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Biomedical subjects

D A Walker

Publications and source records attributed to D A Walker.

At least 109 records · Page 6Linked to original sources
PubMed1979

Huntington's chorea. The basis for long-term prevention.

A long-term programme to reduce the incidence of Huntington's Chorea has been established in South Wales, based on a study of all individuals known to be affected or at high risk in a defined population of 1.7 million people. Systematic but non-directive genetic counselling is being given to all adults at risk, accompanied by regular follow-up to provide both further information and practical support where required. The number of observed and projected new cases of the disorder born between 1900 and 1970 has remained almost constant, despite a progressive reduction in birth-rate in the general population during this period. Prospective monitoring of all births in the high-risk population will allow an estimate to be made of future trends and will show whether preventive measures are having any significant effect on the future incidence of the disorder in the population.

Adolescent↗
PubMed1979

The control of 3-phosphoglycerate reduction in isolated chloroplasts by the concentrations of ATP, ADP and 3-phosphoglycerate.

1. Oxygen evolution by reconstituted chloroplasts with 3-phosphoglycerate as substrate was inhibited by ADP. This inhibition was overcome by increased concentrations of 3-phosphoglycerate or by the addition of excess ATP. 2. The initial rate of 3-phosphoglycerate reduction by chloroplast stromal extracts, measured as 3-phosphoglycerate-dependent oxidation of NADPH, was also dependent on the concentrations of 3-phosphoglycerate, ADP and ATP. 3. Within the range of concentrations of 3-phosphoglycerate, ADP and ATP expected to occur within intact chloroplasts, the rate of 3-phosphoglycerate (PGA) reduction by stromal extract was proportional to the ratio [ATP] [PGA]/[ADP]. 4. These results are consistent with the notion that the substrates and products of 3-phosphoglycerate kinase control the rate of 3-phosphoglycerate reduction via a mass action effect.

Adenosine Diphosphate↗
PubMed1979

Isolation of Protoplasts and Chloroplasts from Flag Leaves of Triticum aestivum L.

Intact protoplasts and chloroplasts have been isolated from mature flag leaves of wheat (Triticum aestivum L.). Both showed high rates of photosynthesis, the best of which equaled those observed in the parent tissue (greater than 150 micromoles O(2) per milligram chlorophyll per hour). The presence of ethylenediaminetetraacetate and an alkaline medium (pH 8.4) were required in the isolation and assay for the achievement of maximum rates of photosynthesis by chloroplasts. Photosynthesis by isolated chloroplasts was inhibited at very low concentrations of external orthophosphate.

Journal Article↗
PubMed1979

Intraocular pressure reduction in chronic simple glaucoma by continuous infusion of dilute pilocarpine solution.

Ten hospital outpatients with bilateral chronic simple glaucoma received a single drop of 2% pilocarpine to one eye and a continuous infusion of 0.1 % pilocarpine at a flow rate of 0.01 ml/min to the other eye (both solutions were at pH 7.2). On another occasion the treatments were reversed. Measurements of intraocular pressure (IOP) and pupil diameter were made at 30 min intervals for 2 hours. The continuous infusion of dilute solution was as effective as the single drop of more concentrated solution in reducing IOP and in constricting the pupil; the drop was somewhat faster in producing its effect.

Aged↗
PubMed1978

Comparison of the kinetic properties of ribulose bisphosphate carboxylase in chloroplast extracts of spinach, sunflower and four other reductive pentose phosphate-pathway species.

Extracts from chloroplasts of spinach, sunflower and four other reductive pentose phosphate (C3)-pathway species were measured spectrophotometrically with or without a modified preactivation procedure. In all six species this modification yielded Km (CO2) values in the range of 7-15 micron and maximum velocities, at 20 degrees C, of 129-431 mumol of CO2 carboxylated/h per mg of chlorophyll. In general, both the carboxylation and electron-transport capacities of sunflower were somewhat greater than that of the other species, and this is discussed in relation to the superior rates of photosynthesis believed to be displayed by the parent tissue.

Brassica↗
PubMed1978

Regulation of photosynthetic carbon assimilation.

