Distribution Of 14C Research Articles

Metabolism of maltose and sucrose by microspores isolated from barley (Hordeum vulgare L.)

The aim of this work was to discover why barley (Hordeum vulgare L.) microspores die when cultured on media containing 40 mM sucrose but undergo embryogenesis on 40 mM maltose. Freshly isolated microspores were cultured for 6–24 h on media containing either [U-14C]maltose or [U-14C]sucrose at 40 mM, and the detailed distribution of 14C was determined. The amounts of glycolytic intermediates, ATP, ADP and AMP, in microspores were also measured. Cultures on sucrose differed from those on maltose in that the initial rate of metabolism was faster but declined rapidly, less 14C was recovered in polymers and more in alanine, there was extensive leakage of assimilated carbon, significant accumulation of ethanol and a lower adenylate energy charge. It is argued that microspores cultured on 40 mM sucrose die because they metabolize the sugar rapidly, become hypoxic and, as a result, accumulate large quantities of ethanol within the cells. Metabolism of maltose is slower and there is sufficient oxygen available to allow cells to survive in culture. Consequently some of the cultured cells undergo embryogenesis.

Gluconeogenesis and Glucuronidation in Liver in Vivo and the Heterogeneity of Hepatocyte Function

In order to examine metabolic zonation in human liver, [2-14C]glycerol, which labels carbons 2 and 5 of glucose-6-P, and [1-14C]lactate, which labels carbons 3 and 4 of glucose-6-P, in the process of gluconeogenesis, were infused intravenously into healthy subjects who ingested acetaminophen and had fasted 36 h. Distributions of 14C were determined in glucose in blood and in the glucuronic acid moiety of acetaminophen glucuronide excreted in urine. Ratios of 14C in carbons 2 and 5 to 14C in carbons 3 and 4 were significantly higher in blood glucose than in glucuronide. Since glucose and glucuronic acid are formed from glucose-6-P in liver without randomization of carbon, the differences in the ratios indicate that the pool of glucose-6-P in liver is not homogeneous. The glucuronide sampled glucose-6-P with more label from lactate than glycerol compared to the glucose-6-P sampled by the glucose. The apparent explanation is the greater decrease in glycerol compared with lactate concentration as blood streams from the periportal to the perivenous zones of the liver lobule. Glucuronidation is then expressed in humans relatively more in the perivenous than periportal zones and gluconeogenesis from glycerol more in the periportal than perivenous zones.

Benthic foraminifera cadmium record from the western equatorial Pacific

The following equations are proposed for the cadmium distribution coefficient between sea water and foraminifera (D ∗) in the Pacific, Antarctic, and Indian Oceans: [1000 m < depth < 4000 m] D ∗ = 1.3 + ( depth − 1000)(1.0/3000) [4000 m < depth] D ∗ = 2.3 . These are slightly lower than the distribution coemcient in the Atlantic. We also propose the 13C distribution coefficients in the Atlantic and other oceans to be 0.99995 and 1.00015, respectively. If these new distribution coefficients are adopted for previously analyzed data (Boyle, 1992), these two tracers corroborate the theory that glacial deep water (water depths between 2000 and 4000 m) was more nutrient-rich than today. We also provide benthic foraminifera Cd Ca records for several sediment cores recovered from the western equatorial Pacific. They suggest that deep water (water depth of 2500 m) cadmium concentration in the glacial Pacific was 30% higher than that of the present-day and that it fluctuated strongly during the period of deglaciation.

Rapid recycling of triose phosphates in oak stem tissue

ABSTRACTWe report the carbon‐13 and oxygen‐18 isotope ratios in cellulose from the early and late wood of pedunculate oak (Quercus robur L.). The δ13 C value of the early wood correlates best with that of the late wood of the previous year. The δ18O value of the early wood correlates best with that of the late wood of the same year. We suggest that a biochemical explanation of these data is that there is a rapid cycle between hexose monophosphates and triose phosphates in oak stem tissue during cellulose synthesis. Evidence in support of this explanation is provided by the intramolecular distribution of 14C in labelled fructose extracted from cores of wood that had been supplied with [1−14C]‐ and [6‐14C]glucose.

