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Biomedical subjects

D A Walker

Publications and source records attributed to D A Walker.

At least 127 records · Page 7Linked to original sources
PubMed1976

Stepwise generation of the natural oxidant in a reconstituted chloroplast system.

Isolated chloroplasts which have lost their envelopes and, in consequence, the soluble components which constitute the stroma, will nevertheless evolve O(2) when supplied with an artificial oxidant (the Hill reaction). They will also evolve O(2) with NADP as the Hill oxidant if supplemented with ferredoxin. With catalytic NADP, continuing O(2) evolution can be maintained by the inclusion of a suitable reaction or reaction sequence which reoxidizes NADPH.In the Benson-Calvin cycle the terminal oxidant is glycerate 1,3-bisphosphate which is generated by phosphorylation of 3-phosphoglycerate, its immediate precursor. Experiments with a reconstituted chloroplast system are described in which this reaction sequence is catalyzed by stromal protein and supported by photophosphorylation of catalytic ADP. In the presence of CO(2), 3-phosphoglycerate can be progressively replaced by ribulose 1,5-bisphosphate, ribose 5-phosphate, fructose 6-phosphate, fructose 1,6-bisphosphate, and finally by glyceraldehyde 3-phosphate. In this last instance the natural oxidant is regenerated from its own reduction product (via the carboxylation step) and the reaction sequence therefore involves the entire photosynthetic carbon cycle.

Journal Article↗
PubMed1976

Assay of parathyroid hormone in human serum and its uses.

The assay of parathyroid hormone has contributed greatly to our understanding of calcium metabolism in health and disease. The most important clinical application of the assay is in the differential diagnosis of hypercalcaemia, which is an increasing clinical problem. PTH assays are of little or no value in the absence of other biochemical data, especially accurate determinations of plasma calcium and phosphate.

Calcium↗
PubMed1976

Continuous infusion of the conjunctival sac with pilocarpine in normal subjects and in patients with chronic glaucoma.

(1) A drug solution can be infused into the conjunctival sac by a simple method that does not cause overflow of solution. (2) At near neutral pH, pilocarpine infused at 1 mug/min was as effective as a single dose of 125 mug as a pupilloconstrictor in normal subjects. (3) At near neutral pH, pilocarpine infused at 10 mug/min was as effective as a single dose of 1 mg as an ocular hypotensive in cases of chronic simple glaucoma. (4) The infusion system described could find application in those situations in which it is essential to maintain a known and constant concentration of drug in the conjunctival sac.

Conjunctiva↗
PubMed1975

Carbon dioxide assimilation by leaves, isolated chloroplasts, and ribulose bisphosphate carboxylase from spinach.

The relationship between rate of photosynthesis and CO(2) concentration has been reinvestigated using isolated spinach (Spinacia oleracea) chloroplasts. The apparently low CO(2) concentration required for half-maximal photosynthesis is shown to result partly from a ceiling imposed by electron transport. In double reciprocal plots of rate against CO(2) concentration, this ceiling results in departures from linearity at high CO(2) concentrations. If these rate limitations are disregarded in extrapolation the "true" CO(2) concentration required for half maximal carboxylation by intact chloroplasts is approximately 46 mum (CO(2)).When assayed under comparable conditions, ribulose bisphosphate carboxylase from these chloroplasts also shows an apparent Km (CO(2)) of approximately 46 mum, suggesting that its characteristics are not modified by extraction. An improved assay for ribulose bisphosphate carboxylase yielded rates of carboxylation considerably higher than those previously reported, the highest maximal velocities recorded approaching 1000 mumoles CO(2) fixed mg(-1) chlorophyll hr(-1) at 20 C. With such Km and V(max), values the carboxylase would be able to achieve, at concentrations of CO(2) less than atmospheric, rates of CO(2) fixation equal to those displayed by the parent tissue or by the average plant under favorable conditions in its natural environment.

Journal Article↗
PubMed1974

Autocatalysis in a reconstituted chloroplast system.

In whole plants and intact chloroplasts, photosynthesis does not reach its full rate immediately upon illumination but only after a lag which is believed to reflect an autocatalytic increase in the concentration of carbon cycle intermediates. Autocatalysis has now been observed in a reconstituted system containing envelope-free chloroplasts augmented with ferredoxin and other stromal proteins but only catalytic amounts of ATP and NADP. With ribose 5-phosphate as substrate, the CO(2) dependent O(2) evolution recorded for such mixtures implies rates of "endogenous" or ferredoxin-dependent photophosphorylation as high as 360 mumoles of orthophosphate esterified mg(-1) chlorophyll hr(-1).

Journal Article↗
PubMed1972

Photosynthesis by isolated chloroplasts. Inhibition by DL-glyceraldehyde of carbon dioxide assimilation.

Photosynthetic carbon assimilation and associated CO(2)-dependent O(2) evolution by chloroplasts isolated from pea shoots and spinach leaves is almost completely inhibited by 10mm-dl-glyceraldehyde. The inhibitor is without appreciable effect on photosynthetic electron transport, photophosphorylation, the carboxylation of ribulose 1,5-diphosphate or the reduction of 3-phosphoglycerate, but apparently blocks the conversion of triose phosphate into ribulose 1,5-diphosphate.

Carbon Dioxide↗
PubMed1971

Phosphoglycerate as a hill oxidant in a reconstituted chloroplast system.

The ability of intact chloroplasts to evolve oxygen when illuminated in the presence of 3-phosphoglycerate is lost following osmotic shock but may be largely restored by the addition of ATP, catalytic amount of nicotinamide adenine dinucleotide phosphate, ferredoxin, and a soluble fraction derived from chloroplasts. In the presence of uncoupling agents and ATP, the rate is faster than that supported by intact chloroplasts. In the absence of uncoupling agents, the ATP requirement may be partially met by photophosphorylation.

Journal Article↗
PubMed1970

Paraquat toxicity.

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Adenosine Triphosphate↗