Summing up: some implications of the neuron as a secreting cell

‘Believing that the nervous system is something more than a mere system of conducting paths, I formed the hypothesis that nerve cells are true secreting cells, and act upon one another and upon the cells of other organs by the passage of a chemical substance of the nature of a ferment or proferment’ (Scott 1905). ‘The most striking morphological feature of the neuron is the tremendous accumulation within its cytoplasm of small granules associated with a well developed endoplasmic reticulum. The same type of association is found in the ergastoplasm of glandular cells... cells which sustain an intense protein production. In the nerve this activity is implicit in chromatolysis and to certain types of generalized stress. The rapid regeneration of axons and the peculiar damming up of axoplasm proximal to a ligature are also reflexions of a continuous and rapid protein synthesis in the perikaryon.... The fact that the structure of the Nissl substance is the same as that of the ergastoplasm in glandular cells means that future analysis... in such readily available cells as those of the pancreas and liver can be profitably applied to the nerve cell’ (Palay & Palade 1955). ‘Neurosecretion should not perhaps be used as a term to describe only the histochemically demonstrable secretory processes of nerves such as those of the hypothalamo-pituitary system. The analogies are sufficient to indicate that similar processes are involved in the production, transport and secretion of acetylcholine from other nerve endings. So these, too, may also be called neurosecretory nerves’ (Hebb 1959). ‘All neurons have a secretory function by which active substances are synthesized and released. Secretion may act over a short distance on specific chemical receptors or on distant receptors by way of the blood stream. Intermediary examples are the adrenergic neuro-effectors ending on smooth muscle. Neurosecretion may be produced all along the neuron or at the nerve endings. In all cases, it is stored within a membrane in vesicles which represent multimolecular quantal units of neurosecretion’ (De Robertis 1964).

Secretion of proteins (chromogranin A and dopamine -hydroxylase) from a sympathetic neuron

Biochemical studies on adrenal chromaffin granules have shown that these particles contain specific soluble proteins, the chromogranins, (see Kirshner & Kirshner 1971) and specific membrane proteins, the chromomembrins (see Winkler 1971). Although the functions of the main components of the chromogranins are not known, the ability to identify these proteins by biochemical and immunochemical methods led directly to the discovery of the release of chromogranins upon stimulation of the adrenal medulla (for reviews see Kirshner & Kirshner 1971; Smith & Winkler 1971) and to the identification of two of these proteins (chromogranin A and dopamine β-hydroxylase) in sympathetic nerves (see Livett, Geffen & Rush 1971).

Linoleate metabolism in multiple sclerosis

(1) The levels of oleate and linoleate in the serum have been measured in 14 patients with multiple sclerosis (MS) and in 14 healthy subjects before, during and after a five day period when the normal diet was supplemented with linoleate. (2) In confirmation of earlier work the mean percentage of linoleate in the serum lipids of the MS patients was significantly lower (P < 0·01) than in the control subjects in the pre-supplementation period. The mean percentage of oleate showed an increase (P < 0·005) in the patients as compared with the controls while on their normal diets. (3) The period of linoleate feeding produced a considerable rise in the percentage of linoleate together with a fall in the percentage of oleate in both the controls and the MS patients. (4) When large amounts of linoleate, as present in sunflower seed oil, are ingested we have been unable to obtain evidence of a defect in absorption from the intestinal lumen.

Civilian vascular injuries: a critical appraisal of three decades of management

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Fatty acid composition of phospholipids from platelets and erythrocytes in multiple sclerosis

The fatty acid composition of the phospholipids of red blood cells and blood platelets has been investigated in multiple sclerosis patients and in normal individuals. The variation in the platelet phospholipid fatty acid pattern in normals has been measured for the first time and has been shown to be small. The relative level of linoleate, expressed as a percentage of the five main fatty acids, was found to be significantly lower in the multiple sclerosis patients than in healthy individuals, both in the red cells and platelets. A highly significant correlation was found between serum linoleate and both platelet and red cell linoleate.