Pentose Cycling and the Distribution of 13C in Trehalose during Glucogenesis from 13C-Labelled Substrates in an Insect

Redistribution of 13C in trehalose (Tre) due to pentose cycling was observed in vivo in Manduca sexta during glucogenesis from [3-13C]alanine (Ala) and [2-13C]glycerol (Gly). The extent of cycling was affected by dietary composition. Larvae maintained on a low-carbohydrate diet (LCD) exhibited approximately 13% cycling, while those on a complete-balanced diet (CBD) or low-fat diet (LFD) displayed much higher rates of cycling. Significant incorporation of 13C via, reversal of the non-oxidative phase was evident on all diets but was greatest on the CBD and LFD. In contrast to conclusions from previous studies with insects, the present results indicate that under normal conditions the pentose pathway is not the principal source of triose phosphates for oxidative catabolism during larval development.

The accumulation and energy load of exogenous lipids in cirrhotic rat liver after partial hepatectomy.

The suitability of three energy substrates, glucose, medium-chain triglycerides (MCT) and long-chain triglycerides (LCT), was studied in cirrhotic rats after a partial hepatectomy. Rats with thioacetamide-induced cirrhosis underwent a 70% hepatectomy, and were divided into three groups. Each group was then injected with 14C-labeled glucose, 14C-labeled MCT or 14C-labeled LCT, respectively. The subsequent tissue distribution of 14C and the cumulative amount of expired 14CO2 were determined. In a second experiment, the 70%-hepatectomized cirrhotic rats received total parenteral nutrition (TPN). The source of the nonprotein calories was 100% glucose (glucose group), 60% MCT + 40% glucose (MCT group), and 60% LCT + 40% glucose (LCT group). The adenylate energy charge and the glycogen content in the liver remnant were determined. The tissue distribution of 14C revealed that the fat emulsions accumulated preferentially in the liver. One hour after the partial hepatectomy, the concentration of 14C-labeled MCT in the liver remnants was threefold higher than in sham-operated controls. Similarly, the concentration of 14C-labeled LCT was twofold higher. The adenylate energy charge in the glucose group with TPN recovered to preoperative levels within 1-hour after the partial hepatectomy, whereas the LCT group with TPN showed a 24-hour delay in their recovery. The MCT group with TPN exhibited an intermediate time course. It is suggested that the specific accumulation of MCT and especially LCT emulsions in the cirrhotic liver remnant acts as an energy load rather than an energy substrate.

Effect of mutual shading on carbon distribution in rice plant

Abstract The 14C distribution among organs of rice plant at the ripening stage was determined under different levels of light intensity and mutual shading. The results obtained were as follows. 1. By shading (63%), dry matter accumulation was remarkably affected, whereas the 14C distribution ratio to shoot, roots, ears, and for respiration was affected slightly by the shading treatment after 14CO2 feeding, but not by the shading treatment before 14CO2 feeding. By prolonged mutual shading in the artificial canopy, dry matter accumulation decreased, but the 14C distribution ratio to each organ and for respiration was affected slightly regardless of growth stages since plants had became adapted to mutual shading for a long period of time. These results indicate that the 14C distribution pattern was basically unaffected by the light intensity.2. When 14CO2 was assimilated in a fully expanded leaf of a tropical rice cultivar Peta under the condition that leaves at a lower position below the fully expanded leaf were completely shaded by aluminum foil, 14C compounds from the fully expanded leaf translocated slowly into other organs, but a large amount of 14C compounds from the fully expanded leaf was lost by respiration. When 14CO2 was assimilated to the flag leaf of temperate rice plants at the late maturation stage, and the side of the plant was shaded at 50% with a cloth, a large amount of 14C compounds was consumed by respiration in the 14C-fed leaf, but the 14C compounds were not translocated from the upper leaf to the lower leaves. Thus, it is considered that the lower leaves did not consume the carbohydrates which were assimilated in upper leaves regardless of growth conditions, and that the respiratory activity of the fully light-exposed upper leaf increased due to the imbalance of source ability.3. By mutual shading, the photosynthetic activity of the lower leaf decreased, and the distribution ratio of the 14C compounds from leaves in the upper position to organs in a lower position also decreased. Therefore, it was assumed that the root activity decreased by mutual shading, which may account for the retardation of plant growth.

