Amine transport at the plasmalemma of Riccia fluitans

Green thallus cells of the aquatic liverwort, Riccia fluitans, are rapidly depolarized in the presence of 1–20 μM NH₄Cl and 5–100 μM CH₃NH₃Cl, respectively. Simultaneously, the membrane conductance is increased from 0.41 to 1.2 S · m^?2. Uptake of [¹⁴C]methylamine is stimulated by increasing [K⁺]_o and inhibited by increasing [Na⁺]_o or [H⁺]_o, is highly voltage sensitive, and saturates at low amine concentrations.Double-reciprocal plots of (a) maximal membrane depolarization and (b) methylamine uptake vs. external amine concentration give apparent K_m values of 2 ± 1 μM ammonia and 25–50 μM methylamine; K_m values for changes in conductance and membrane current are greater and voltage dependent. Whereas the amine transport into the cell is strongly inhibited by CN^?, the amine efflux is stimulated.The current-voltage characteristics of the ammonia transport are represented by a sigmoid curve with an equilibrium potential of ?60 mV, and this is understood as a typical carrier curve with a saturation current of about 70 mA · m^?2. It is further concluded that the evidently carrier-mediated transport is competitive for the two amines tested, and that ammonia and methylamine are transported in the protonated form as NH₄⁺ and CH₃NH₃⁺ into the cytoplasm.     

Biliary lipid secretion in hypercholesterolemia.

A report on the effects of primary bile acid ingestion alone or in combination with plant sterols on serum cholesterol levels, biliary lipid secretion, and bile acid metabolism. Biliary bile acid and cholesterol secretion were measured in four patients with type IIa hypercholesterolemia before and after randomized treatment periods. During these periods either a bile acid mixture (cholic-chenodeoxycholic 2:1, a proportion similar to that endogenously synthesized in health), at a level of 20 mg/kg, or the same mixture plus sitosterols, 200 mg/kg, was fed. Serum cholesterol and the cholesterol saturation of fasting-state bile was also measured. Pretreatment biliary lipid secretion was within normal limits. Bile acid kinetic measurements were also recorded before treatment and showed that cholic acid synthesis was disproportionately decreased relative to that of chenodeoxycholic acid, a finding previously reported by others. Administration of the bile acid mixture increased biliary bile acid secretion in 3 of 4 patients, but did not influence biliary cholesterol secretion. The combination of sitosterol-bile acid, however, caused a relative decrease in cholesterol secretion in bile, and fasting-state bile became unsaturated in all patients. No change in fecal neutral sterol excretion occurred during the beta-sitosterol-bile acid regimen, suggesting that simultaneous bile acid feeding blocks the compensatory increase in cholesterol synthesis known to be induced by beta-sitosterol feeding in hypercholesterolemic patients. Serum cholesterol levels also fell modestly during the sitosterol-bile acid regimen, the decrease averaging 15%. We conclude that the abnormally low rate of bile acid synthesis in patients with type IIa hyperlipoproteinemia does not influence biliary lipid secretion; that increasing the input of the two primary bile acids into the enterohepatic circulation does not increase biliary cholesterol secretion or lower serum cholesterol levels in such patients; and that the usual increase in cholesterol synthesis induced by beta-sitosterol feeding does not occur if bile acids are administered simultaneously.     

Régulation de la biosynthèse de l'aspartate-ammonium lyase (aspartase) de Pseudomonas fluorescens

Variations in aspartasic activity in various media are due to aspartate-ammonium lyase induction and to regulation of the biosynthesis of this enzyme. Evidence for neosynthesis of the enzyme is provided by labelling and separation of the protein. The inducer appears to be aspartic acid. The biosynthesis is subject to pronounced catabolic repression. The physiological function of aspartate-ammonium lyase is discussed.     

Innervation of the cultured fetal rat pancreas

Summary The fetal rat pancreas, explanted at 18 days of gestation and cultured up to ten days, contains numerous acetylcholinesterase-positive neurons. These nerves usually appear in small ganglia although single nerve cells are encountered. The axons of these intrapancreatic nerves appear to terminate only in the islet tissue and not on any exocrine components of the expiant. It is concluded that the fetal rat pancreas contains an islet-specific group of cholinergic neurons.We gratefully acknowledge the skilled technical assistance of Dan Whitehead and the secretarial assistance of Mary Pat Brady     

Alkaloids of Papaver bracteatum: 14-β-hydroxycodeinone, 14-β-hydroxycodeine and N-methylcorydaldine

Three new alkaloids have been identified from Papaver bracteatum, 14-β-hydroxycodeinone, 14-β-hydroxycodeine and N-methylcorydaldine. The presence of alpinigenine was also confirmed.     

Purification and properties of pig kidney glutamate dehydrogenase.

Glutamate dehydrogenase from pig kidney has been purified to homogeneity by means of affinity chromatography on matrix bound Cibacron Blue F3G-A and gel chromatography on Sepharose 6B. The enzyme exhibits allosteric properties with the substrates alpha-ketoglutarate, ammonium, and NADH, respectively. GTP is a strong inhibitor which strengthened the cooperative interactions between the ammonium binding sites. ADP as an activator relieves the inhibition by GTP. Like glutamate dehydrogenase from bovine liver, glutamate dehydrogenase from pig kidney shows the ability of self-association, too. The sedimentation coefficient increases from 13.5 S at 0.07 mg protein/ml to 19.4 S at 1.32 mg protein/ml. In the sodium dodecylsulphate gel electrophoresis the enzyme migrates as a single band with a molecular-weight at 51000.     

The influence of chronic ethanol feeding to rats on the integrity of liver mitochondrial membrane as assessed with the Mg2+-stimulated ATPase enzyme.

The mouse submaxillary proteases (A + D), the isolation and properties of which were previously described by us, hydrolyze only arginyl bonds in proteins. Formol titrations and peptide mapping on digests of polyarginine, polylysine, lysozyme, histone, and insulin suggested this specificity. Amino acid compositions of peptides from lysozyme and insulin showed that most but not all arginyl bonds were hydrolyzed but that no lysyl bonds were split. The proteases should be useful in the sequencing of proteins.     

Fructose-2,6-bisphosphate increases the binding affinity of yeast phosphofructokinase to AMP

At saturating concentrations of AMP four molecules of this ligand are bound per octamer of yeast phosphofructokinase. Fructose-2,6-bisphosphate increases the binding affinity of the enzyme to AMP. This indicates synergistic cooperation of the two allosteric activators in the binding process. The stoichiometry of binding is not altered by fructose-2,6-bisphosphate.