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Biomedical subjects

H Beevers

Publications and source records attributed to H Beevers.

At least 91 records · Page 5Linked to original sources
PubMed1968

Characterization of glyoxysomes from castor bean endosperm.

Electron micrographs are presented which establish the identity of the components of the 3 major bands observed after sucrose density centrifugation of the crude particulate fraction from the endosperm of germinating castor bean seedlings. These are: mitochondria (density 1.19 g/cc), proplastids (density 1.23 g/cc) and glyoxysomes (density 1.25 g/cc). Further evidence is provided on the enzymatic composition of the glyoxysomes. Essentially all of the particulate malate synthetase, isocitrate lyase, catalase, and glycolic oxidase is present in these organelles. The distribution of glyoxysomal enzymes on sucrose density gradients is contrasted with that of the strictly mitochondrial enzymes fumarase, NADH oxidase, and succinoxidase. Malate dehydrogenase and citrate synthetase are present in both organelles. The functional role of glyoxysomes and their relationship to cytosomes from other tissues is discussed.

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PubMed1967

Gluconeogenesis from amino acids in germinating castor bean endosperm and its role in transport to the embryo.

During germination of the castor bean all of the contents of the endosperm are ultimately transported to the embryo through the cotyledon or respired. A net loss of nitrogen from the endosperm begins about the fourth day, i.e. at the time when embryo growth and fat breakdown are also beginning. Amino acid analysis of the exudate from the cotyledons, still enclosed in the endosperm, showed that the amounts of aspartate, glutamate, glycine, and alanine were very low and that glutamine made up 40% of the amino acids in the exudate.Amino acids labeled with (14)C were applied to intact excised endosperms to follow utilization. Aspartate, glutamate, alanine, glycine, serine, and leucine were converted to sugar to varying extents. Proline, arginine, valine, and phenylalanine were not appreciably converted to sugars. Proline and glutamate were converted to glutamine. When (14)C-glutamate, aspartate, and alanine were added to the outer endosperm of intact seedlings, only sugars and glutamine contained appreciable label in the exudate. When (14)C-valine was added, it was virtually the only labeled compound in the exudate.The results show that amino acids which on deamination can give rise to intermediates in the pathway of conversion of fat to sucrose are largely converted to sucrose and the nitrogen transported as glutamine. Other amino acids released from the endosperm protein are transported intact into the seedling axis. Some carbon from the gluconeogenic amino acids is also transported as glutamine.

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PubMed1967

Sugar uptake and translocation in the castor bean seedling I. Characteristics of transfer in intact and excised seedlings.

Changes in the dry weights of various parts of the castor bean seedling showed that the rates of transfer of material through the cotyledons to the embryonic axis exceeded 2 mg/hour after 5 to 6 days of germination. The sugar present in the endosperm was predominantly, and in the cotyledon almost exclusively, sucrose. Anatomical features were described which contribute to the efficiency of the cotyledons as organs of absorption and transmittal of sucrose to the embryonic axis, where hexoses are much more prevalent.The ability of the cotyledons to absorb sucrose survived removal of the endosperm from the seedling. A series of experiments is described in which the cotyledons of such excised seedlings were immersed in sucrose-(14)C and measurements made of uptake and of translocation to various parts of the seedling. Increasing rates of absorption were observed as the sucrose concentration was raised to 0.5 m and these rates were maintained for several hours. Removal of the embryonic axis (hypocotyl plus roots) drastically altered both the response to sucrose concentration and the time course of absorption by the cotyledons.More than 80% of the sugar normally entering the cotyledons from the endosperm is transmitted to the embryonic axis and this extensive turnover was seen also in pulse/chase experiments with excised seedlings. The cotyledons of excised seedlings absorbed sucrose against high apparent concentration gradients. The absorption was stimulated by phosphate and had a pH optimum at about pH 6.4. It was inhibited by arsenate, azide and 2,4-dinitrophenol.

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PubMed1967

Sugar Uptake and Translocation in the Castor Bean Seedling II. Sugar Transformations During Uptake.