Fermentation of radiolabelled substrates by batch cultures of caecal microflora maintained in a continuous‐flow culture*

Glucose- and lactose-based media containing either 14C-labelled glucose, galactose, lactose or lactic acid were inoculated with anaerobic cultures of chicken caecal bacteria maintained for 121 d in a continuous-flow (CF) culture. The culture was previously shown to reduce Salmonella colonization in the caeca of chicks inoculated with the culture and the reduction was associated with increases in volatile fatty acids. The distributions of 14C were determined among the fermentation products, especially acetic, propionic and lactic acids. After 12 h of incubation and fermentation, variations were observed in the total amounts of 14C, from each 14C-labelled substrate, detected as acetic and propionic acids in the glucose- and lactose-based media, respectively: 92.9 and 89.4% of 14C-glucose, 77.0 and 44.0% of 14C-galactose, 0.0 and 76.9% of 14C-lactose, and 76.5 and 93.2% of lactic acid in the two media, respectively. The results from this study indicate that carbohydrate metabolism by the CF culture was a major source of acetic and propionic acids and that lactic acid was an important metabolic intermediate for the synthesis of the volatile fatty acids.

Stable isotope determination of plasma lactate conversion into glucose in fasting infants.

To quantify lactate gluconeogenesis, we developed a gas chromatography-mass spectrometry method based on the infusion of [6,6-2H2]glucose and [3-13C]lactate tracers to 12 infants aged 1-25 mo fasting for 11.5 +/- 1.5 h. Both rates of appearance of plasma glucose (26.7 +/- 2.6 mumol.kg-1.min-1, 4.8 +/- 0.5 mg.kg-1.min-1) and lactate (30.8 +/- 3.1 mumol.kg-1.min-1, 2.8 +/- 0.3 mg.kg-1.min-1) were remarkably elevated compared with adult values. The interconversion of plasma lactate and glucose was determined by 1) measuring the incorporation of 13C from [3-13C]lactate into plasma glucose; 2) correcting for the metabolic exchange of carbon atoms in the tricarboxylic acid cycle. For this purpose, an additional group of six infants was infused with [3-13C]lactate, and the distribution of 13C at specific carbon positions in the glucose molecule was determined using relevant ions in the electron-impact mass spectrum of its 1,2,5,6-diisopropylidene-3-O-acetyl-alpha-furanosyl derivative; and 3) measuring the reverse conversion of glucose to lactate in five other infants infused with [1-13C]glucose. We found that 54 +/- 2% of glucose was derived from plasma lactate (14.4 +/- 1.3 mumol.kg-1.min-1, 2.6 +/- 0.2 mg.kg-1.min-1). Lactate and glucose rates of appearance were correlated (r = 0.58, P < 0.05) and decreased with fasting duration (r = 0.66, P < 0.02). The correction factor for carbon exchange in the tricarboxylic acid cycle was 1.14 +/- 0.11.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of radiocarbon as a test of global carbon cycle models

Accurate global carbon cycle models are needed to estimate the future change of atmospheric CO2 for specified scenarios of CO2 emissions. Model accuracy cannot be tested directly because of the difficulty in estimating the carbon flux to the oceans and the terrestrial biosphere. However, one test of model consistency is the requirement that the model reproduce past changes and spatial distributions of 14C. A model for carbon exchange within and among the atmosphere, oceans, and terrestrial biosphere is found to satisfy this test. The ocean is modeled as an upwelling‐diffusion column capped by a mixed layer with recirculation of the polar bottom water to complete the thermohaline circulation. This ocean advection scheme contains only two key dynamic parameters, the vertical eddy diffusivity κ and the upwelling velocity w, which are calibrated to match the vertical distribution of preanthropogenic 14C. The thermocline depth scale κ/w = 1343 m found by calibration is considerably deeper than that required to match the steady vertical temperature profile (500 m). This is consistent with the hypothesis that isopycnal mixing, which is much more rapid than diapycnal mixing, has a stronger effect on 14C than on temperature since isopycnals are nearly isothermal. This model is found to match measured values, within measurement error, of the prebomb decrease in 14C in the atmosphere and the mixed layer due to the Suess effect, the bomb 14C in the mixed layer, the bomb 14C penetration depth, the bomb 14C ocean inventory, and the vertical distribution of total carbon. Results are compared to those of other schematic carbon cycle models as well as those of ocean general circulation models.