Membranes of the adrenal medulla. Behaviour of insoluble proteins of chromaffin granules on gel electrophoresis

Washed membranes of bovine adrenal chromaffin granules contained most of the cholesterol and phospholipids of the particle and 22% of the total protein. The protein/lipid ratio was about 0.45 (w/w). Dopamine(3,4-dihydroxyphenethylamine)beta-hydroxylase, Mg(2+)-activated nucleoside triphosphatase and cytochrome b-559 activities were present in the membrane. ATP was the best substrate for the nucleoside triphosphatase, whose pH optimum was 6.4, K(m) 7x10(-4)m and V(max.) 1.8mumol/h per mg of protein. Treatment of the membranes with various detergents caused a preferential solubilization of protein compared with lipids. Membranes dissolved in sodium dodecyl sulphate or phenol-acetic acid-urea were subjected to polyacrylamide-gel electrophoresis at alkaline and acid pH respectively. The electrophoretic patterns given by the proteins of the chromaffin granule membrane were distinct from those given by the chromogranins, and from those given by mitochondrial and microsomal membrane proteins.

Lysosomal phospholipases A1 and A2 of bovine adrenal medulla

1. [(32)P]Lecithin and [(32)P]phosphatidylethanolamine were prepared by incubating rat liver mince with [(32)P]phosphate. With these (32)P-labelled phospholipids conditions for the quantitative assay of phospholipase A activity were established. 2. The distribution of phospholipase A activity between subcellular fractions of the bovine adrenal medulla was determined. Phospholipases A(1) and A(2), with pH optima at 4.2 and 6.5 respectively, were found in the large-granule fraction. By means of sucrose-density-gradient centrifugation it was shown that both these phospholipases were localized in lysosomes. 3. Lysosomal phospholipase A(1) catalysed the hydrolysis of [(32)P]lecithin and [(32)P]phosphatidylethanolamine at the same rate. The enzymic activity was inhibited by 70% in the presence of 10mm-calcium chloride. 4. Lysosomal phospholipase A(2) catalysed the hydrolysis of [(32)P]phosphatidylethanolamine more rapidly than it hydrolysed [(32)P]lecithin. The hydrolysis of [(32)P]phosphatidylethanolamine, but not that of [(32)P]lecithin, by phospholipase A(2) was activated by 0.8mm-calcium chloride. However, the hydrolysis of both substrates was inhibited by 8mm-calcium chloride. 5. The significance of the presence of phospholipase activity in lysosomes is discussed in relation to the functions of lysosomes in general and in the adrenal medulla.

The formation of ornithine from proline in animal tissues

1. Homogenates of liver or kidney from rat, mouse, dog and guinea pig formed ornithine from proline but not from glutamate. Rat kidney was most active in this reaction and was used for further studies. 2. The overall reaction was found to be catalysed by proline oxidase to yield glutamic gamma-semialdehyde, followed by transamination of this product with glutamate as catalysed by ornithine-keto acid aminotransferase. 3. The unfavourable equilibrium of the ornithine-keto acid aminotransferase reaction was overcome chiefly by glutamate dehydrogenase in the tissue, which removed the alpha-oxoglutarate produced, by reduction with endogenous ammonia and NADH. 4. Aspartate aminotransferase in these preparations also aided in the removal of alpha-oxoglutarate. In this case the overall reaction was driven also by the rapid decarboxylation of oxaloacetate. 5. No evidence could be found for a pathway of ornithine synthesis involving acylated intermediates as has been observed in some micro-organisms. 6. The rate of ornithine synthesis in homogenates of several rat tissues paralleled the activity of ornithine-keto acid aminotransferase in these tissues, indicating that this enzyme was rate-determining for the synthesis. 7. The possible influence of these reactions on urea synthesis is discussed.

