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Biomedical subjects

C R Benedict

Publications and source records attributed to C R Benedict.

At least 127 records · Page 7Linked to original sources
PubMed1977

Fractionation of stable carbon isotopes by phosphoenolpyruvate carboxylase from c(4) plants.

The active species of "CO(2)" and the amount of fractionation of stable carbon isotopes have been determined for a partially purified preparation of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) from corn (Zea mays) leaves. The rates of the enzyme reactions, using substrate amounts of HCO(3) (-), CO(2) or CO(2) plus carbonic anhydrase, show that HCO(3) (-) is the active species of "CO(2)" utilized by PEP carboxylase. The K(m) values for CO(2) and HCO(3) (-) are 1.25 mm and 0.11 mm, respectively, which further suggest the preferential utilization of HCO(3) (-) by PEP carboxylase. The amount of fractionation of stable carbon isotopes by PEP carboxylase from an infinite pool of H(12)CO(3) (-) and H(13)CO(3) (-) was -2.03 per thousand. This enzyme fractionation (delta), together with the fractionation associated with absorption of CO(2) into plant cells and the equilibrium fractionation associated with atmospheric CO(2) and dissolved HCO(3) (-) are discussed in relation to the fractionation of stable carbon isotopes of atmospheric CO(2) during photosynthesis in C(4) plants.

Journal Article↗
PubMed1977

Adult coarctation of the aorta: Anaesthesia and postoperative management.

The complications following surgery for the correction of aortic coarctation are reviewed. The immediate postoperative increase in blood pressure is associated with a very high (750%) increase in plasma noradrenaline concentrations. The pre-operative management and preparation of the patient, the use of induced hypotension during surgery with careful haemodynamic control to prevent the hypoperfusion of the lower segment of the aorta, and the postoperative management with special reference to adequate control of blood pressure are discussed.

Adult↗
PubMed1977

A sensitive radioenzymatic assay for adrenaline and noradrenaline in plasma.

1 An existing radioenzymatic assay for plasma catecholamines using catechol-o-methyl transferase and [3H]-S-adenosyl-methionine has been modified resulting in a more sensitive assay for the measurement of plasma adrenaline and noradrenaline. 2 The lower limit of sensitivity for this method is 25 pg for adrenaline and 30 pg for noradrenaline/ml of plasma. 3 Resting supine (60 min) plasma adrenaline concentration was (mean +/- s.d.) 124 +/- 76 pg/ml(n=11) in males and 130 +/- 71 pg/ml (n=7) in females; plasma noradrenaline concentrations were respectively 444 +/- 129 pg/ml and 550 +/- 87 pg/ml. 4 The changes in plasma catecholamine concentrations in response to 40 degrees head-up tilt have been determined in a group of healthy normal subjects and have been shown to be related to changes in blood pressure and heart rate.

Adult↗
PubMed1976

Development of ribulose 1,5-diphosphate carboxylase in nonphotosynthetic endosperms of germinating castor beans.

Ribulose 1,5-diphosphate (RuDP) carboxylase has been partially purified from dark-grown nonphotosynthetic endosperms of germinating castor beans (Ricinus communis var. Hale). The Km values for RuDP, HCO(3) (-), and Mg(2+) are 0.51, 33, and 1.78 mm, respectively. The pH optimum for the carboxylation reaction is pH 7.5. Germination is required for the development of the carboxylase in the endosperms. The enzyme reaches a maximal activity in 4- to 5-day-old dark-grown seedlings (which have an endosperm weight of approximately 0.75 g fresh weight/bean) and then declines. Total endosperm carboxylase activity is 1230 nmoles/min.g fresh weight which is 25 and 50% of the total activity developed in soybean and maize leaves, respectively. Specific activity of the carboxylase in crude soluble endosperm preparations (which contain enzymic and storage protein) is 0.05 mumole/min.mg protein. This is 5 times greater than the specific activity of RuDP carboxylase in soluble preparations from etiolated leaves. During germination the V(max) of the endosperm carboxylase for RuDP increases 10-fold. Development of the enzyme is inhibited 90% by the exposure of the endosperm to 2 mug/ml cycloheximide or 50 mug/ml chloramphenicol. Light (or phytochrome Pfr) is not required for the synthesis of the enzyme. Electron photomicrographs of dark-grown endosperm cells (with peak RuDP carboxylase activity) show proplastids with several invaginations of the inner membrane but no prolamellar-like structures.

Journal Article↗
PubMed1976

Photosynthetic carbon metabolism of a marine grass.