Partitioning of14C Assimilates between Sources and Sinks During Different Growth Stages in the Sympodial Thin-Leaved Orchid Oncidium Goldiana

Distribution of 14C assimilates from test leaves of a sympodial thin-leaved orchid hybrid, Oncidium Goldiana, was mapped using 14CO2 feeding at three growth stages. Transport time of 33 h was suitable for studying patterns of 14C assimilates partitioning to all plant parts on a single shoot. Oncidium Goldiana has a highly integrated pattern of source-sink assimilate partitioning in which all test leaves supplied 14C assimilates to every part of the plant. Both the inflorescence and the axillary bud are major sinks for all the leaves tested. Generally, all test leaves within each growth stage supplied similar amounts of 14C assimilates to the respective major sinks. Pseudobulbs usually received significantly more 14C assimilates from upper leaves (L1 and L2) than lower leaves (L3 and L4). Within each growth stage, the lower leaf (L4) supplied more 14C assimilates to the roots than the other leaves (L1, L2, and L3). Mature leaves are an important sink, especially after 6 h of 14CO2 feeding. The time-course ...

Absorption, translocation, and fate of propyzamide in witloof chicory (Cichorium intybus L.) and common amaranth (Amaranthus retroflexus L.)

SummaryWitloof chicory (Cichorium intybus L.) is tolerant to propyzamide and common amaranth (Amaranthus retroflexus L.) is sensitive. The absorption, translocation, and metabolism of propyzamide was studied in seedlings of witloof chicory and common amaranth to determine if differences in these processes cause the differential sensitivity. At 24,48, and 72 h after root treatment, there was no difference in the concentration of 14C (g−1 plant dry wt) in com‐mon amaranth and witloof chicory. Approximately 50% of the absorbed 14C was translocated out of the roots to shoots of both species at 24 and 48 h after treatment. After 72 h about 55 and 74% of the absorbed 14C was translocated to shoots of witloof chicory and common amaranth, respectively. Distribution of 14C (g−1 plant dry wt) in plant parts of witloof chicory and common amaranth seedlings was similar. Roots of both species accumulated the highest concentration of total 14C, whereas shoots contained the lowest. Thin layer chromatography revealed that the herbicide was metabolized in neither species 48 h after treatment. No differences were found in absorption, translocation, or metabolism between witloof chicory and common amaranth with regard to propyzamide.

Root decay and turnover of rhizodeposits in field-grown winter wheat and spring barley estimated by 14C pulse-labelling

Root decay and turnover of rhizodeposits in field-grown winter wheat and spring barley were estimated from the difference in 14C distribution between a sampling carried out 3 wk after pulse-labelling and a sampling 15–20 days before crop harvest. Plants were labelled at tillering and at ear emergence on plots under conventional (CONV) and integrated (INT) management. The latter was characterised by reductions in nutrient input, soil tillage and use of biocides. Of the roots formed at the tillering stage, 43% had decayed by the end of crop growth in winter wheat, independent of the management practice. Root decay in spring barley was higher than in wheat, and was higher in CONV (55%) than in INT (49%). Since most of the roots were produced around tillering, these values were used as estimates of accumulated annual root decay. Of the total rhizodeposition of assimilates fixed at tillering 87% was respired by microbes by the end of the growing season with winter wheat, 69% with barley in CONV and 54% with barley in INT. Decay of roots formed at ear emergence and the associated microbial respiration of rhizodeposits was lower than that of roots produced at tillering, but the same crop and management effects were observed. The assumptions underlying the calculations are discussed and the results are compared to data from minirhizotron observations.

