Effect of somatomedin A and insulin on cyclic AMP generation in isolated rat hepatocytes

Insulin and somatomedin A were shown to have inhibitory action on glucagon stimulated but not basal cyclic AMP production in isolated rat hepatocytes. The inhibition was dose-dependent and the potency per mol was about 100 fold higher for insulin than for somatomedin A.     

Purification and some properties of the phosphoprotein from rat incisors.

The phosphoprotein of rat incisors has been purified by successive gel and ion-exchange chromatography. The product gave a single band on polyacrylamide gel electrophoresis and contained approximately 34% phosphoserine and 32% aspartic acid. Alkaline elimination experiments showed all the phosphate to be present as phosphoserine. Ultraviolet spectra in the presence or absence of ATP showed that the phosphoprotein did not contain an nucleotide moiety as suggested by Veis, A., Spector, A. R. and Zamoscianyk, H. ((1972) Biochim. Biophys. Acta 257, 404-413) for bovine dentin phosphoprotein.     

Purification and physicochemical properties of superoxide dismutase from two photosynthetic microorganisms.

Superoxide dismutase (EC 1.15.1.1) has been isolated and characterised from the blue-green alga Spirulina platensis and from aerobically-grown Rhodopseudomonas spheroides, a purple, non-sulphur bacterium. The former enzyme contains 1 gatom of iron and the latter 1 gatom of manganese per mol; both enzymes have a molecular weight of 37 000-38 000, being composed of two non-covalently joined subunits of equal size. Various spectral studies have been carried out including absorbance, circular dichroism and electron spin resonance. Catalytic activity has been studied as a function of pH and shows a decrease at alkaline pH values. The manganoenzyme is generally more stable to various potentially denaturing conditions and is resistant to inactivation by hydrogen peroxide. Amino acid compositions and N-terminal residue determinations are presented.     

Hormonal mechanisms for differentiation in plant tissue culture

Summary Cells possess extraordinary powers to organize their molecular processes not only to maintain a cell in a given steady state but also to recognize that state during differentiation. Regulation of these organizational forces appears to be under the control of chemical factors, and a hormonal concept of regulation has evolved. Hormones have been considered to act by reacting with a specific target site. This may be part of their mode of action, but I would like to suggest that a hormone enters and becomes part of a total molecular resonance system. In so doing, the entire molecular system of the cell is modified. Of the known plant hormones, the cytokinins, because of their role in experimentally induced cell division and differentiation, serve as a probe of hormonal involvement in differentiation. Cultured somatic cells of tobacco plants can be induced to undergo differentiation by addition of cytokinin and auxin to the medium. Studies of the cytokinin hormones show a series of diverse molecular involvements. The archetype cytokinin, N⁶-(Δ²-isopentenyl) adenosine (i⁶Ado), occurs in some molecular species of tRNA where it plays a vital role in the codon-anticodon interaction of tRNA and m-RNA. i⁶Ado under-goes extensive metabolism in the tobacco tissue. It is either degraded to adenosine or converted to derivatives that possess biological activity. It is perhaps, therefore, more correct to consider the hormone function as being derived from this total metabolic web. The normal somatic cells of tobacco cultures spontaneously change occasionally into an autonomous form that requires no external growth factors. This line of cells synthesizes i⁶Ado. The metabolic web of the hormone-dependent strain can be perturbed by added auxin but such is not the case in the autonomous strain. These data provide some insight into the altered state of cytokinin activity in which a cell line changes into an autonomous form. Curiously, in become independent of the requirement for exogenous cytokinin, the autonomous tissue becomes sensitive to added cytokinin. i⁶Ado also inhibits the growth of lines of mammalian cancer cells grown in culture. Presented in the formal symposium on Information Transfer in Eukaryotic Cells, at the 26th Annual Meeting of the Tissue Culture Association, Montreal, Quebec, June 2–5, 1975.     

Acetylcholine receptors in normal and denervated rat diaphragm muscle. I. Purification and interaction with [125I]-alpha-bungarotoxin.

