Cyproheptadine metabolites inhibit proinsulin and insulin biosynthesis and insulin release in isolated rat pancreatic islets

The contribution of drug metabolites to cyproheptadine (CPH)-induced alterations in endocrine pancreatic -cells was investigated by examining the inhibitory activity of CPH and its biotransformation products, desmethylcyproheptadine (DMCPH), CPH-epoxide and DMCPH-epoxide, on hormone biosynthesis and secretion in pancreatic islets isolated from 50-day-old rats. Measurement of (pro)insulin (proinsulin and insulin) synthesis using incorporation of ³H-leucine showed that DMCPH-epoxide, DMCPH and CPH-epoxide were 22, 10 and 4 times, respectively, more potent than CPH in inhibiting hormone synthesis. The biosynthesis of (pro)insulin was also inhibited by CPH and DMCPH-epoxide in islets isolated from 21-day-old rat fetuses. The inhibitory action of CPH and its metabolites was apparently specific for (pro)insulin, and the synthesis of other islet proteins was not affected. Other experiments showed the metabolites of CPH were active in inhibiting glucose-stimulated insulin secretion but were less potent than the parent drug in producing this effect. CPH and its structurally related metabolites, therefore, have differential inhibitory activities on insulin synthesis and release. The observation that CPH metabolites have higher potency than CPH to inhibit (pro)insulin synthesis, when considered with published reports on the disposition of the drug in rats, indicate that CPH metabolites, particularly DMCPH-epoxide, are primarily responsible for the insulin depletion observed when the parent compound is given to fetal and adult animals.Abbreviations CPH cyproheptadine - CPH-epoxide cyproheptadine-10-11-epoxide - DMCPH desmethylcyproheptadine - DMCPH-epoxide desmethylcyproheptadine-10,11-epoxide - HPLC high-performance liquid chromatography - KBB Krebs biocarbonate buffer Recipient of a Society of Toxicology Predoctoral Research Fellowship.Present address: Department of Biochemistry, The University of Hong Kong, Hong Kong.     

Synthesis of polypeptides by microwave heating II. Function of polypeptides synthesized during repeated hydration-dehydration cycles

Summary Polypeptides, synthesized from a mixture of amino acid amides by microwave heating during repeated hydration-dehydration cycles, showed hydrolase- and oxidoreductase-like catalytic activities. Poly(GAVDH), polypeptides synthesized from an equimolar mixture (each 0.1 M) of glycinamide,l-alaninamide,l-valinamide,l-aspartic acid -amide, andl-histidinamide, catalyzed the hydrolysis of PNPAc. The hydrolytic rate of PNPAc with poly(GAVDH) was the quadruple of that ofl-histidine alone. Though the k_cat values of different resulting polypeptides were 10³–10⁶ times less than those of native hydrolases, the K_m value of the polypeptides further containing phenylalanine residues was nearly equal to that of the esterase. This result indicates the presence of hydrophobic interaction between a substrate and the polypeptides. Resulting polypeptides also showed catalytic activity for the reduction of ferricyanide ion [Fe(CN)^3–] with NADH. The polypeptides seemed to have a strong affinity for adenine nucleotides, because the reaction was inhibited by adenine derivatives such as NAD⁺ and AppA. A hypothesis for the emergence of primitive protein enzymes is discussed.     

Effect of oxygen pressure on synthesis and export of nitrogenous solutes by nodules of cowpea