Heat stress effects on sink activity of developing maize kernels grown in vitro

The objective of this study was to determine the effect of short‐term (4 days) and long‐term (8 days) heat stress (35°C) on sink activity of maize (Zea mays L.) kernels. Beginning at 3 days after pollination (DAP) kernels were grown in vitro at 25°C and 24 h later were transferred to 35°C for either 4 or 8 days. Each treatment had a control that was maintained continously at 25°C. Two experiments were designed to examine the uptake and distribution of 14C among hexoses, sucrose and starch in the pedicel placento‐chalazal (pedicel/p‐c). endosperm, and pericarp tissues of kernels exposed to heat stress for 4 or 8 days. Kernels cultured in vitro were placed in 14C‐sucrose medium either during the period of heat stress (experiment 1; 5 to 13 DAP) or immediately following heat‐stress treatments (experiment 2; 10 to 22 DAP). In both experiments no significant effect of heat stress was observed on the total radioactivity accumulated in the kernels until about 17 DAP, after which heat‐stressed kernels accumulated less 14C than the control. During the linear fill period, the endosperm of kernels exposed to heat stress accumulated more radioactivity associated with hexoses and sucrose and less radioactivity incorporated into starch, as compared to the control. Kernels heat stressed for 4 days showed a partial recovery in starch synthesis by 21 DAP, but to levels of only 65% of that of the control. Kernels heat stressed for 8 days did not recover. When 14C‐sucrose was supplied during the heat stress period (5–13 DAP). kernels from all treatments accumulated more hexoses that sucrose in the pedicel/p‐c. However, during the period following heat stress (10–22 DAP), pedicel/p‐c accumulated sucrose, but only in kernels exposed to long‐term heat stress. Soluble invertase activity was inhibited by both short‐term and long‐term heat stress, whereas the activity of insoluble invertase was affected only by long‐term heat stress. These results support the hypothesis that the disruption of kernel growth and more particularly endosperm starch biosynthesis, in response to heat stress, is mainly associated with changes in carbon utilization and partitioning between the different nonstructural carbohydrates within the endosperm rather than with a limitation in carbon supply to the kernel. Therefore, the effect on sink activity does not seem to be attributable to a thermal disruption of kernel uptake of sugars, but rather it is a consequence of heat perturbation of other physiological processes such as endosperm sugar metabolism and starch biosynthesis.

Effect of Vesicular-Arbuscular Mycorrhizal Fungi on the Growth, Photosynthesis, Transpiration and the Distribution of Photosynthates of Bearing Satsuma Mandarin Trees.

The response of satsuma mandarin (Citrus unshiu Marc. cv. Okitsu wase) on trifoliate orange (Poncirus trifoliata Raf.) rootstocks to inoculation of vesicular-arbuscular mycorrhizal (VAM) fungi, such as Glomus ambisporum Smith and Schenck, Gloumus fasciculatum (Thaxter) Gerdemann and Trappe emend. Walker and Koske, Glontus mosseae (Nicolson and Gerdemann) Gerdemann and Trappe, and Gigaspora ramisporophara Spain was studied at low phosphorus (P) application.Under high air temperature stress conditions in August, the photosynthetic (Pn) and transpiration (Tr) rates of VAM-inoculated trees were faster than non VAM ones. But in September when air temperature decreased, the Pn and Tr rates did not differ significantly between VAM-inoculated trees and non infected ones. In general, infected trees had larger leaf area and higher leaf P concentration and the tree growth was more vigorous than was that of non infected ones.When 13CO2 was fed to fruit-bearing trees under the optimal air temperature conditions, fruits were the strongest sink for photosynthates and new leaves were the second strongest. The leaves of VAM and non VAM trees assimilated 13CO2 equally fast per unit leaf area, and the distribution of 13C into various parts of the bearing trees did not differ. VAM trees, however, had 3 times more photoassimilates per tree than uninfected ones because the former had a leaf area 3 times larger than the latter and grew more vigorously.