Acetylcholine receptors have been purified from junctional regions of normal rat diaphragm muscle and from extrajunctional regions of denervated diaphragm. The reaction of purified receptors with [122I]-alpha-bungarotoxin has been investigated by kinetic methods. The toxin-receptor complexes dissociated in a biphasic manner at 35 degrees with a rapidly dissociating component (t1/2 = 4 hr) and a slowly dissociating component (t1/2 is greater than or equal to 100 hr). The association reaction between toxin and receptor did not obey simple second-order kinetics but could be analyzed in terms of two classes of binding sites corresponding to the two rates of dissociation. This treatment of the data allowed derivation of association rate constants for the two sites. Value obtained for the dissociation constants were 3.7 times 10(-10) and less than or equal to 0.4 times 10(-10) M for the junctional receptor and 1.7 times 10(-10) and is less than or equal to 0.2 times 10(-10) M for the extrajunctional receptor. In each case it is the more tightly binding component that associates and dissociates more slowly. Receptors present in crude preparations were comparable to purified receptors in their reaction with [125I-alpha-bungarotoxin. The validity of the two site model is discussed in relation to the kinetic studies.     

Acetylcholine receptors in normal and denervated rat diaphragm muscle. II. Comparison of junctional and extrajunctional receptors.

Acetylcholine (ACh) receptors have been purified separately from normal rat diaphragm muscle (junctional receptors) and from extrajunctional regions of denervated diaphragm (extrajunctional receptors) in order to compare their properties. The toxin-receptor complexes of the two receptors were indistinguishable by gel filtration and by zone sedimentation in sucrose gradients, and showed identical precipitation curves with rabbit antiserum to the eel ACh receptor. Both toxin-receptor complexes bind concanavalin A and are therefore probably glycoproteins. Low concentrations of d-tubocuratine (dTC) were more effective in decreasing the rate of toxin binding to junctional than to extrajunctional receptors. The apparent dissociation constant for dTC binding to the junctional receptor was 4.5 X 10 minus 8 M, whereas the value for the extrajunctional receptor was 5.5 X 10 minus 7 M. When the complexes were analyzed by isoelectric focusing, the junctional complex focused at approximately 0.15 pH unit lower than the extrajunctional complex. This result was also found with crude preparations of receptor. We conclude that junctional and extrajunctional receptors are similar but distinct molecules. The properties of receptors present in neonatal diaphragm muscle were also examined and found to be similar to those of receptors in denervated muscle, as shown by dTC inhibition and isoelectric focusing.     

Oscillation phenomena in black lipid membranes induced by a single alamethicin pore

In this paper we show how alamethicin (a small cyclic peptide of molecular weight 1691) can produce voltage oscillations in black lipid membranes and how a nonactin-alamethicin oscillator can be constructed. Alamethicin alone induces oscillations only with an applied bias current, but with nonactin and appropriate salt solutions oscillations occur with no bias current. Both kinds of oscillations can be quantitatively understood in terms of the known properties of alamethicin and nonactin and both depend on the statistical nature of the formation of pores in the membrane by alamethicin.     

Methyl galactosidase activity: an alternative evolutionary destination for the ebgA0 gene.

Previous studies (Campbell et al., 1973; Hall and Hartl, 1974; Hall and Hartl, 1975) have shown that the ebgA0 gene, whose product does not hydrolyze lactose may evolve so that its product does hydrolyze lactose; i.e., lactase activity is one evolutionary destination of the ebgA0 gene. Beginning with a strain that synthesizes ebgA0 gene product constitutively and grows extremely slowly (doubling time, 30 to 50 h) on methyl-beta-D-galactopyranoside (MG), a derivative was selected capable of growth on MG at a moderate rate (doubling time, 5.9 h). Genetic evidence is presented showing that the gene that permits growth on MG is an allele of ebgA. A comparison among strains bearing several alleles of ebgA shows that the new allele, termed ebgAmg, synthesizes a product specific for MG and thus represents a true alternative evolutionary destination for the ebgA0 gene.