It may be concluded that the conversion of PGA to DPGA plays a key role in induction and in the regulation of cycle activity. The high concentrations of PGA in actively photosynthesizing chloroplasts reflect this role and the control exerted by adenylate ratios. Thus the cycle can operate at its maximum rate only in the presence of high PGA and low ribulose 5-phosphate concentrations. Once induction is complete, the reductive pentose phosphate pathway will continue to function at its maximum rate if sink activity within the cytoplasm makes available sufficient Pi to support rapid export of triose phosphate. If triose phosphate tends to build up in the straoma, it will favor pentose monophosphate accumulation. A relative excess of ribulose 5-phosphate would, in turn, inhibit PGA reduction (and hence its own formation) by drawing too heavily on the available ATP.

Adenosine Diphosphate↗
PubMed1978

Photosynthesis by isolated protoplasts, protoplast extracts, and chloroplasts of wheat: influence of orthophosphate, pyrophosphate, and adenylates.

Protoplasts, protoplast extracts (intact chloroplasts plus extrachloroplastic material), and chloroplasts isolated from protoplasts of wheat (Triticum aestivum) have rates of photosynthesis as measured by light-dependent O(2) evolution of about 100 to 150 micromoles of O(2) per milligram of chlorophyll per hour at 20 C and saturating bicarbonate. The assay conditions sufficient for this activity were 0.4 molar sorbitol, 50 millimolar N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid KOH (pH 7.6), and 10 millimolar NaHCO(3) with protoplast, plus a requirement of 1 to 10 millimolar ethylenediaminetetraacetate (EDTA) and 0.2 to 0.5 millimolar inorganic orthophosphate (Pi) with protoplast extracts and chloroplasts. Protoplast extracts evolved approximately 6 micromoles of O(2) per milligram of chlorophyll before photosynthesis became largely dependent on exogenous Pi while photosynthesis by chloroplasts had a much stronger dependence on exogenous Pi from the outset.Photosynthesis by chloroplasts from 6-day-old wheat plants under optimum levels of Pi was similar to that with the addition of 5 millimolar inorganic pyrophosphate (PPi) plus 0.2 millimolar adenosine-5'-diphosphate (ADP). Either PPi or ADP added separately inhibited photosynthesis. When chloroplasts were incubated in the dark for 2 to 6 minutes, photosynthesis was strongly inhibited by 5 millimolar PPi and this inhibiting was relieved by including adenosine-5'-triphosphate (ATP) or ADP (0.2 to 0.6 millimolar). Chloroplasts from 9-day-old wheat leaves were slightly less sensitive to inhibition by PPi and showed little or no inhibition by ADP.Chloroplasts isolated from protoplasts and assayed with 0.3 millimolar Pi added before illumination have an induction time from less than 1 minute up to 16 minutes depending on the time of the assay after isolation and the components of the medium. In order to obtain maximum rates of photosynthesis and minimum induction time, NaHCO(3) and chelating agents, EDTA or PPi (+ATP), are required in the chloroplast isolation, resuspension and assay medium. With these inclusions in the isolation and resuspension medium the induction time decreased rapidly during the first 20 to 30 minutes storage of chloroplasts on ice. Requirements for isolating intact and photosynthetically functional chloroplasts from wheat protoplasts are discussed.

Journal Article↗
PubMed1977

Photosynthesis by isolated pea chloroplasts: some effects of adenylates and inorganic pyrophosphate.

When added singly to chloroplasts isolated from young pea (Pisum sativum) leaves, both inorganic pyrophosphate (PPi) and small quantities (0.2 mm) of ADP inhibit photosynthesis, but when added together they cause a marked stimulation. ATP (at 0.2 mm) is less inhibitory (or not inhibitory) when added alone, but like ADP, stimulates when added in the presence of PPi. This behavior is in marked contrast to that of spinach (Spinacia oleracea) chloroplasts which are normally stimulated rather than inhibited by PPi and which are largely unresponsive to exogenous adenylates. The inhibitory behavior of PPi with pea chloroplasts was observed under conditions where external hydrolysis to Pi is negligible. It is proposed that the exchange of organic and PPi across the chloroplast envelope may be more rapid in chloroplasts from young pea leaves than in chloroplasts from spinach and that interaction between these two processes could account for the principal observations.

Journal Article↗
PubMed1977

Parathyroid carcinoma in familial hyperparathyroidism.

Two families with hereditary hyperparathyroidism are described. One member of each family developed a parathyroid carcinoma. In one case this recurred locally and metastasised. This patient showed hyperplasia of one of the three other parathyroid glands. It is possible that the different parathyroid lesions found in familial hyperparathyroidism may be the result of a progression from hyperplasia to formation of benign or malignant tumours. The remainiing hyperplastic glands may be suppressed by hypercalcaemia. There was no evidence of multiple endocrine neoplasia in either family. Three members of a first family had ichthyosis and both affected members of the second had tumours of the jaw, one of which was an ossifying fibroma, suggesting a possible association of these conditions with familial hyperparathyroidism.