Nodules of cowpea plants (Vigna unguiculata (L.) Walp. cv. Vita 3 :Bradyrhizobium CB756) cultured for periods of 23 d with their root systems maintained in atmospheres containing a range of partial pressures of O₂ (pO₂; 1–80%, v/v, in N₂) formed and exported ureides (allantoin and allantoic acid) as the major products of fixation at all pO₂ tested. In sub-ambient pO₂ (1 and 2.5%) nodules contained specific activities of uricase (urate: O₂ oxidoreductase; EC 1.7.3.3) and allantoinase (allantoin hydrolyase; EC 3.5.2.5) as much as sevenfold higher than in those from air. On a cell basis, uninfected cells in nodules from 1% O₂ contained around five times the level of uricase. Except for NAD: glutamate synthase (EC 1.4.1.14), which was reduced in sub-ambient O₂, the activities of other enzymes of ureide synthesis were relatively unaffected by pO₂. Short-term effects of pO₂ on assimilation of fixed nitrogen were measured in nodules of air-grown plants exposed to subambient pO₂ (1, 2.5 or 5%, v/v in N₂) and¹⁵N₂. Despite a fall in total¹⁵N₂ fixation, ureide synthesis and export was maintained at a high level except in 1% O₂ where formation was halved. The data indicate that in addition to the structural and diffusional adaptations of cowpea nodules which allow the balance between O₂ supply and demand to be maintained over a wide range of pO₂, nodules also show evidence of biochemical adaptations which maintain and enhance normal pathways for the assimilation of fixed nitrogen. This work was supported by a grant from the Australian Research Council (to C.A.A.) and an Australian Development Assistance Bureau postgraduate fellowship (to F.D.D.).     

Current perceptions of Photosystem II

In the last few years our knowledge of the structure and function of Photosystem II in oxygen-evolving organisms has increased significantly. The biochemical isolation and characterization of essential protein components and the comparative analysis from purple photosynthetic bacteria (Deisenhofer, Epp, Miki, Huber and Michel (1984) J Mol Biol 180: 385–398) have led to a more concise picture of Photosystem II organization. Thus, it is now generally accepted that the so-called D1 and D2 intrinsic proteins bind the primary reactants and the reducing-side components. Simultaneously, the nature and reaction kinetics of the major electron transfer components have been further clarified. For example, the radicals giving rise to the different forms of EPR Signal II have recently been assigned to oxidized tyrosine residues on the D1 and D2 proteins, while the so-called Q₄₀₀ component has been assigned to the ferric form of the acceptor-side iron. The primary charge-separation has been meaured to take place in about 3 ps. However, despite all recent major efforts, the location of the manganese ions and the water-oxidation mechanism still remain largely unknown. Other topics which lately have received much attention include the organization of Photosystem II in the thylakoid membrane and the role of lipids and ionic cofactors like bicarbonate, calcium and chloride. This article attempts to give an overall update in this rapidly expanding field.     

急性缺氧性缺氧时心脏功能的改变

F_(IO_2)(吸入气氧浓度)为12.35、9.87及7.7l％,分别吸入10、8及5min时,心功能呈代偿性增强改变。F_(IO_2)为9.37％、吸入20min时心功能的变化趋势与9.87％8min时仍基本相同。继发性缺二氧化碳对缺氧引起的心功能代偿性增强,在一定程度上起抵消作用。F_(IO_2)为9.87％时的缺氧程度约相当于18km高空加压供氧总压值为15.3kPa(115mmHg)时的缺氧。单纯从缺氧因素考虑,将总压值由常用的17.3kPa(130mmHg)降低为15.3kPa是可允许的。     

Calcium-dependent Modulation of the Agonist Affinity of the Mammalian Olfactory Cyclic Nucleotide-gated Channel by Calmodulin and a Novel Endogenous Factor

The calcium-dependent modulation of the affinity of the cyclic nucleotide-gated (CNG) channels for adenosine 3′,5′-cyclic monophosphate (cAMP) was studied in enzymatically dissociated rat olfactory receptor neurons, by recording macroscopic cAMP-activated currents from inside-out patches excised from their dendritic knobs. Upon intracellular addition of 0.2 mm Ca²⁺ (0.2 Ca) the concentration of cAMP required for the activation of half-maximal current (EC₅₀) was reversibly increased from 3 μm to about 30 μm. This Ca²⁺-induced affinity shift was insensitive to the calmodulin antagonist, mastoparan, was abolished irreversibly by a 2-min exposure to 3 mm Mg²⁺+ 2 mm EGTA (Mg + EGTA), and was not restored by the application of calmodulin (CAM). Addition of CAM plus 0.2 mm Ca²⁺ (0.2 Ca + CAM), further reversibly shifted the cAMP affinity from 30 μm to about 200 μm. This affinity shift was not affected by Mg + EGTA exposure, but was reversed by mastoparan. Thus, the former Ca²⁺-only effect must be mediated by an unknown endogenous factor, distinct from CAM. Removal of this factor also increased the affinity of the channel for CAM. The affinity shift induced by Ca²⁺-only was maintained in the presence of the nonhydrolyzable cAMP analogue, 8-bromo-cAMP and the phosphatase inhibitor, microcystin-LR, ruling out modulation by phosphodiesterases or phosphatases. Our results indicate that the olfactory CNG channels are modulated by an as yet unidentified factor distinct from CAM. Received: 26 December 1995/Revised: 14 March 1996     

