Dynamics of ligand binding to myoglobin.

Myoglobin rebinding of carbon monoxide and dioxygen after photodissociation has been observed in the temperature range between 40 and 350 K. A system was constructed that records the change in optical absorption at 436 nm smoothly and without break between 2 musec and 1 ksec. Four different rebinding processes have been found. Between 40 and 160 K, a single process is observed. It is not exponential in time, but approximately given by N(t) = (1 + t/to)-n, where to and n are temperature-dependent, ligand-concentration independent, parameters. At about 170 K, a second and at 200 K, a third concentration-independent process emerge. At 210 K, a concentration-dependent process sets in. If myoglobin is embedded in a solid, only the first three can be seen, and they are all nonexponential. In a liquid glycerol-water solvent, rebinding is exponential. To interpret the data, a model is proposed in which the ligand molecule, on its way from the solvent to the binding site at the ferrous heme iron, encounters four barriers in succession. The barriers are tentatively identified with known features of myoglobin. By computer-solving the differential equation for the motion of a ligand molecule over four barriers, the rates for all important steps are obtained. The temperature dependences of the rates yield enthalpy, entropy, and free-energy changes at all barriers. The free-energy barriers at 310 K indicate how myoglobin achieves specificity and order. For carbon monoxide, the heights of these barriers increase toward the inside; carbon monoxide consequently is partially rejected at each of the four barriers. Dioxygen, in contrast, sees barriers of about equal height and moves smoothly toward the binding site. The entropy increases over the first two barriers, indicating a rupturing of bonds or displacement of residues, and then smoothly decreases, reaching a minimum at the binding site. The magnitude of the decrease over the innermost barrier implies participation of heme and/or protein. The nonexponential rebinding observed at low temperatures and in solid samples implies that the innermost barrier has a spectrum of activation energies. The shape of the spectrum has been determined; its existence can be explained by assuming the presence of many conformational states for myoglobin. In a liquid at temperatures above about 230 K, relaxation among conformational states occurs and rebinding becomes exponential.     

Equation for model intracranial aneurysm with consideration of small dissipation term

An electric circuit model of a ‘model intracranial aneurysm’ has been developed from previous experimental work showing a non linear pressure-volume relationship. This leads to a revised formulation of the following non linear differential equation which simulates the response of the model aneurysm to a pulsatile driving function

$\text{V}\text{?}\text{+?}\text{υ}\text{?}\text{+ζmυ+ξnυ}{}^2\text{=F cos(ωt + B)}$

, where V = velocity of flow within the aneurysm.Solution of this equation using the principle of harmonic balance shows that discontinuous jumps in amplitude of velocity of flow leading to turbulence or increased turbulence can occur with an increase in pulse rate or pulse pressure. It is postulated that turbulence is dangerous due to both the large increase in velocity of flow and also due to the possible stasis effect on large molecules and platelets from eddy formation.     

SOME PHYSICAL PROPERTIES OF BOVINE ADRENAL MEDULLARY DOPAMINE β-HYDROXYLASE

(1) Dopamine, β-hydroxylase (EC 1.14.2.1) was purified from bovine adrenal medullae according to the method of Foldes , Jeffrey , Preston and Austin (1972). (2) The kinetics, pH optimum and the effect of Cu²⁺ ions on the purified enzyme were found to resemble those of the enzyme isolated by more involved procedures. (3) The sedimentation coefficient (s₂₀) of the homogeneous enzyme in 10 mM-phosphate buffer, pH 7·2, containing 0·1 M-NaCI was found to be 10·24 ± 0·12 (S.E.M. of 10 determinations). (4) The effect of pH on the mol. wt. of the enzyme was investigated and no large deviation was found from the native mol. wt. of 290,000 in the pH range 3·9 to 11·1. (5) The amino acid analysis of dopamine β-hydroxylase is presented, and is contrasted to that of chromogranin A purified from the same chromaffin granule lysate. (6) Treatment with either 8 M-urea or 0·1% (w/v) sodium dodecyl sulphate was found to dissociate the enzyme into three similar, non-active subunits, each of mol. wt. of the order of 100,000.     

The turnover of protein in discrete areas of rat brain

1. Rats were injected serially with [(14)C]glucose to obtain a constant specific radioactivity of brain amino acids. Measurements with this system for periods of up to 8h gave an apparent mean half-life for protein in whole brain of 85h (indicating the presence of a protein fraction with much more rapid turnover than this). 2. The half-lives of proteins in the granule-cell, molecular and white-matter layers of cerebellum were also determined. These had values of 33, 59 and 136h respectively. In addition, the incorporation into protein in six layers of the cerebral cortex, subjacent white matter and five layers of Ammon's horn was studied. All cell-body layers incorporated amino acids at about the same rate irrespective of location, and these rates were considerably higher than those for incorporation into proteins in areas rich in dendrites or fibre tracts. 3. A new method for measuring small amounts of glutamate with a cyclic enzyme system is presented.     

Secretion of Dopa in Phaeochromocytoma

In a series of 11 cases with phaeochromocytoma, two patients were found in whom the tumour secreted dopa in addition to dopamine, noradrenaline, and adrenaline. These two patients were normotensive in spite of high plasma and urinary levels of noradrenaline and adrenaline. This raises the possibility that dopa may be able to protect against the hypertensive action of noradrenaline. Such a mechanism would also explain the absence of hypertension in many cases of neuroblastoma.     

Interaction of apoprotein from porcine high-density lipoprotein with dimyristoyl lecithin. 1. The structure of the complexes.

The morphology and structural organisation of the complexes formed from the apoprotein of porcine high-density lipoprotein and dimyristoyl phosphatidylcholine (lecithin) have been studied using the technique of small-angle X-ray scattering. Scattering measurements made in solvents of varying electron density were interpreted in terms of a scattering-equivalent model for the structure of the complex. This model is described by an oblate ellipsoidal morphology with dimensions at 20 degrees C: major axis 11.0 nm, minor axis 5.5 nm. Within this overall shape the lipid hydrocarbon chains are organised in an apolar core whilst the lipid polar head groups and protein are located in a outer shell 0.85 nm in thickness. The oblate morphology demonstrates that the structure of the complex is directed by the fundamental bilayer organisation of the lecithin. The dimension of the minor axis (5.5 nm) indicates that phospholipid hydrocarbon chains are orientated perpendicular to the interface.     

Intra-specific fusions in Solanum tuberosum

Summary Plants were regenerated from callus arising from protoplast fusion of two S. tuberosum diploids. Tetraploid progeny from the fusion of the two diploid partners had increased vigor. Isozyme analysis confirmed the presence of proteins from both partners in the fusion progeny. Pigmentation of tubers and anthers was heightened substantially in the fusion products. This fusion, the first intra-specific fusion within S. tuberosum, indicates that somatic fusion may be useful for transferring traits within this group.     

Studies in metachromatic leukodystrophy: XV. Purification of normal and mutant arylsulfatase A from human liver

In this report we describe a method to purify both normal and abnormal (inactive) arylsulfatase A. The abnormal enzyme protein was isolated both from cases of late infantile and early juvenile forms of metachromatic leukodystrophy. Conventional protein isolation methods reported earlier were followed by size exclusion high-performance liquid chromatography in the final purification stages. Both the mutant enzyme and the normal enzyme had the same HPLC elution behavior. They thus appeared to self-associate in a similar pH-dependent fashion. Both could be followed by their reaction to a rabbit antibody to normal human arylsulfatase A. The amount of homogenous protein obtained from about 500 grams of liver was 300-400 micrograms.