Adolescent↗
PubMed1976

Photosynthesis in a reconstituted chloroplast system from spinach. Some factors affecting CO2-dependent oxygen evolution with fructose-1,6-bisphosphate as substrate.

When envelope-free spinach chloroplasts are incubated with stromal protein, catalytic NADP, catalytic ADP, radioactive bicarbonate and fructose 1,6-bisphosphate, 14CO2 fixation starts immediately upon illumination but oxygen evolution is delayed. The delay is increased by the addition of fructose 6-phosphate and by a variety of factors known (or believed) to increase fructose bisphosphatase activity (such as dithiothreitol, more alkaline pH, higher [Mg] and antimycin A). Conversely, the lag can be decreased or eliminated by the addition of an ATP-generating system. Bearing in mind the known inhibition, by ADP, of sn-phospho-3-glycerate (3-phosphoglycerate) reduction it is concluded that the lag in O2 evolution results from the production of ribulose 5-phosphate from fructose bisphosphate and that this in turn inhibits the reoxidation of NADPH by adversely affecting the ADP/ATP ratio. The results are discussed in their relation to the mode of action of antimycin A and to regulation of the reductive pentose phosphate pathway.

Carbon Dioxide↗
PubMed1976

Transient inhibition by ribose 5-phosphate of photosynthetic O2 evolution in a reconstituted chloroplast system.

Photosynthetic oxygen evolution by a reconstituted chloroplast system utilising sn-phospho-3-glycerol (3-phosphoglycerate) ceases upon the addition of ribose 5-phosphate even though the presence of this metabolite permits a rapid and immediate CO2 fixation. The period of cessation is appreciable at 0.1 mM ribose 5-phosphate. It is lengthened as the amount of added ribose 5-phosphate is increased and by the addition of dithiothreitol, a known activator of ribulose-5-phosphate kinase. Ribulose 1,5-bisphosphate is without effect. A similar interruption of O2 evolution may also be brought about by the addition of ADP or by ADP-generating systems such as glucose plus hexokinase. Spectrophotometric experiments indicate that the reoxidation of NADPH in the presence of sn-phospho-3-glycerol is similarly affected. The transient inhibition by ribose 5-phosphate is not observed in the presence of an active ATP-generating system or in the presence of sufficient DL-glyceraldehyde to inhibit ribulose-5-phosphate kinase activity. It is concluded that ribose 5-phosphate inhibits photosynthetic O2 evolution by adversely affecting the steady-state ATP/ADP ratio and consequently the reduction of sn-phospho-3-glycerol to glyceraldehyde 3-phosphate. The results are discussed in their relation to ADP regulation of photosynthetic carbon assimilation and metabolite transport.

Carbon Dioxide↗
PubMed1976

Inhibition of spinach phosphoribulokinase by DL-glyceraldehyde.

Spinach chloroplast phosphoribulokinase is inhibited by DL-glyceraldehyde. The inhibition is non-competitive with respect to ribulose 5-phosphate (Ki 19mM) and ATP (Ki 20mM). The inhibition is discussed in relation to a previously reported inhibition of CO2 assimilation in intact and envelope-free chloroplasts by DL-glyceraldehyde. It is concluded that the inhibition of phosphoribulokinase is insufficient to account for the inhibition, by DL-glyceraldehyde, of O2 evolution with ribose 5-phosphate as substrate and that a further site of inhibition is also present in this system.

Adenosine Triphosphate↗
PubMed1976

Localization of inhibition by adenosine diphosphate of phosphoglycerate-dependent oxygen evolution in a reconstituted chloroplast system.

ADP was shown to inhibit phosphoglycerate-dependent O2 evolution in a simplified reconstituted chloroplast system containing 3-phosphoglycerate kinase and triose phosphate dehydrogenase. Rates of O2 evolution in the simplified system are comparable with those obtained by using stromal protein rather than purified enzymes. ADP does not inhibit O2 evolution with glycerate 1,3-biphosphate as substrate nor does it inhibit triose phosphate dehydrogenase. The inhibitory effect of ADP is attributed to an increase in the rate of conversion of glycerate biphosphate into phosphoglycerate. The results are discussed in terms of control by ADP of phosphoglycerate-dependent oxygen evolution.

Adenosine Diphosphate↗