Rabbit muscle phosphofructokinase. Modification of molecular and regulatory kinetic properties with the affinity label 5'-p-(fluorosulfonyl)benzoyl adenosine.

The affinity label 5'-p-(fluorosulfonyl)benzoyl adenosine modifies rabbit muscle phosphofructokinase to the extent of one group/subunit. Modification appears to occur at a binding site specific for AMP, cyclic AMP, and ADP, i.e. those adenine nucleotides which are activators under conditions where regulatory kinetic behavior is obtained. The consequences of the modification are consistent with the model proposed previously for correlation between the pK of specific ionizable groups, regulatory kinetic behavior, ligand binding, and the reversible cold inactivation of the enzyme (Frieden, C., Gilbert. H. R., and Bock, P. E. (1976) J. Biol. Chem. 251, 5644-5647). Thus, the modification shifts the apparent pK of the essential ionizable groups from 6.9 to 6.4 at 25 degrees C, with the result that regulatory kinetic behavior at pH 6.9 and 25 degrees C is lost. Furthermore, the apparent affinity of a site (other than the active site) for ATP, as measured by ATP-dependent quenching of intrinsic protein fluorescence at pH 6.9 and 25 degrees C, is decreased by the modification. Regulatory kinetic behavior for both substrates is obtained with the modified enzyme at a lower pH, consistent with the downward shift in the pK of the ionizable groups, but sensitivity to cAMP activation is abolished by the modification. The loss of regulatory kinetic behavior upon modification of sulfhydryl groups does not appear to be the same as that due to modification by the affinity label.     

EFFECTS OF ARSENATE ON GROWTH OF NITROGEN-AND PHOSPHORUS-LIMITED CHLORELLA VULGARIS (CHLOROPHYCEAE) ISOLATES1

The effects of arsenate on the growth characteristics of five isolates of the freshwater alga, Chlorella vulgaris Beij., were examined. Two field isolates originated from arsenic-contaminated sites in Yukon, Canada and Kyushi, Japan; two reference isolates were obtained from the University of Texas Culture Collection. One isolate was selected for arsenic-tolerance in the laboratory. All five strains survived in culture solutions containing high arsenate concentrations. Arsenate (1–25 mM As) reduced photosynthesis and cell growth, as reflected by induced lag periods, slower growth rates, and lower stationary cell yields. Field isolates had shorter lag periods, higher growth rates, and enhanced cell yields compared to lab isolates when exposed to the same arsenic concentrations. Growth of the phosphorus-limited field strains was stimulated by the addition of arsenic. The cell yield of phosphorus-limited C. vulgaris Yukon, when treated with arsenic, was two times that of the phosphorus-limited control. This pattern was not evident when photosynthesis was used as a measure of cell response.     

Cardiovascular effects of training for a marathon run in unfit middle aged men.

The effects of a 30 week exercise programme on serum lipid values, blood pressure, and cardiac function were assessed in a group of sedentary men aged 35-50 training for their first marathon. Mean serum cholesterol concentration (n = 33) fell by 12% from 6.54 (SE 0.18) to 5.76 (0.15) mmol/l (mean fall 0.78 mmol/l; 95% confidence interval 0.52 to 1.04 mmol/l), serum triglyceride concentration (n = 33) by 22% from 1.56 (0.17) to 1.21 (0.09) mmol/l (mean fall 0.34 mmol/l; 95% confidence interval 0.12 to 0.56 mmol/l), and mean blood pressure (n = 27) by 10% from 102 (2) to 92 (2) mm Hg (mean fall 10 mm Hg; 95% confidence interval 7 to 13 mm Hg). These changes were not explained by changes in body composition. Peak exercise left ventricular end diastolic volume (n = 16) increased with training; as a result of this and an increased exercise left ventricular ejection fraction peak exercise cardiac output increased from 19.9 (1.2) to 23.1 (3.0) l/min (mean rise 3.2 l/min; 95% confidence interval 1.5 to 5.0 l/min). Maximum oxygen consumption increased from 33.9 (1.6) to 39.0 (1.3) ml/kg/min (mean rise 5.0 ml/kg/min; 95% confidence interval 1.8 to 8.2 ml/kg/min). This study showed favourable effects on coronary risk factors and cardiac function and supports the place of regular exercise in coronary prevention programmes.     

Development of ionic conductances in neurons of the inferior olive in the rat: an in vitro study

Neurons of the inferior olive of the rat were studied at different stages of their postnatal (PN) development by using the current clamp technique in slices maintained in vitro. Antidromic and synaptic activation of inferior olivary neurons could be achieved in preparations as young as PN day 2. Neurons at this age already exhibited a variety of ionic conductances which included fast sodium-dependent spikes, high-threshold and low-threshold calcium spikes, potassium-dependent currents, Ca-dependent after-hyperpolarizing potentials (AHPS), and both instantaneous and time-dependent inward rectification at hyperpolarized levels of membrane potential. The two types of Ca-dependent responses recorded in olivary neurons during the first postnatal week were graded with the magnitude of the depolarization imposed on the cells. Furthermore, the high-threshold Ca spikes were only clearly observed during this early period when K conductances were depressed by the injection of caesium into the cells or by bath application of 4-aminopyridine. In contrast, the high-threshold Ca spikes could be obtained without suppression of K currents and were all-or-none in character in some neurons after PN day 8 and in all neurons after PN day 11. The observations suggest that the balance between K and Ca currents changes throughout maturation and is largely in favour of the K current until about the end of the first PN week. At all ages studied, the low-threshold Ca spikes were much less sensitive to the Ca channel blocker cadmium than were the high-threshold Ca spikes. Finally, spontaneous, regular oscillations of the membrane potential were observed for the first time at PN day 16 and were only commonly observed after PN day 19, suggesting a late development of electrotonic coupling between olivary neurons.     

