Asunto(s)

RESUMEN

Photorespiration in Chlorella pyrenoidosa Chick. was assayed by measuring (18)O-labeled intermediates of the glycolate pathway. Glycolate, glycine, serine, and excreted glycolate were isolated and analyzed on a gas chromatograph/mass spectrometer to determine isotopic enrichment. Rates of glycolate synthesis were determined from (18)O-labeling kinetics of the intermediates, pool sizes, derived rate equations, and nonlinear regression techniques. Glycolate synthesis was higher in high CO(2)-grown cells than in air-grown cells when both were assayed under the same O(2) and CO(2) concentrations. Synthesis of glycolate, for both types of cells, was stimulated by high O(2) levels and inhibited by high CO(2) levels. Glycolate synthesis in 1.5% CO(2)-grown Chlorella, when exposed to a 0.035% CO(2) atmosphere, increased from about 41 to 86 nanomoles per milligram chlorophyll per minute when the O(2) concentration was increased from 21% to 40%. Glycolate synthesis in air-grown cells increased from 2 to 6 nanomoles per milligram chlorophyll per minute under the same gas levels. Synthesis was undetectable when either the O(2) concentration was lowered to 2% or the CO(2) concentration was raised to 1.5%. Glycolate excretion was also sensitive to O(2) and CO(2) concentrations in 1.5% CO(2)-grown cells and the glycolate that was excreted was (18)O-labeled. Air-grown cells did not excrete glycolate under any experimental condition. Indirect evidence indicated that glycolate may be excreted as a lactone in Chlorella. Photorespiratory (18)O-labeling kinetics were determined for Pavlova lutheri, which unlike Chlorella and higher plants did not directly synthesize glycine and serine from glycolate. This alga did excrete a significant proportion of newly synthesized glycolate into the media.

RESUMEN

A method was devised to quantify short-term photorespiratory rates in terrestrial plants using (18)O-intermediates of the glycolate pathway, specifically glycolate, glycine, and serine. The pathway intermediates were isolated and analyzed on a GC/MS to determine molecular percent (18)O-enrichment. Rates of glycolate synthesis were determined from (18)O-labeling kinetics of the intermediates, derived rate equations, and nonlinear regression techniques. Glycolate synthesis in wheat (Triticum aestivum L.), a C(3) plant, and maize (Zea mays L.), a C(4) plant, was stimulated by high O(2) concentrations and inhibited by high CO(2) concentrations. The synthesis rates were 7.3, 2.1, and 0.7 micromoles per square decimeter per minute under a 21% O(2) and 0.035% CO(2) atmosphere for leaf tissue of wheat, maize seedlings, and 3-month-old maize, respectively. Photorespiratory CO(2) evolution rates were estimated to be 27, 6, and 2%, respectively, of net photosynthesis for the three groups of plants under the above atmosphere. The results from maize tissue support the hypothesis that C(4) plants photorespire, albeit at a reduced rate in comparison to C(3) plants, and that the CO(2)/O(2) ratio in the bundle sheath of maize is higher in mature tissue than in seedling tissue. The pool size of the three photorespiratory intermediates remained constant and were unaffected by changes in either CO(2) or O(2) concentrations throughout the 10-minute labeling period. This suggests that photorespiratory metabolism is regulated by other mechanism besides phosphoglycolate synthesis by ribulose-1,5-bisphosphate carboxylase/oxygenase, at least under short-term conditions. Other mechanisms could be alternate modes of synthesis of the intermediates, regulation of some of the enzymes of the photorespiratory pathway, or regulation of carbon flow between organelles involved in photorespiration. The glycolate pool became nearly 100% (18)O-labeled under an atmosphere of 40% O(2). This pool failed to become 100% (18)O-enriched under lower O(2) concentrations.

RESUMEN

Of 68 patients who presented between the ages of 10 and 35 years with elevated intraocular pressure, 25 were classified as juvenile ocular hypertension and 43 as juvenile primary open-angle glaucoma. Blacks constituted a greater proportion of the primary open-angle glaucoma patients (47%) than of the ocular hypertensives (20%) and in both groups presented at younger ages than did whites. Myopia was present in 59% of the ocular hypertensives and 73% of the primary open-angle glaucoma patients, of whom 39% had more than 6 diopters of myopia. All eyes of black patients with more than 3 diopters of myopia had glaucomatous defects compared with 52% of such eyes of white patients. Our data suggest that myopia is strongly associated with juvenile open-angle glaucoma and that young black patients with elevated intraocular pressure, especially when myopic, are more susceptible to glaucomatous damage than are whites.

Asunto(s)

RESUMEN

The effects of various calcium ion antagonists and ion transport inhibitors on photosynthetic O(2) evolution of corals, isolated zooxanthellae, sea anemone tentacles, and Chlorococcum oleofaciens were measured. Only the phenothiazine drugs were effective at inhibiting photosynthesis. Trifluoperazine, a calcium ion antagonist drug, inhibited at low concentrations, with 10(-4) molar and 8 x 10(-6) molar completely abolishing photosynthesis in the intact corals and isolated zooxanthellae, respectively. Net photosynthetic O(2) evolution of C. oleofaciens was eliminated by concentrations of trifluoperazine as low as 2.8 x 10(-5) molar.

RESUMEN

The marine diatom Amphora sp. was grown at a light intensity of 7.0 x 10(15) quanta centimeter(-2) second(-1). Light saturation of photosynthesis for these cells was between 6.0 and 7.0 x 10(16) quanta centimeter(-2) second(-1). At light intensities greater than saturation, photosynthetic (14)CO(2) fixation was depressed, while P700 unit size (chlorophyll a concentration/P700 activity) increased and number of P700 units per cell decreased. After a 1-hour exposure of Amphora sp. to a photoinhibitory light intensity of 2.45 x 10(17) quanta centimeter(-2) second(-1), there was a 45 to 50% decrease in the rate of (14)CO(2) fixation relative to the rate at the culture light intensity. There also was a 25% increase in P700 unit size and a 30% reduction in the number of P700 units per cell but no change in total chlorophyll a concentration. Following this period of photoinhibition, the cells were returned to a light regime similar to that in the original culture conditions. Within 1 hour, both number of P700 units per cell and P700 unit size returned to levels similar to those of cells which were kept at the culture light intensity. The rates of photosynthesis did not recover as rapidly, requiring 2 to 3 hours to return to the rate for the nonphotoinhibited cells. Our results indicate that a decrease in P700 activity (with a resultant increase in P700 unit size) may be partially responsible for the photoinhibition of algal photosynthetic carbon dioxide fixation.

RESUMEN

The effect of external pH on two laboratory-cultured acid-intolerant species (Chlorella pyrenoidosa Chick and Scenedesmus quadricauda Turp. Bréb.) and one acid-tolerant species from a natural population (Euglena mutabilis Schmitz) was examined by measuring internal pH. These measurements were made with the weak acid (14)C-dimethyloxazolidine-2,4-dione after cells had been incubated for 2 and 6 hours at external pH levels from 3.0 to 8.0. Photosynthetic and respiration rates of the three species were also measured over the range of external pH levels.All three species regulated their internal pH levels over the 6-hour incubation time. C. pyrenoidosa and S. quadricauda had internal pH levels around 7.0, regardless of external pH. E. mutabilis had a wider internal pH range, from 5.0 at low external pH to 8.0 at high external pH. External pH had no effect on either photosynthetic or respiration rates. Statistical comparisons showed that there was a significant difference between the acid-intolerant and acid-tolerant species with regard to the level of internal pH maintained and the response of internal pH to external pH.

Asunto(s)