Evolutionary analysis of the multigene pregnancy-specific β1-Glycoprotein family: Separation of historical and nonhistorical signals

The pregnancy-specific ₁-glycoproteins (PSG) form a large family of closely related proteins. Using newly developed methods of sequence analysis, in combination with protein modeling, we provide a framework for investigating the evolution and biological function of genes like the PSG. Evolutionary trees, based on C-terminal sequence, group PSG genes in a manner consistent with their genomic organization. Trees constructed using the N-terminal domain sequences are unreliable as an indicator of phylogeny because of non-neutral processes of sequence change. During duplication of the PSG genes, evolutionary pressures have resulted in a gradient of constrained change across each gene. The N-terminal domains show a nonrandom pattern of amino acid substitutions clustered in the immunoglobulin complementarity-determining region (CDR)-like regions, which appear to be important in the function of the protein.     

Overexpression of MerT, the mercuric ion transport protein of transposon Tn501, and genetic selection of mercury hypersensitivity mutations

The small (116 amino acids) inner membrane protein MerT encoded by the transposon Tn501 has been overexpressed under the control of the bacteriophage T7 expression system. Random mutants of MerT were made and screened for loss of mercuric ion hypersensitivity. Several mutantmerT genes were selected and sequenced: Cys24Arg and Cys25Tyr mutations abolish mercury resistance, as do charge-substitution mutations in the first predicted transmembrane helix (Glyl4Arg, Glyl5Arg, Gly27Arg, Ala18Asp), and the termination mutations Trp66Ter and Cys82Ter.     

Effects of sound direction on the processing of amplitude-modulated signals in the frog inferior colliculus

Single-unit recordings were made from 143 neurons in the frog (Rana p. pipiens) inferior colliculus (IC) to investigate how free-field sound direction influenced neural responses to sinusoidal-amplitude-modulated (SAM) tone and/or noise. Modulation transfer functions (MTFs) were derived from 3 to 5 sound directions within 180° of frontal field. Five classes of MTF were observed: low-pass, high-pass, band-pass, multi-pass, and all-pass. For 64% of IC neurons, the MTF class remained unchanged when sound direction was shifted from contralateral 90° to ipsilateral 90°. However, the MTFs of more than half of these neurons exhibited narrower bandwidths when the loudspeaker was shifted to ipsilateral azimuths. There was a decrease in the cut-off frequency for neurons possessing low-pass MTFs, an increase in cut-off frequency for neurons showing high-pass MTFs, or a reduction in the pass-band for neurons displaying bandpass MTFs. These results suggest that sound direction can influence amplitude modulation (AM) frequency tuning of single IC neurons.Since changes in periodicity of SAM tones alter both the temporal parameters of sounds as well as the sound spectrum, we examined whether directional effects on spectral selectivity play a role in shaping the observed direction-dependent AM selectivity. The directional influence on AM selectivity to both SAM tone and SAM noise was measured in 62 neurons in an attempt to gain some insight into the mechanisms that underlie directionally-induced changes in AM selectivity. Direction-dependent changes in the shapes of the tone and noise derived MTFs were different for the majority of IC neurons (55/62) tested. These data indicate that a spectrally-based and a temporally-based mechanism may be responsible for the observed results.Abbreviations AM amplitude modulation - CF characteristic frequency - DI direction index - FR isointensity frequency response - GABA gamma-aminobutyric acid - IC inferior colliculus - ICc central nucleus of the inferior colliculus - ITD interaural time difference - MTF modulation transfer function - PSTH peri-stimulus time histogram - SAM sinusoidal-amplitude-modulated - SC synchronization coefficient - CN cochlear nucleus