During sucrose uptake by the cotyledons of castor bean seedlings excised from their endosperms, small amounts of glucose and fructose accumulate in the medium. When these hexoses were supplied separately, it was found that the rate of fructose uptake exceeded that of glucose at concentrations up to 0.5 m. Sucrose uptake exceeded that of both hexoses combined, particularly at concentrations greater than 0.1 m. Only minute amounts of labeled hexoses were recovered from the cotyledons after exposure to glucose-U-(14)C or fructose-U-(14)C; sucrose contained virtually all of the (14)C in the sugar fraction. The fructose and glucose moieties of the extracted sucrose were equally labeled when glucose-U-(14)C was supplied, and the glucose moiety contained 41% of the (14)C when fructose-U-(14)C was the substrate.Several considerations led to the conclusion that by far the greater part of sucrose absorbed by the cotyledons was not hydrolyzed prior to or during uptake. These included the differential responses of hydrolysis and uptake to pH, and to Tris(hydroxymethyl) aminomethane, the absence of interference of the hexoses on sucrose absorption and particularly the demonstration that when the cotyledons were supplied with fructosyl-(14)C sucrose, 90% of the (14)C was retained in the fructose residue of sucrose in the cotyledons.

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PubMed1967

Compartmentation of Organic Acids in Corn Roots. III. Utilization of Exogenously Supplied Acids.

The rates of utilization of exogenously supplied (14)C labeled acids by corn roots was compared to the utilization of these acids generated endogenously in the mitochondria from acetate-(3)H. (14)C-labeled citrate, pyruvate, succinate, glutamate or aspartate were supplied with acetate-(3)H in a 15 minute pulse and the (14)C and (3)H contents of extracted acids were measured over a 4 hour period. It was found, in contrast to previous experiments with malate, that these exogenously added acids were used as rapidly as the endogenous forms. Apparently, therefore, these acids penetrate readily into the mitochondria and do not enter cytoplasmic pools which are not in ready equilibrium with those in the mitochondria. Small amounts of labeled glutamate were produced from succinate-2,3-(3)H by corn root tissue. Since glutamate would not be expected to be labeled by reactions of the tricarboxylic acid cycle it was concluded that it was produced rather directly from succinate. The minor pool of glutamate generated in this way retained its radioactivity while that generated in the cycle was rapidly lost. An extra-mitochondrial location of this pool of glutamate is therefore suggested.

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PubMed1966

Compartmentation of organic acids in corn roots I. Differential labeling of 2 malate pools.

Bicarbonate-(14)C and acetate-(3)H were simultaneously provided to corn roots to give 2 isotopic forms of malate in the tissue, malate-(14)C produced by dark fixation reactions and malate-(3)H produced by reactions of the tricarboxylic acid cycle. Following a short pulse of exposure to the isotopes, the dissimilation of both isotopic forms of malic acid was followed. The rate of utilization of malate-(3)H was much faster than that of malate-(14)C.These results are interpreted as showing that the malate produced from (14)CO(2) is in a pool physically separated from that in the tricarboxylic acid cycle. The introduction of the 2 isotopes through distinct metabolic pathways produced the differential labeling of 2 distinct pools of malate.

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PubMed1966

Compartmentation of Organic Acids in Corn Roots II. The Cytoplasmic Pool of Malic Acid.

The major conclusion drawn was that malate generated in corn roots during a 15-minute period of CO(2) fixation and malate introduced into the tissue during a similar period from the bathing medium share a common extramitochondrial compartment, the cytoplasmic pool. The utilization of these 2 forms of malate is normally much slower than that of malate generated in the mitochondria by the tricarboxylic acid cycle. By lowering the pH of the medium or treating the tissue with malonate or 2,4-dinitrophenol, similar increases in the rates of utilization of both forms of cytoplasmic malate were brought about. Changes in (A) the demand for acetyl acceptors in the mitochondria and (B) mitochondrial permeability were invoked to account for the increased utilization of the cytoplasmic malate under the various experimental treatments.

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PubMed1966

Metabolism of corn roots in malonate.

The rate of O(2) uptake by sub-apical corn roots is largely resistant to 0.1 m Na-malonate at pH 5.0. The resistance of this tissue, in which the tricarboxylic acid cycle is very active, is not due to the compensatory induction of another oxidative pathway as seems to be the case in fresh potato slices. In corn roots malonate inhibits succinate utilization as expected and the smallness of the effect on O(2) uptake is due to the utilization of endogenous (cytoplasmic) malate as acetyl acceptor and its conversion to succinate. Malonate uptake stops after 2 to 3 hours when only a fraction (roughly 20%) of the root volume has equilibrated with external malonate. After this time the accumulated succinate is apparently able to overcome the malonate block, the ability to oxidize acetate to CO(2) is largely regained and O(2) uptake is maintained at about 80% of the control level.Malonate sensitivity at high external concentrations of malonate and conditions appropriate for its uptake is therefore fully expressed only under conditions where the production or availability of extra-mitochondrial malate (or perhaps other precursors of oxaloacetate) is at a minimum.

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