Enzymes of glyoxylate in conifers

The high level of lipids in seeds of some species of conifers suggested that the glyoxylate cycle might have a role in conifer seed metabolism.

Six species (Pinus pinea, Pinus pinaster, Pinus canariensis, Pinus strobus, Abies alba, and Cupressus sempervirens) were investigated for their lipid content and malate synthase and isocitrate lyase level. The fatty acid composition of the triglyceride fraction was also investigated. The correlation between lipid content of germinating seed with the presence of the cycle was confirmed. The enzymes of the glyoxylate cycle were not detected in Cupressus sempervirens where the lipid content is very low.

     

Muscarinic inhibition of high-voltage-activated calcium channels in excised membranes of rat hippocampal neurons

The effect of muscarine on voltage-gated calcium channels was investigated in outside-out patches from rat hippocampal neurons in culture. By clamping the excised patches at –60 mV holding potential, single and multiple Ca channel currents were recorded, and these displayed features similar to the high-voltage-activated Ca current, with unitary conductance of 6.4 pS in 50 mM external Ca²⁺. These channels turned out to be insensitive both to Bay K 8644 and to -conotoxin. In excised patches muscarine caused a marked reduction in the probability of opening of this class of Ca channels without significant changes in the unitary current amplitude. Interestingly, the degree of current inhibition was reduced by depolarization, thus suggesting a voltage-dependent inhibitory action of the agonist. We conclude that in hippocampal neurons one of the possible ways of HVA Ca channel modulation by muscarine occurs through activation of a substratum localized within the plasma membrane of the cell, independent of the involvement of diffusible intracellular messengers. Correspondence to: M. Toselli     

The interaction of 3-phosphoglycerate and other substrate analogs with the glyoxylate- and succinate-binding sites of isocitrate lyase

The gene for isocitrate lyase from Escherichia coli has recently been cloned and sequenced. However, knowledge of this enzyme from E. coli is limited. Because of the possible role of 3-phosphoglycerate as a metabolic inhibitor of isocitrate lyase in E. coli, a detailed analysis of this compound as an inhibitor is reported in this paper. Kinetic data suggest that 3-phosphoglycerate is an analog of isocitrate (or glyoxylate) and also that it competes with succinate, or succinate analogs, by interfering with their binding to the enzyme. This could be due to the steric bulk of the phosphate moiety of 3-phosphoglycerate extending in the direction of and over the succinate-binding site. The interaction of other substrate analogs, including glycolate, oxalate, phosphoenolpyruvate, and cis-aconitate, with isocitrate lyase from E. coli is also characterized.     

D-glucose uptake in intestinal brush-border membrane vesicles of rachitic rats

We have previously reported the metabolic consequences of feeding rats Steenbock and Black's rickets-inducing diet, deficient in vitamin D and with an altered Ca/P ratio. Using isolated brush-border membrane vesicles prepared from the jejunum, ileum and duodenum of control and rachitic rats, we have demonstrated a marked decrease of Na+-dependent D-glucose uptake at jejunum-ileum level of rachitic rats. At duodenum level Na+-dependent D-glucose transport was not influenced by rickets. A lack of any significant difference between the two animal groups was observed studying the facilitated transport of D-glucose, the diffusion of L-glucose and the Na+-dependent uptake of phenylalanine and aspartate.     

Comparative study of the kinetic properties of the neutral maltase of human granulocytes and of the kidney maltase of the rat

Neutral maltase from human granulocytes has a different substrate specificity from the human neutral maltase of kidney, though it has been reported that these two enzymes are immunologically similar. We report here that human granulocyte neutral maltase is similar to the neutral maltase from rat's kidney as regards the substrate specificity and the inhibition by Tris and maltodextrins. We also report a different thermal stability that might imply some structural differences between the two enzymes.     

Identification of a novel mouse Dbl proto-oncogene splice variant: Evidence that SEC14 domain is involved in GEF activity regulation

The Rho guanine nucleotide exchange factor protoDbl is involved in different biochemical pathways affecting cell proliferation and migration. The N-terminal sequence of protoDbl contains negative regulatory elements that restrict the catalytic activity of the DH-PH module. Here, we report the identification of a new mouse protoDbl splice variant lacking exon 3. We found that the splice variant mRNA is expressed in the spleen and bone marrow lymphocytes, adrenal gland, gonads and brain. The protoDbl variant protein was detectable in the brain. The newly identified variant displays the disruption of the SEC14 domain, positioned on exons 2 and 3 in the protoDbl N-terminal region. We show here that an altered SEC14 sequence leads to enhanced Dbl translocation to the plasma membrane and to augmented transforming and exchange activity.