Interaction of a phosphorylated site of muscle phosphofructokinase with the activation of the enzyme by NH4+ and K+.1

The crystal structure of the cyclic peptide disulfide

has been determined by X-ray diffraction. The peptide crystallizes in the space group P2₁2₁2₁, with a = 8.646(1), b = 18.462(2), c = 19.678(3)Å and Z = 4. The molecules adopt a highly folded compact conformation, stabilized by two intramolecular 4→ 1 hydrogen bonds between the Cys (1) and Pro (2) CO groups and the Cys (4) and methylamide NH groups, respectively. The backbone conformational angles for the peptide lie very close to those expected for a 3₁₀ helix. The S-S bridge adopts a right handed twist with a dihedral angle of 82°. The structure illustrates the role of stereochemically constrained residues, in generating novel peptide conformations.     

The sulfhydryl groups of the thiol-dependent cytolytic toxin from Bacillus alvei evidence for one essential sulfhydryl group

Alveolysin, an extracellular protein toxin (M_r ? 63,000) excreted by Bacillus alvei and purified to homogeneity was shown to contain four cysteine residues. All thiol groups of the hemolytically active toxin preparation were free as found by direct titration by 5,5′-dithiobis (2-nitrobenzoic acid) and confirmed by the absence of disulfide bond. Toxin alkylation with tosyl lysine chloromethyl ketone resulted in the complete loss of hemolytic activity and the disappearance of only one thiol group with no modification of histidine residues. These results support the conclusion that one essential thiol group is implicated in the membrane-disrupting activity of alveolysin.     

A respiration-dependent primary sodium extrusion system functioning at alkaline pH in the marine bacterium Vibrioalginolyticus

The membrane potential generated at pH 8.5 by K⁺-depleted and Na⁺-loaded $\text{Vibrio}\text{alginolyticus}$ is not collapsed by proton conductors which, instead, induce the accumulation of protons in equilibrium with the membrane potential. The generation of such a membrane potential and the accumulation of protons are specific to Na⁺-loaded cells at alkaline pH and are dependent on respiration. Extrusion of Na⁺ at pH 8.5 occurs in the presence of proton conductors unless respiration is inhibited while it is abolished by proton conductors at acidic pH. The uptake of α-aminoisobutyric acid, which is driven by the Na⁺-electrochemical gradient, is observed even in the presence of proton conductors at pH 8.5 but not at acidic pH. We conclude that a respiration-dependent primary electrogenic Na⁺ extrusion system is functioning at alkaline pH to generate the proton conductor-insensitive membrane potential and Na⁺ chemical gradient.     

Unusual patterns of benzo[a]pyrene metabolites and DNA-benzo[a]pyrene adducts produced by human placental microsomes in vitro

Human placental microsomes were incubated with [³H]benzo[a]pyrene (BP) and Salmon sperm DNA and the resulting metabolite-nucleoside complexes resolved by Sephadex LH-20 chromatography. The metabolite pattern was analyzed by high-pressure liquid chromatography (HPLC). The incubates were also co-chromatographed with extracts obtained from incubates with rat liver microsomes and [¹⁴C]BP. Phenols, quinones and 7,8-dihydrodiol were detected in the placental incubates. Both 9,10- and 4,5-dihydrodiols were very low as compared with control rat liver samples. Placental microsomes catalyzed the binding of BP metabolites to DNA in vitro, giving rise to two main complexes which co-chromatographed with rat liver-produced peaks attributable to 7,8-diol-9,10-epoxide and 7,8-oxide and/or quinones when metabolized further. The nucleoside metabolite peaks attributable to 4,5-oxide and 9-phenol-4,5-oxide were lacking when compared with the binding pattern catalyzed by rat liver. Both the total binding and specific metabolite-nucleoside adducts in the placenta correlated with fluorometrically measured aryl hydrocarbon hydroxylase (AHH) activity and with the amount of dihydrodiol formed. The results demonstrate that both the metabolite pattern and the nucleoside-metabolite complexes formed by the placental microsomes in vitro differed greatly from those produced by rat liver microsomes. These studies also suggest that it is not possible to predict specific patterns of DNA binding from AHH measurements or even from BP metabolite patterns, especially when comparing different tissues and species.