Metabolic signals produced by purine ribonucleosides stimulate proinsulin biosynthesis and insulin secretion.

Inosine, guanosine and adenosine strongly stimulated proinsulin biosynthesis and insulin secretion in isolated mouse pancreatic islets. None of the purine ribonucleosides stimulated insulin secretion in rat islets, although as reported [jain & Logothetopoulos (1977) Endocrinilogy 100, 923-927] inosine and guanosine, but no adenosine, were potent stimulants of proinsulin biosynthesis in this species. The purine bases had no effect in either species. D-Ribose, which enhanced proinsulin biosynthesis at 0.3 and 0.6 mM but not at 5mM in rat pancreatic islets [jain & Logothetopoulos (1977) Endocrinology 100, 923-927], produced no secretory signals in rat islets and was without any effect on proinsulin biosynthesis and insulin secretion in mouse islets. The rates of oxidation of 14C-labelled purine ribonucleosides and D-ribose in islets of the two species correlated well with their effectiveness as inducers of insulin secretion and proinsulin biosynthesis. Specific inhibitors of purine ribonucleoside phosphorylase, adenosine deaminiase and of purine ribonucleoside transport suppressed the stimulatory effects of nucleosides in pancreatic islets without altering the effect of D-glucose. The same inhibitors also markedly diminished the oxidation rats of the labelled purine ribonucleosides. The experiments clearly indicate that porinsulin biosynthesis and insulin secretion are modulated through metabolic signals and not through interactions of intact substrate molecules with cell receptors.     

Visualization of drug-nucleic acid interactions at atomic resolution: II. Structure of an ethidium/dinucleoside monophosphate crystalline complex,ethidium:5-iodocytidylyl (3′–5′) guanosine

Ethidium forms a second crystalline complex with the dinucleoside monophosphate 5-iodocytidylyl(3′–5′)guanosine (iodoCpG). These crystals are monoclinic, P2₁, with $\text{a = 14.06}\text{A}\text{?}\text{, b = 32.34}\text{A}\text{?}\text{, c = 16.53}\text{A}\text{?}\text{, β = 117.8 °}$. The structure has been solved to atomic resolution using rigid-body Patterson vector search and Fourier methods, and refined by full matrix least-squares to a residual of 0.16 on 3180 observed reflections. The structure consists of two ethidium molecules, two iodoCpG molecules, 27 water molecules and four methanol molecules, a total of 165 atoms (excluding hydrogens) in the asymmetric unit. Both iodoCpG molecules are hydrogen-bonded together by guanine · cytosine Watson-Crick base-pairing. Adjacent base-pairs within this paired iodoCpG structure and between neighboring iodoCpG molecules in adjoining unit cells are separated by 6.7 Å. This distance reflects the presence of an ethidium molecule intercalated between base-paired iodoCpG molecules and another ethidium molecule stacked above (and below) the dinucleotide. Approximate 2-fold symmetry is used in the interaction; this reflects the pseudo-2-fold symmetry axis of the phenanthridinium ring system in ethidium coinciding with the approximate 2-fold axis relating base-paired iodoCpG molecules. The phenyl and ethyl groups of the intercalated ethidium molecule lie in the narrow groove of the miniature iodoCpG double-helix. The stacked ethidium, however, lies in the opposite direction, its phenyl and ethyl groups neighboring iodine atoms on cytosine residues. Base-pairs within the paired nucleotide units are related by a twist of about 8 °. The magnitude of this angular twist reflects conformational changes in the sugar-phosphate chains accompanying intercalation. These primarily reflect the differences in ribose sugar ring puckering that are observed (i.e. both iodocytidine residues have C3′ endo sugar conformations, while both guanosine residues have C2′ endo sugar conformations), and alterations in the glycosidic torsional angles that describe the base-sugar orientation.The information provided by this structure analysis (along with the accompanying one (ethidium:iodoUpA), described in the previous paper) has led to an understanding of the general nature of intercalative drug binding to DNA. This is described in the third paper of this series.     