The delta(13)C value of a tropical marine grass Thalassia testudinum is -9.04 per thousand. This value is similar to the delta(13)C value of terrestrial tropical grasses. The delta(13)C values of the organic acid fraction, the amino acid fraction, the sugar fraction, malic acid, and glucose are: -11.2 per thousand, -13.1 per thousand, -10.1 per thousand, -11.1 per thousand, and -11.5 per thousand, respectively. The delta(13)C values of malic acid and glucose of Thalassia are similar to the delta(13)C values of these intermediates in sorghum leaves and attest to the presence of the photosynthetic C(4)-dicarboxylic acid pathway in this marine grass. The inorganic HCO(3) (-) for the growth of the grass fluctuates between -6.7 to -2.7 per thousand during the day. If CO(2) fixation in Thalassia is catalyzed by phosphoenolpyruvate carboxylase (which would result in a -3 per thousand fractionation between HCO(3) (-) and malic acid), the predicted delta(13)C value for Thalassia would be -9.7 to -5.7 per thousand. This range is close to the observed range of -12.6 to -7.8 per thousand for Thalassia and agree with the operation of the C(4)-dicarboxylic acid pathway in this plant. The early products of the fixation of HCO(3) (-) in the leaf sections are malic acid and aspartic acid which are similar to the early products of CO(2) fixation in C(4) terrestrial plants.Electron microscopy of the leaves of Thalassia reveal thick walled epidermal cells exceedingly rich in mitochondria and C(3)-type chloroplasts. The mesophyll cells have many different shapes and surround air lacunae which contain O(2) and CO(2). The mesophyll cells are highly vacuolated and the parietal cytoplasm contains an occasional chloroplast. This chloroplast contains grana but the lamellar system is not as developed as the system in epidermal chloroplasts. Extensive phloem tissue is present but the xylem elements are reduced in this aquatic grass. The vascular tissue is not surrounded by bundle sheath cells.This work does not establish the exact relation between structure and function in Thalassia, but it does show the C(4)-type photosynthetic carbon metabolism in this grass involves epidermal and mesophyll cells and internally produced O(2) and CO(2) in the air lacunae.

Journal Article↗
PubMed1974

Elimination of the lag period in chloroplast development in a chlorophyll mutant of peanuts.

The mutation of a nuclear gene in peanut (Arachis hypogaea L.) plants results in a reduced light-dependent development of chloroplast fine structure, soluble protein, ribulose-1, 5-diP carboxylase, NADP-glyceraldehyde-3-P dehydrogenase, fructose-1, 6-diP aldolase, glycerate-3-P kinase, phosphoenolpyruvate carboxylase, malate dehydrogenase, and dark respiration during the 72-hour lag period of chlorophyll synthesis in dark-grown leaves exposed to continuous light. The mutation has pleiotropic affects. Kinetic analysis shows there is also a 72-hour lag period in the light-dependent development of NADP-glyceraldehyde-3-P dehydrogenase and fructose-1, 6-diP aldolase in the mutant leaves, whereas there is no lag in the development of NAD-malate dehydrogenase and dark respiration. There is minimal development of the chloroplast during the 72-hour mutationally induced lag period, but there is pronounced cytoplasmic and mitochondrial activity during this phase. There is a 24-hour lag period in the light-dependent enlargement of the mutant leaves. At the completion of leaf enlargement, chloroplast differentiation is initiated. The mutation does not result in any chloroplast deletions, it only affects the timing of the synthesis of these components.Elimination of the lag period in leaf enlargement and chloroplast development (potentiation) requires a preliminary 72- to 96-hour dark period before exposing the dark-grown leaves to continuous light. There is extensive development of the etioplasts during this dark period. These results establish that the nuclear gene mutation affects the early stages of plastid development and not the light-dependent synthesis of plastid components. The nuclear gene may code for the regulation of the synthesis of a component (nutrient) in the dark (or during the lag phase in the light) which is essential for the development of mesophyll cells and plastids. Although, the chloroplast is a semi-autonomous organelle, nuclear gene control of chloroplast differentiation may not be independent of cellular growth.

Journal Article↗
PubMed1974

The development of isocitric lyase activity in germinating cotton seed.

