Electric field-induced orientation of l- and dl-phosphatidylcholine bilayers

Membrane orientation induced by an alternating electric field has been examined for the l-enantiomer and racemic dipalmitoylphosphatidylcholine (DPPC) bilayers. The orientation effect was measured by bending curvature of hairpin-like deformation of the multilamellar cylindrical tubes with varying field-strength, frequency and tube size. It has been observed that both l- and dl-DPPC tubes are similar in the profiles of field-strength dependence and frequency dependence on the curvature deformation, but different in the deformed curvatures. dl-DPPC tubes deform largely as compared with l-DPPC tubes. The square of the deformed curvature of dl-DPPC tubes is larger than that of l-DPPC by about 37% on average. The result indicates that the racemic membrane is responsive to the electric field as compared with the l-enantiomer membrane. This suggests that a hybrid arrangement of head groups of the racemic lipid leads an effective response of the membrane due to the head group orientation.     

Ionic control of renal gluconeogenesis IV. Effect of extracellular phosphate concentration

Isolated rat renal tubules from glucose from pyruvate, malate, glycerol and α-ketoglutarate. The rate of glucose formation from all but glycerol is enhanced by an increase in Ca²⁺ concentration. Because changes in inorganic phosphate concentrations influence the uptake and retention of calcium by isolated cells, the effect of changes in phosphate concentration upon renal gluconeogenesis was examined. It was found that changing phosphate concentration altered the metabolism of isolated rat renal tubules in three ways which dependend upon the Ca²⁺ concentration. In the absence of Ca²⁺, increasing phosphate concentration from 0.07 to 1.2 mM led to a stimulation of the decarboxylation of [U-¹⁴C]malate, [1-14C]pyruvate, [2-¹⁴C]-pyruvate, α-keto[5-¹⁴C]glutarate and [1,3-¹⁴C₂]glycerol, and to an increase in ATP concentration but had no effect upon the rate of glucose formation from malate, pyruvate, α-ketoglutarate but a slight stimulation of glucose production from glycerol. A further increase in phosphate above 1.2 mM had no effect on any of these parameters. In the presence of either low (0.2 mM) or high (2.0 mM) Ca²⁺, changing phosphate concentration had no effect upon the decarboxylation of any of these substrates except glycerol whose decarboxylation was stimulated by increasing medium phosphate concentration. In the presence of calcium, increasing phosphate concentration led to an inhibition of glucose formation from malate, pyruvate and α-ketoglutarate but not from glycerol. Also in the presence of calcium both parathyroid hormone and cyclic AMP stimulated glucose formation, and under these conditions increasing phosphate concentration led to an inhibition of glucose formation. In tubules treated with parathyroid hormone an increase in phosphate concentration from 0.07 to 6.0 mM led to a significant increase in cyclic AMP concentration even though the rate of glucose formation decreased.Analysis of metabolite concentrations and rates of substrates decarboxylations, under a variety of conditions, revealed that P_i altered renal gluconeogenesis at a site different from those controlled by changes in Ca²⁺ concentration. The P_i-control site was tentatively identified as the glyceraldehyde phosphate dehydrogenase-glycerate kinase reaction sequence. However, the effect of changing P_i concentration upon parathyroid hormone-induced alterations in cyclic AMP concentration could not be explained by this action of P_i, and was probably due to an effect of P_i upon cellular calcium distribution. Thus, changes in P_i concentration appear to have two cellular effects, only one of which is related to a change in cellular calcium metabolism.     

Further Evidence for Inactivation of Fructose-1,6-bisphosphatase at the Beginning of SO2 Fumigation. Increase in Fructose-1,6-bisphosphate and Decrease in Fructose-6-phosphate in SO2-Fumigated Spinach Leaves

The substrate level of the photosynthetic reductive pentosephosphate cycle in spinach leaves during SO₂ fumigation wassurveyed. At the beginning of SO₂ fumigation, fructose-1,6-bisphosphateincreased and fructose-6-phosphate decreased, while ribulose-1,5-bisphosphateremained unchanged and 3-phosphoglyceric acid rapidly decreased.These results suggested that the inhibition of photosynthesisin spinach leaves with SO₂ might be due to inactivation of fructose-1,6-bisphosphatase. (Received May 26, 1982; Accepted September 27, 1982)     

A Simple Bile-Disk Method for the Identification of Bacteroides fragilis

A simple method for the separation and identification of Bacteroides fragilis from other Bacteroides species through the use of oxgall-impregnated filter paper disks is described. As in the antibiotic sensitivity test, filter paper disks containing defined amounts of bile salt were placed on a lawn of bacterial cells on Gifu anaerobic medium agar plates. Bile-resistant bacteria were identified as strains of Bacteroides fragilis after incubation in anaerobic jars containing hydrogen and carbon dioxide generators at 35 C for 24 hr. The optimum concentration of bile salt, yielding results very similar to those of the conventional tube method, was determined to be 25 mg per disk, empirically. Since this method is easy to perform, time saving, economical and gives clear results, it may be readily used as a daily routine test in the clinical microbiology laboratory.