Anthropogenic 14C Marine Geochemistry in the Vicinity of a Nuclear Fuel Reprocessing Plant

The British Nuclear Fuels plc reprocessing plant at Sellafield in Cumbria, Northwest England, discharges between 1 and 3 TBq of 14C per annum as low-level waste via a pipeline into the Eastern Irish Sea. Our results demonstrate 14C activities in excess of the current ambient level (i.e., ca. 260 Bq kg−1 carbon) in a range of both biotic and abiotic samples with evidence that a significant percentage of the discharges are in the form of dissolved inorganic carbon (DIC). There also appears to be a significant seasonal influence on the uptake of the DIC by the phytoplankton, most likely linked to their bloom periods. Results from different aspects of the research are integrated to provide an opportunity to consider the overall distribution and behavior of 14C in the Eastern Irish Sea. We also discuss the results in terms of existing sediment chronologies and sedimentation rates and consider the possible consequences of phytoplankton blooms on the cycling of the other radionuclides.

Metabolic Fate of Dorzolamide, a New Carbonic Anhydrase Inhibitor. (I): Absorption, Distribution, Metabolism and Excretion of (14C)Dorzolamide after Single Administration to Rats.

The absorption, distribution, metabolism and excretion of [14C]dorzolamide hydrochloride (MK-507) were studied in rats after single administration. In vitro plasma protein binding was also investigated.1. Following intravenous administration of [14C]dorzolamide hydrochloride to male rats at doses ranging from 0.1 to 5 mg/kg, blood AUC values increased proportionally with doses up to 0.5 mg/kg, but no increase in the values was observed for the dose range of 1 to 5 mg/kg. Most of the radioactivity in blood was present in erythrocytes. The elimination of radioactivity from the blood was very slow and the estimated t1/2 was 10.5-11.3 days.2. No sex differences were observed in blood concentration profiles regardless route of administration, oral or intravenous. After oral administration, almost all [14C]dorzolamide was absorbed.3. After intravenous administration at the dose of 0.5 mg/kg, most of the radioactivity was distributed in blood. At 30 min after dosing, the highest radioactivity was found in the kidney, followed by lung, spleen, bone marrow, stomach, liver, caecum and large intestine. The concentration-time profiles of most tissues were similar to that of blood.4. Within 31 days after intravenous administration at the dose of 0.5 mg/kg, the excretion of radioactivity accounted for 74.7% in urine and 11.8% in feces. Within 24 hr after oral administration at the dose of 0.5 and 5 mg/kg, 24.7 and 73.8% of administered radioactivity were excreted in urine, respectively.5. N-deethylated dorzolamide was found in the blood, urine and tissues. The ratio of the radioactive N-deethylated metabolite to the total radioactivity in blood or in tissues after administration at the dose of 5 mg/kg was larger than those after dosing at the dose of 0.5 mg/kg.6. The in vitro binding of dorzolamide and N-deethylated dorzolamide to plasma proteins accounted for less than 30% in all species studied, and these compounds were mainly bound to albumin.

Metabolic Fate of Fluvastatin, an Inhibitor of HMG-CoA Reductase. (1): Absorption, Distribution and Excretion of (14C)Fluvastatin after Single Administration in Rats.