In cotyledons of germinating cotton (Gossypium hirsutum L. var. Stoneville 213) seedlings, in the dark, isocitric lyase (EC 4.1.3.1) activity peaks after 2 days and thereafter slowly declines to a negligible value after 8 days. The maximum activity of this enzyme in cotyledons of 2-day-old seedlings was 16.2 mumoles of glyoxylate formed/15 min.10 cotyledon pairs. Actinomycin D at a concentration of 10 mug/ml, if added to the imbibing solution, completely prevents the development of isocitric lyase activity in these germinating seed. In cotyledons of germinating cotton seedlings, in the light, isocitric lyase activity peaks after 2 to 3 days and sharply declines to a negligible value after 4 days. The maximum activity of this enzyme in cotyledons of 2- to 3-day-old seedlings was 13.2 mumoles of glyoxylate formed/15 min.10 cotyledon pairs. Actinomycin D at a concentration of 10 mug/ml, if added to the imbibing solution, severely inhibits the development of enzyme activity.In germinating seed, in the light, the synthesis of chlorophyll and glyceraldehyde-3-P dehydrogenase is also limited by the addition of low concentrations of actinomycin D. The new synthesis of fructose-1, 6-diP aldolase, which is detectable after 1 to 2 days of germination, is inhibited by 10 mug/ml of actinomycin D. We, therefore, conclude that the synthetic events leading to the development of chlorophyll, some glyoxysomal and chloroplast enzymes in germinating cotton seedlings depend on newly transcribed mRNA.

Journal Article↗
PubMed1973

The presence of ribulose 1,5-diphosphate carboxylase in the nonphotosynthetic endosperm of germinating castor beans.

Ribulose 1,5-diphosphate carboxylase was detected in extracts of germinating castor bean (Ricinus communis var. Hale) endosperms. This is the first report of this enzyme in a nonphotosynthetic (no chlorophyll) plant tissue. Radioactive 3-phosphoglyceric acid has been identified as the principle product resulting from the enzymatic condensation of (14)C-bicarbonate and ribulose-1,5-diP in endosperm extracts. The Km values of bicarbonate and ribulose-1,5-diP for the endosperm carboxylase are 1.14 x 10(-2)m and 7.5 x 10(-5)m, respectively. The carboxylase activity peaks at 4 days in endosperms of castor beans germinated in the dark. The specific activity of the carboxylase at this stage of germination is 4.3 mumoles of 3-phosphoglycerate formed/mg protein.hr. The presence of ribulose-1,5-diP carboxylase and other enzymes of the reductive pentose phosphate pathway show the potential of this pathway in castor bean endosperms.

Journal Article↗
PubMed1972

High photosynthetic rate of a chlorophyll mutant of cotton.

In a chlorophyll mutant (virescent) and wild-type cotton (Gossypium hirsutum L.), a number of photosynthetic parameters have been measured and compared with those published for other chlorophyll mutants. (a) The photosynthetic rates at 230 w/m(2) (400-700 nm) from a tungsten lamp were 36.8 mg CO(2) fixed/dm(2).hr (virescent) and 39.5 mg CO(2) fixed/dm(2).hr (wild-type). On a chlorphyll basis, the photosynthetic rates were 36.8 and 12.1 mg CO(2) fixed/mg chl.hr, respectively. (b) The photosynthetic rates at 13 w/m(2) (400-700 nm) from a tungsten source were 7.1 mg CO(2) fixed/dm(2).hr (virescent) and 7.4 mg CO(2) fixed/dm(2).hr (wild-type). On a chlorophyll basis, the photosynthetic rates were 6.0 and 1.4 mg CO(2) fixed/mg chl.hr, respectively. (c) The chlorophyll a/b ratios of the virescent and wild-type leaves were 3.3 and 4.1 (d) The chlorophyll/carotenoid ratios for the virescent and wild-type leaves were 3.2 and 7.3, respectively. (e) The photosynthetic carbon metabolism of the chlorophyll mutant was through the reductive pentose phosphate cycle. (f) The CO(2) compensation points for the virescent and wild-type plants were similar. (g) The mutant and wild-type leaves have the same quantum yield in the red part of the visible spectrum, but the virescent leaves have a lower quantum yield in the blue part of the spectrum. (h) Virescent and wild-type leaves contain similar levels on a protein basis of several reductive pentose phosphate cycle enzymes.

Journal Article↗
PubMed1972

Nuclear gene affecting greening in virescent peanut leaves.

Chlorophyll synthesis induced by continuous illumination of dark-grown seedlings has been followed in wild-type and virescent peanut leaves. Compared to the wild-type leaves, chlorophyll synthesis in the virescent leaves shows a 72-hour lag period before the onset of a phase of rapid chlorophyll accumulation. The development of chloroplast grana and the activity of many enzymes of the reductive pentose phosphate cycle, phosphoenolpyruvate carboxylase, and malate dehydrogenase are reduced in the virescent leaves during the lag phase of chlorophyll accumulation. Although nucleic acid synthesis in the virescent leaves in normal, there is a distinctly lower rate of protein synthesis. The low level of protein synthesis during the lag period might limit the synthesis of a factor(s) essential for the development of both cell and chloroplast constituents.

Journal Article↗