing sites within the transforming region of bovine papillomavirus type 1.

The mRNAs present in bovine papillomavirus type 1 (BPV-1)-transformed C127 cells were studied by primer extension. The results show that two internal promoters are present in the E region of BPV-1 in addition to the previously identified promoter at coordinate 1 (H. Ahola, A. Stenlund, J. Moreno-López, and U. Pettersson, Nucleic Acids Res. 11:2639-2650, 1983). One, located at coordinate 31, generated a set of mRNAs with heterogeneous 5' ends, which may encode the major transforming protein of BPV-1, the E5 protein. The second promoter, which is located at coordinate 39, generates colinear mRNAs which encode either the E4 protein or a truncated form of the E2 protein. Unlike the cottontail rabbit papillomavirus (O. Danos, E. Georges, G. Orth, and M. Yaniv, J. Virol. 53:735-741, 1985), BPV-1 appears to lack a separate promoter for expression of the E7 protein. The major splice sites in the transforming region (E region) of the BPV-1 genome were also identified by nucleotide sequence analysis.     

A rapid-equilibrium model for the control of the Calvin photosynthesis cycle by cytosolic orthophosphate

1. A simple model based on rapid-equilibrium assumptions is derived which relates the steady-state activity of the Calvin cycle for photosynthetic carbohydrate formation in C3 plants to the kinetic properties of a single cycle enzyme (fructose bisphosphatase) and of the phosphate translocator which accounts for the export of photosynthate from the chloroplast. Depending on the kinetic interplay of these two catalysts, the model system may exhibit a single or two distinct modes of steady-state operation, or may be unable to reach a steady state. 2. The predictions of the model are analysed with regard to the effect of external orthophosphate on the steady-state rate of photosynthesis in isolated chloroplasts under conditions of saturating light and CO2. Due to the possible existence of two distinct steady states, the model may account for the stimulatory as well as the inhibitory effects of external phosphate observed in experiments with intact chloroplasts. Stability arguments indicate, however, that only the steady-state case corresponding to phosphate inhibition of the rate of photosynthesis could be of physiological interest. 3. It is concluded that chloroplasts under physiological conditions most likely operate in a high-velocity steady state characterized by a negative Calvin cycle flux control coefficient for the phosphate translocator. This means that any factor enhancing the export capacity of the phosphate translocator can be anticipated to decrease the actual steady-state rate of photosynthate export due to a decreased steady-state rate of cyclic photosynthate production.     

Effect of evolution on the kinetic properties of enzymes

1. The likely effect of a selective pressure in the direction of higher reaction fluxes on rate parameters for enzyme reactions confirming to Michaelis-Menten kinetics has been analyzed on the basis of relationships which take into account the changes in metabolite concentrations that must be associated with mutational changes of the kinetic properties of enzymes participating in metabolic pathways. 2. Arguments are presented to show that such a pressure should tend to increase kcat, whereas Km may decrease or increase depending on what stage of evolutionary development the enzyme has reached. While the early evolution of enzymes must have been associated with decreasing Km values, an increase of both kcat and Km is mandatory for enhancement of the rate performance of extensively developed enzymes which exhibit kcat/Km ratios approaching the diffusion-control limit. The latter limit is dependent on the equilibrium constant for the catalysed reaction. 3. Enzymes which have reached the diffusion-control limit for their second-order rate performance cannot be considered as perfectly evolved catalysts, but may well undergo further development towards a higher catalytic efficiency in response to the improvement of other enzymes in the metabolic pathway with regard to the criterion of an enhanced reaction flux. Such evolution is associated with an increase of the metabolite levels in the pathway, and a simple model system is examined in order to illustrate the ultimate limits for the metabolite levels and reaction flux that may obtain. 4. The theoretical evidence presented lends no support to previous proposals that certain enzymes (e.g. triosephosphate isomerase), or enzymes showing certain kinetic characteristics (e.g. kcat/Km quotients approaching 10(9) s-1 M-1), have reached the end of their evolutionary development. A claim that any specific enzyme has reached catalytic perfection would provide the unreasonable inference that all enzymes participating in intermediary metabolism have reached catalytic perfection.     

Metabolites controlling the rate of starch synthesis in the chloroplast of C3 plants

The extent to which different stromal metabolites affecting ADPglucose pyrophosphorylase control the rate of photosynthetic starch production in the chloroplast of C3 plants has been examined by kinetic model studies. The results indicate that ATP, glucose 1-phosphate, 3-phosphoglycerate, fructose 6-phosphate, and orthophosphate may provide significant contributions to the starch synthesis rate changes induced by variation of the external concentration of orthophosphate, the detailed control situation being dependent on the actual concentration of the external metabolite.