Lipids of the adrenal medulla. Lysolecithin, a characteristic constituent of chromaffin granules

1. The lipid composition of microsomes, mitochondria and chromaffin granules, obtained from homogenates of bovine adrenal medulla, has been investigated. 2. The three types of particle showed characteristic differences of phospholipid and cholesterol content. The lipid composition of microsomes and mitochondria resembled that of corresponding particles from other tissues. The chromaffin granules contained 19% of the cholesterol and 14% of the phospholipids of the low-speed supernatant. 3. Thin-layer chromatography indicated the presence of these phospholipids in extracts from each particle: lecithin, lysolecithin, phosphatidylethanolamine (partly plasmalogen), phosphatidylserine, phosphatidylinositol and sphingomyelin. 4. On quantitative analysis of the phospholipids, chromaffin granules were found to contain a high concentration of lysolecithin (17% of the lipid phosphorus). Mitochondria and microsomes, on the other hand, contained very little lysolecithin (less than 2% of the lipid phosphorus).

Purification and properties of an acidic protein from chromaffin granules of bovine adrenal medulla

1. A soluble protein has been purified from an aqueous extract of bovine adrenal chromaffin granules by chromatography on Sephadex G-200. This protein comprises 25% of the total protein of the granules and gave a single band on gel electrophoresis. 2. The protein is unusually rich in acidic amino acids, notably glutamic acid (26.0%, w/w); it is also relatively rich in proline (8.6%, w/w) but poor in cystine (0.35%, w/w). 3. A molecular weight of 77000 was obtained from sedimentation and diffusion measurements on the protein, and approach-to-equilibrium measurements gave apparent molecular weights of the same order. 4. A molecular weight 7 times that given above was estimated from the results of chromatography on a column of Sephadex G-200 that had been calibrated with globular proteins. However, good agreement between the ultracentrifuge and Sephadex experiments was obtained on the assumption that Sephadex chromatography depends on the effective hydrodynamic radii of proteins and not on their molecular weights. 5. The hydrodynamic properties of the protein differed from those of a typical globular protein. Thus the protein had a high intrinsic viscosity, a high frictional ratio and a large effective hydrodynamic volume. 6. The hydrodynamic properties of the protein, but not its molecular weight, were dependent on the ionic strength of the solvent. Increasing the ionic strength caused an increase in the sedimentation and diffusion coefficients, but a decrease in the intrinsic viscosity and in the frictional ratio of the protein. 7. Optical-rotatory-dispersion measurements indicated that only a small part of the polypeptide chain was in an alpha-helical conformation. 8. These results are compatible with the protein's having a conformation approaching that of a random-coil polypeptide, the volume occupied by the molecule being determined by electrostatic repulsion between the excess of negative charges.

A simple method for the isolation of adrenal chromaffin granules on a large scale

Highly purified chromaffin granules can be obtained from homogenates of either ox, pig, horse or rat adrenal medullae by ultracentrifugation of the large-granule fraction layered on 1.6m-sucrose solution, by using angle-head rotors. The chromaffin granules are obtained as a pink sediment that is only slightly contaminated by mitochondria and lysosomes.

The localization of lysosomal enzymes in chromaffin tissue

1. An homogenate of bovine adrenal medulla contains significant amounts of six acid hydrolases: acid ribonuclease, acid deoxyribonuclease, cathepsin, acid phosphatase, beta-glucuronidase and arylsulphatase. Most of the activity of each enzyme could be sedimented in the large-granule fraction at 242,000 g-min.2. Differential centrifugation indicated the presence of three populations of particles, which sedimented at slightly different rates; these are, in order of decreasing sedimentation rate, mitochondria, particles containing the acid hydrolases, and chromaffin granules.3. The three types of particle could be separated by ultracentrifuging the large-granule fraction in a sucrose density gradient. Most of the activity of each hydrolase was recovered in a layer intermediate between those formed by mitochondria and chromaffin granules.4. The large-granule fraction therefore contains particles which are defined by their enzyme content as lysosomes.5. Highly purified chromaffin granules, containing less than 5% of the activity of each acid hydrolase, were obtained from the gradient.

Cyanide, vitamin B-12, experimental demyelination and tobacco amblyopia

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