The chemistry of vitamin B 12. The co-ordination of biologically important molecules

The following equilibrium constants (given as logK in units of m(-1)) were determined for the substitution of co-ordinated H(2)O in aquocobalamin by glycine (bound through N) 5.8, cysteine (bound through S) 6.0 or 8.3, depending on the value chosen for the pK of the thiol group, and phenolate 2.9. The spectrum of the phenolate cobalamin shows an additional intense absorption band at 468nm with a molar extinction coefficient of 1.1x10(4), which is assigned to a charge transfer from the phenolate to the cobalt ion. Equilibrium constants have also been determined for the equilibria between adenylcobamide cyanide and CN(-), HO(-) and H(+), which show that the adenine is more easily displaced by CN(-) and HO(-) than is 5,6-dimethylbenziminazole in vitamin B(12), but can be protonated by acid while still remaining co-ordinated to the cobalt. It is shown that in the binding of corrinoids to proteins and polypeptides the formation of hydrogen bonds is far more important than co-ordination by the metal.

Site of origin of the hormone responsible for gastric hypersecretion associated with portacaval shunt

Previous studies in our laboratory established that the intestinal phase of gastric secretion and the acid hypersecretion associated with portacaval shunt are mediated by a potent hormone in both dogs and humans. The present investigation was designed to localize the site of origin of the intestinal phase hormone by measuring the 5 1/2 hour gastric secretory response to the instillation of a standard meal into isolated segments of jejunum, ileum and colon before and after portacaval shunt. Twenty-four dogs with Thirty-Villa fistulas and Heidenhain pouches underwent 48 intestinal meal tests prior to portacaval anastomosis, and 46 tests following shunt. Instillation of food into the isolated jejunum produced substantial gastric secretion before portacaval shunt, and sustained and highly significant acid hypersecretion after shunt (p = < 0.001). The magnitude of the hypersecretion averaged over six times the basal rate of acid production. Food in the isolated ileum stimulated modest gastric secretion in unshunted dogs, but portacaval shunt failed to enhance acid output. Food in the colon did not stimulate gastric acid production in either unshunted or shunted dogs. It is concluded that the jejunum is the major source of the hormone responsible for the intestinal phase of gastric secretion and for the profound acid hypersecretion associated with portacaval shunt.

Freezing injury and resistance in spinach chloroplast grana

Spinach grana appear to be injured by the same mechanism and by the same degree of dehydration and volume reduction that injures animal cells. Winter-hardened or artificially protected grana avoid injury by permitting a reversible influx of solute which forestalls excessive dehydration and shrinkage.

Analysis of the visible spectra of some sperm-whale ferrimyoglobin derivatives

The visible spectra of sperm-whale metmyoglobin and its fluoride, hydroxide, cyanide, azide, cyanate, formate, acetate, thiocyanate and nitrite derivatives were measured and resolved into Gaussian components. A linear correlation between the intensity of the band at 18500cm.(-1) and the magnetic susceptibility is found, suggesting that the band is the beta-band of the low-spin form. The analysed intensity of this band can be used to determine the relative proportions of the two spin states present. The spectra of the pure high-spin and low-spin components of each derivative can be reconstructed. Correlations between the energy of the near-infrared band and the visible bands of the high-spin form, and between the position and intensity of the 16000cm.(-1) band, are demonstrated. The relationship between the position of the Soret band and the magnetic susceptibility can be explained in terms of the equilibrium between spin states.

The biosynthesis of cyclic carotenes

1. The incorporation of (3RS)-[2-(14)C,(4R)-4-(3)H(1)]mevalonic acid into various cyclic carotenes in the fruit of the tomato mutant delta has been studied. The results confirm our previous view that the alpha-ionone ring of alpha-carotene does not arise by isomerization of a beta-ionone residue, and show that the same is also true for the alpha-ionone ring of delta- and in-carotene and alpha-zeacarotene. 2. The incorporation of (3RS)-[2-(14)C,2-(3)H(2)]mevalonic acid into alpha- and beta-carotene in carrot roots has been studied. The results show that the beta-ionone ring of beta-carotene does not arise by isomerization of the alpha-ionone residue of alpha-carotene. 3. These experiments show that alpha- and beta-ionone rings in cyclic carotenes are formed independently, probably by elimination of different protons from the same carbonium ion intermediates.

The stereospecific biosynthesis of phytoene and polyunsaturated carotenes

1. The incorporation of [2-(14)C,(5R)-5-(3)H(1)]mevalonic acid and [2-(14)C,5-(3)H(2)]-mevalonic acid into phytoene, phytofluene, zeta-carotene, neurosporene, alpha-, beta-, gamma- and delta-carotene and lycopene by slices of fruit from two tomato mutants (delta and tangerine) and into alpha- and beta-carotene by bean leaves has been studied. 2. In the formation of phytoene, all the pro-R-hydrogen atoms from C-5 of mevalonic acid are retained whereas two pro-S-hydrogen atoms are lost. 3. Possible mechanisms for the condensation of two molecules of all-trans-geranylgeranyl pyrophosphate are outlined. 4. In each dehydrogenation step from phytoene to the fully unsaturated carotenes, one pro-R-hydrogen atom from C-5 of mevalonic acid is lost, indicating that the sequential dehydrogenation is stereospecific and in the same sense at each step.