Restricted range of ocular accommodation in barn owls (Aves:Tytonidae)

Summary In an examination of the focusing abilities of 15 species of owls, the North American barn owl, Tyto alba pratincola (Bonaparte 1838), was an outstanding accommodator, having a range of accommodation exceeding 10 diopters (Murphy and Howland 1983). Using comparable methods, we examined the accommodation of 4 specimens of the Australian barn owl, Tyto alba delicatula (Gould 1837). We failed to elicit accommodation greater than two diopters, and most stimuli failed to evoke any discernable accommodation at all. Furthermore, examination of other Australian tytonid owls, the grass owl, T. longimembris, the sooty owl, T. tenebricosa, and both the mainland and Tasmanian subspecies of the masked owl, T. novaehollandiae novaehollandiae and T. novaehollandiae castanops, also failed to reveal anything but very moderate accommodative ranges. We conclude that the outstanding accommodative ability of the American barn owl is truly an exception to the modest accommodative abilities of the tytonid owls generally.     

Bovine liver dihydropyrimidine amidohydrolase: pH dependencies of inactivation by chelators and steady-state kinetic properties

Dihydropyrimidine amidohydrolase (EC 3.5.2.2) catalyzes the reversible hydrolysis of 5,6-dihydropyrimidines to the corresponding beta-ureido acids. Previous work has shown that incubation of this Zn2+ metalloenzyme with 2,6-dipicolinic acid, 8-hydroxyquinoline-5-sulfonic acid, or o-phenanthroline results in inactivation by Zn2+ removal by a reaction pathway involving formation of a ternary enzyme-Zn2+-chelator complex which subsequently dissociates to yield apoenzyme and the Zn2+-chelate (K. P. Brooks, E. A. Jones, B. D. Kim, and E. G. Sander, (1983) Arch. Biochem. Biophys. 226, 469-483). In the present work, the pH dependence of chelator inactivation is studied. The equilibrium constant for formation of the ternary complex is strongly pH dependent and increases with decreasing pH for all three chelators. There is a positive correlation between the value of the equilibrium constant observed for each chelator and the value of its stability constant for formation of Zn2+-chelate. The affinity of the chelators for the enzyme increases in the order 8-hydroxyquinoline-5-sulfonic acid greater than o-phenanthroline greater than 2,6-dipicolinic acid. The first-order rate constant for breakdown of the ternary complex to yield apoenzyme and Zn2+-chelate is invariant with pH for a given chelator but is different for each chelator, increasing in the reverse order. The pH dependence of the inactivation shows that two ionizable groups on the enzyme are involved in the inactivation. On the other hand, the steady-state kinetic behavior of the enzyme is well-described by ionization of a single group with a pK of 6.0 in the free enzyme. The basic form of the group is required for catalysis; protonation of the group decreases both Vmax and the apparent affinity for substrate. Conversely, binding of substrate decreases the pK of this group to about 5. L-Dihydroorotic acid is shown to be a competitive inhibitor of dihydropyrimidine amidohydrolase. Binding of L-dihydroorotic acid increases the pK of the ionizable group to 6.5. The agreement between the pK in the enzyme-L-dihydroorotic acid complex and the higher pK observed in the pH dependence of inactivation by chelators suggests that the same group is involved in the binding of acid, and chelators. The different effects of substrate and L-dihydroorotic acid on the pK suggest that the binding modes of these two ligands may be different and suggest a structural basis for the mutally exclusive substrate specificities of dihydropyrimidine amidohydrolase and dihydroorotase.     

Structural basis for the variation of pH-dependent redox potentials of Pseudomonas cytochromes c-551

The redox potentials of many c-type cytochromes vary with pH over the physiological pH range. We have investigated the pH dependence of redox potential for the four homologous cytochromes c-551 from Pseudomonas aeruginosa, Pseudomonas stutzeri strain 221, Pseudomonas stutzeri strain 224, and Pseudomonas mendocina . The pH dependence is due to an ionizable group that ionizes with pKox in ferricytochrome c-551 but with a higher pK, pKred , in ferrocytochrome c-551. For P. aeruginosa cytochrome c-551 it has been shown that this ionizable group is one of the heme propionic acid substituents [Moore, G. R., Pettigrew , G. W., Pitt , R. C., & Williams, R. J. P. (1980) Biochim. Biophys. Acta 590, 261-271]but the values of pKox and pKred are significantly lower in this protein than in the other three cytochromes. NMR and chemical modification studies show that for the two P. stutzeri cytochromes c-551 and P. mendocina cytochrome c-551, this propionic acid substituent is again important for the pH dependence of the redox potential. However, a histidine occurring at position 47 in their sequences hydrogen bonds to the propionic acid and thereby raises its pK. In P. aeruginosa cytochrome c-551, His-47 is substituted by Arg-47. Hydrogen-bonding schemes involving His-47 and the propionic acid are proposed.     

The purification and amino acid sequence of cytochrome c-552 from Euglena gracilis

Cytochrome c-552 from Euglena gracilis was purified and the amino acid sequence determined. The protein is a single peptide chain of 87 residues with the haem prosthetic group bound through two thioether linkages to two cysteine residues near the amino-terminal region. The amino acid sequence shows some similarities to mitochondrial cytochrome c and to two prokaryote c-type cytochromes. The sequence, taken with the known characteristics of cytochrome c-552, indicates that it is an f-type cytochrome. The possible functional and evolutionary significance of these features in common is discussed. Detailed evidence for the amino acid sequence of Euglena cytochrome f has been deposited as Supplementary Publication SUP 50027 at the British Library, Lending Division (formerly the National Lending Library for Science and Technology), Boston Spa, Yorks. LS23 7QB, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1973) 131, 5.