An automatic method for determination of urinary 3-methylhistidine: normal values.

A system for automatic analysis of urinary 3-methylhistidine is described, applying ion-exchange chromatography and using an automatic sample injector, a motoric selector valve, and a diode programmer, which controls the analytical system. The method permits a sampling rate of 22 samples/day. 3-Methylhistidine was completely separated from histidine in 37 min whereas 1-methylhistidine was eluted together with ammonia. The 3-methylhistidine concentration was linear up to 150 nmol/ml and no appreciable sample interaction was found at automatic sequential runs. The error, in a single determination based on duplicate samples, was 4.61% and, in duplicated determinations, 3.26%. The mean urinary 3-methylhistidine output was 299.4 ± 23.8 μmol/day in 12 healthy females and 545.5 ± 35.2 μmol/day in 12 healthy males. The 3-methylhistidine excretion was significantly higher in males than in females, when expressed as the absolute daily output or as the estimated ratio to body weight, body surface area, or creatinine.     

Charge-transfer complexes of some linear conjugated polyenes.

On adsorption of some electron-acceptor molecules on the solid films of all-trans-beta-carotene, beta-apo-8'-carotenal, astacene and methylbixin a new absorption band appears on the longer-wavelength side of the spectrum in addition to the original bands. The position of this new band is dependent on the electron affinity (EA) of the acceptor molecules, and the intensity of this band increases with the amount of adsorbed acceptor molecules. A linear relationship between the vmax. of the new band and EA was observed. The value of the ionization potential of the polyenes estimated from such linear relationship agrees satisfactorily with the value obtained by other methods. It has been concluded that the polyenes behave as electron donor and first form molecular charge-transfer complexes (of type [polyene . I2] with iodine) with electron acceptors, these finally dissociating to yield ionic complexes (of type [polyene . I+] with iodine).     

Phosphatidylcholesterol bilayers. A model for phospholipid-cholesterol interaction

Aqueous dispersions of monovalent and divalent cation salts of O-(1,2-dipalmitoyl-sn-glycero-3-phosphoryl) cholesterol form multilamellar vesicles as shown by freeze-fracture electron microscopy, by electron micrographs of the negatively stained liposomes, and by swelling curves of liposomes in hypoosmotic medium. Differential scanning calorimetry reveals that aqueous dispersions of divalent metal salts of O-(1,2-dipalmitoyl-sn-glycero-3-phosphoryl)-cholesterol undergo a characteristic thermotropic phase transition with a relatively large cooperative unit (n > 250 for the calcium salt). In contrast, monovalent cation salts of O-(1,2-dipalmitoyl-sn-glycerol-3-phosphoryl)cholesterol do not show a thermotropic phase transition under comparable conditions. The molecular area of O-(1,2-dipalmitoyl-sn-glycero-3-phosphoryl)cholesterol in a monolayer is the same in the presence and absence of Ca²⁺, and is virtually equal to the area of an equimolar mixture of dipalmitoyl phosphatidic acid and cholesterol. To account for the novel state induced by Ca²⁺ on aqueous dispersions of O-(1,2-dipalmitoyl-sn-glycero-3-phosphoryl)cholesterol (i.e., bilayer organization and highly cooperative phase transition), a linear array model is proposed in which Ca²⁺ bridges adjacent arrays of O-(1,2-dipalmitoyl-sn-glycero-3-phosphoryl)cholesterol molecules, thus freezing the acyl chains in their normal state. One of the main corollaries of the model is that the cooperative unit for a thermotropic phase transition is essentially one-dimensional, rather than a two-dimensional matrix. O-(1,2-Dipalmitoyl-sn-glycero-3-phosphoryl)cholesterol is proposed as an orientationally and conformationally restricted analog of glycerophospholipid plus cholesterol in bilayers.