Fluvastatin (FV) is a potent competitive inhibitor of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase. Absorption, distribution and excretion of the drug were investigated in rats after single administration of [14C]FV.1. After oral administration of [14C]FV at doses of 0.25, 1, 5, 10 and 20 mg/ kg, the radioactivity in the blood reached the maximum concentration (Cmax) between 1 and 2 hr, and declined with half-lives range of 7 ?? 11 hr. The values of both Cmax and AUC increased in a dose-dependent manner. [14C]FV was absorbed mainly from the intestine, and the extent of absorption was estimated to be 61 ?? 84%.2. Concentration of the unchanged FV in the plasma reached the maximum at 0.5 hr after oral administration of [14C]FV, and decreased with half-life of 3 ?? 4 hr. Bioavailability of the drug was about 46% at 1 mg/kg.3. Distribution of the radioactivity to tissues was rapid and the maximum concentrations were attained within 2 hr after oral administration (1 mg/kg). Majority of the radioactivity was found in the liver and gastro-intestinal contents. At 24 hr, the levels of the radioactivity in the liver and intestinal contents were low, and those in another tissues were negligible.4. Plasma protein binding of [14C]FV was over 97 ?? 99% in rat, dog and human. Binding ratio of [14C]FV to human serum albumin showed to be 94 ?? 97%. After oral administration of [14C]FV to rats, binding ratio of the radioactivity to plasma protein was 97 ?? 99%.5. Excretion of the radioactivity in urine and feces within 144 hr after oral dosing was 4% and 95% of the dose, respectively. In bile duct-cannulated rat, biliary excretion was 83%, and the 58% of the radioactivity excreted into bile was reabsorbed.6. No sex difference was observed in the pharmacokinetic parameters.

Preparation and evaluation of the homogeneity of milk as a candidate reference material for carbon-14

A pilot project was initiated to study the feasibility of preparing milk as a candidate reference material for 14C near environmental levels. Two materials, MK-B at natural level of 14C and MK-C4 at an elevated level, have been prepared from pasteurized 2% dairy milk. MK-C4 was spiked with an appropriate amount of 14C-methylated casein tracer to achieve the elevated level. Several samples from MK-B and MK-C4 have been analyzed to test the homogeneity of these materials for the distribution of 14C. The samples were combusted in oxygen under 20 atmospheres pressure using a Parr bomb. The 14C concentrations were determined by liquid scintillation counting using Carbo-Sorb/Permafluor E+ cocktail. The results indicate that these materials are homogeneous with respect to 14C concentration even in sub-sample sizes of 0.25 g of the freeze-dried material. The precision of our 14C measurements, as expressed by the % relative standard deviation, is within 5%. The accuracy has been tested by analyzing replicate samples of the IAEA 14C quality assurance materials, C-3 (cellulose) and C-6 (ANU sucrose) and found to be within 3%. The lower limits of detection are 0.08, 0.05 and 0.02 Bq.g−1 of carbon for 20 ml of liquid scintillation mixture (Carbo-Sorb/Permafluor E+ = ∼0.67) loaded with up to 0.4 g of carbon from the sample and counted for 3 cycles of 60, 180 and 1000 min each, respectively. Our measurements of 14C specific activities of MK-B and MK-C4 are 0.26±0.01 and 15.3±0.4 Bq.g−1 of carbon, respectively.

The role of the hexose transporter in the chloroplasts of Arabidopsis thaliana L.

The metabolism of wild-type Arabidopsis thaliana L. and its mutant TC265 were compared in order to reveal the role of the chloroplast glucose transporter. Plants were grown in a 12-h photoperiod. From 20 to 40 days after germination, starch per gram fresh weight of shoot in the mutant was four times that in the wild type. The extent of this difference did not alter during this period. Stereological analysis showed that the chloroplasts in the mutant were larger than those in the wild type; the thylakoids appeared to be distorted by the high starch content. [U-14C]Glucose and [U-14C]glycerol were supplied, separately, to excised leaves in the dark. [U-14C]Glucose was a good precursor of sucrose in the wild type and mutant; [U-14C]glycerol was a poor precursor of sucrose in both. The distribution of 14C in the wild type was used to calculate that the net flux was from hexose monophosphates to triose phosphates, not vice versa. During the first 4 h of the night the sugar content (75% sucrose, 20% glucose) of the leaves of the mutant dropped sharply, and at all times during the night it was less than that of the wild-type leaves. This drop in sugar coincided with a decrease in the rate of respiration. The growth rate of the mutant was less than that of the wild type. Addition of sucrose restored the rate of respiration at night and increased the rate of growth. It is argued that a major function of the glucose transporter in Arabidopsis chloroplasts is export of the products of starch breakdown that are destined for sucrose synthesis at night.