The homologous tryptophan critical for cytochrome c peroxidase function is not essential for ascorbate peroxidase activity

The crystal structures of ascorbate peroxidase (APX) and cytochrome c peroxidase (CCP) show that the active site structures are nearly identical. Both enzymes contain a His-Asp-Trp catalytic triad in the proximal pocket. The proximal Asp residue hydrogen bonds with both the His proximal heme ligand and the indole ring nitrogen of the proximal Trp. The Trp is stacked parallel to and in contact with the proximal His ligand. This Trp is known to be the site of free radical formation in CCP compound I and also is essential for activity. However, APX forms a porphyrin radical and not a Trp-centered radical, even though the His-Asp-Trp triad structure is the same in both peroxidases. We found that conversion of the proximal Trp to Phe has no effect on APX enzyme activity and that the mutant crystal structure shows that changes in the structure are confined to the site of mutation. This indicates that the paths of electron transfer in CCP and APX are distinctly different. The Trp-to-Phe mutant does alter the stability of the APX compound I porphyrin radical, by a factor of two. Electrostatic calculations and modeling studies show that a potassium cation located about 8?Å from the proximal Trp in APX, but absent in CCP, makes a significant contribution to the stability of a cation Trp radical. This underscores the importance of long-range electrostatic effects in enzyme catalyzed reactions.     

The influence of the diel climatic cycle on the depth-time distribution of phytoplankton and photosynthesis in a shallow equatorial lake (Lake Baringo,Kenya)

Lake Baringo is a shallow equatorial lake. This paper reports a diel study of the depth-time distribution of phytoplankton and photosynthesis at one location in Lake Baringo on 10 March 1989. The water column shows a pattern of diurnal stratification probably accentuated by the high turbidity of the water and therefore rapid attenuation of solar energy. This stratified pattern breaks down at night due to atmospheric cooling and the regular onset of winds in the early evening. The phytoplankton is dominated byMicrocystis aeruginosa with some associated epiphytes. It concentrates in the narrow euphotic zone during the diurnal period of stratification due to buoyancy of theMicrocystis; evening breakdown of the thermocline results in the phytoplankton being mixed throughout the water column. A series of measurements of photosynthesis throughout the diurnal period gives an areal rate of 3.8 g O₂ m⁻² d⁻¹. The relationship between this value and the level of fish exploitation in Lake Baringo is discussed. The diel cycle in Lake Baringo is interpreted as dominating over any seasonal limnological cycle in the lake.     

Five new microsatellite polymorphisms at the q21 region of human chromosome 21

Five clones, containing polymorphic CA-repeat sequences, have been isolated from a specific human chromosome 21 phage library and have been localised to band q21 of chromosome 21 using a somatic cell hybrid panel. These highly repetitive sequences (D21S1263, D21S1264, D21S1415, D21S1417 and D21S1420) have been characterised in the CEPH reference parents and have heterozygosities ranging from 0.30 to 0.81 and an average polymorphism information content (PIC) of 0.62. The relative order of these markers, based on the somatic cell hybrid panel, is cen-D21S1417, D21S1420-D21S1263, D21S1415-D21S1264-tel. The most polymorphic marker (D21S1264) has been included in the chromosome 21 genetic map. They have also been localised in the CEPH/ Généthon YAC panel, providing a refined localisation of these polymorphic sequences. These five CA-repeat markers should provide a better characterisation of the q21 region of chromosome 21.     

Genetics of high level penicillin resistance in clinical isolates of Streptococcus pneumoniae

Abstract Mosaic penicillin-binding proteins (PBP) 1A, 2X and 2B genes were cloned from four clinical isolates of Streptococcus pneumoniae with levels of susceptibility to penicillin ranging from 1.5 to 16 μg benzylpenicillin ml⁻¹. In each instance it was possible to transform either the penicillin-sensitive laboratory strain R6 or a sensitive clinical isolate 110K/70 to the full level of penicillin resistance with these three penicillin-binding proteins alone. Until now it has not been possible to clearly determine whether alterations to PBP1A, 2X and 2B alone were sufficient to attain high level penicillin resistance.     

A minimal,compartmental model for a dendritic origin of bistability of motoneuron firing patterns

Various nonlinear regenerative responses, including plateau potentials and bistable repetitive firing modes, have been observed in motoneurons under certain conditions. Our simulation results support the hypothesis that these responses are due to plateau-generating currents in the dendrites, consistent with a major role for a noninactivating calcium L-type current as suggested by experiments. Bistability as observed in the soma of low- and higher-frequency spiking or, under TTX, of near resting and depolarized plateau potentials, occurs because the dendrites can be in a near resting or depolarized stable steady state. We formulate and study a two-compartment minimal model of a motoneuron that segregates currents for fast spiking into a soma-like compartment and currents responsible for plateau potentials into a dendrite-like compartment. Current flows between compartments through a coupling conductance, mimicking electrotonic spread. We use bifurcation techniques to illuminate how the coupling strength affects somatic behavior. We look closely at the case of weak coupling strength to gain insight into the development of bistable patterns. Robust somatic bistability depends on the electrical separation since it occurs only for weak to moderate coupling conductance. We also illustrate that hysteresis of the two spiking states is a natural consequence of the plateau behavior in the dendrite compartment.     

The plasma protease inhibitor system (Pi) of Standardbred horses

The plasma protease inhibitor system (Pi) of Standardbred horses was studied by thin-layer, high-voltage, acid polyacrylamide gel electrophoresis (pH 4.6) followed by protein staining and staining for trypsin and chymotrypsin inhibition. In addition to the eight Thoroughbred alleles (PiF, G, I, L, N, S1, S2, U), another 10 alleles, designated PiH, J, K, O, P, Q, R, V, X, Z, were postulated to account for the 98 Pi types which were observed in Standardbreds. Detailed inhibitory spectra of the 'new' alleles were determined and further exceptions to the Pi1, Pi2 classification of Juneja et al. (1979) were found. Limited family data demonstrated the genetic nature of the 'new' variants and confirmed the allelic inheritance of the 'new' Pi variants.