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Biomedical subjects

M C Evans

Publications and source records attributed to M C Evans.

At least 19 recordsLinked to original sources
PubMed1992

An e.x.a.f.s. study of the manganese O2-evolving complex in purified Photosystem II membrane fractions. The S1 and S2 states.

Manganese K-edge X-ray spectra have been obtained for Photosystem II samples isolated using Triton X-100 detergent and samples further purified by n-heptyl beta-D-thioglucoside detergent treatment to remove light-harvesting polypeptides and low-affinity calcium. The structure of the manganese complex is very similar for the two detergent preparations used. Analysis of the e.x.a.f.s. spectra for samples in the S1 and S2 states indicate changes in bond lengths for the shells of oxygen/nitrogen atoms. For the S1 state, oxygen shells at 0.181 and 0.193 nm (1.81 and 1.93 A) were observed and one manganese shell at 0.270 nm (2.70A). In the S2 state the oxygen bond lengths are longer at 0.184 and 0.200 nm (1.84 and 2.00 A). Additionally a shell of scatterers at 0.37 nm (3.7 A) was observed in both states which could be fitted to models with calcium scatterers at this distance.

Cell Membrane↗
PubMed1992

Investigation of the origin of the "S3" EPR signal from the oxygen-evolving complex of photosystem 2: the role of tyrosine Z.

The origin of the "S3" EPR signal from calcium-depleted photosystem 2 samples has been investigated. This signal is observed after freezing samples under illumination and has been assigned to an interaction between the manganese cluster and an oxidized histidine radical [Boussac et al. (1990) Nature 347; 303-306]. In calcium-depleted samples prepared by three different methods, we observed the trapping of the tyrosine radical YZ+ under conditions which also formed the "S3" signal. An "S3"-type signal and YZ+ were also formed in PS2 samples treated with the water analogue ammonia. Following illumination at 277 K, the "S3" and YZ+ signals decayed at the same rate at 273 K in the dark. Both the YZ+ and "S3" signals decayed on storage at 77 K and could be subsequently regenerated by illumination at 8-77 K. No evidence to support histidine oxidation was found. The effects of DCMU, chelators, and alkaline pH on the dark-stable multiline S2 and the "S3" signals from calcium-depleted samples were determined. Both signals required the presence of EGTA or citrate for maximum yield. The addition of DCMU caused a reduction in the yield of "S3" generated by freezing under illumination. Incubation at pH 7.5 resulted in the loss of both signals. We propose that a variety of treatments which affect calcium and chloride binding cause a stabilization of the S2 state and slow the reduction of YZ+. This allows the trapping of YZ+, the interaction with the manganese cluster (probably in the S2 state) resulting in the "S3" signal. The data allow the position of the manganese cluster to be estimated as within 10 A of tyrosine Z (D1-161).

Ammonia↗
PubMed1992

Lateral spine densitometry is a more sensitive indicator of glucocorticoid-induced bone loss.

Osteoporosis is a common complication of glucocorticoid therapy. Bone density measurement is now commonly used in assessing which steroid-treated patients require specific interventions to reduce fracture risk. The recently developed techniques for the measurement of bone mineral density (BMD) of the vertebral body alone, by dual-energy x-ray absorptiometry (DXA) in the lateral projection, may be particularly useful in this context since steroid-induced bone loss is most marked in trabecular-rich regions like the vertebral body. This possibility has been assessed in the present study by the measurement of BMD in the lateral and anterioposterior (AP) projections in 28 women receiving chronic glucocorticoid treatment. The two BMD measurements were significantly related (r = 0.62, p < 0.001). When expressed in relation to age-appropriate normal values, lateral BMDs were lower than AP BMDs both in percentage terms (70.8 +/- 4.4 versus 90.3 +/- 2.6%, p < 0.001) and in terms of Z scores (-1.42 +/- 0.22 versus -0.91 +/- 0.24, p = 0.027). AP BMD Z scores classified 12 patients as osteopenic, whereas a further 7 were so categorized by lateral BMD Z score. It is concluded that lateral DXA scanning is a more sensitive indicator of glucocorticoid-induced osteopenia than conventional BMD measurement in the AP projection.

Absorptiometry, Photon↗
PubMed1992

Bone mineral density of the proximal femur and lumbar spine in glucocorticoid-treated asthmatic patients.

The importance of the proximal femur as a site of osteoporotic fractures, the development of techniques for bone mineral density (BMD) measurement at this site and the apparent selectivity of the osteopenic effects of glucocorticoids have focused attention on the assessment of proximal femoral BMD in steroid-treated subjects. We have, therefore, measured BMD (Lunar DPX) in the lumbar spine and proximal femur of 31 asthmatic patients receiving long-term glucocorticoid therapy (mean +/- SEM dose 16 +/- 1 mg prednisone/day, mean duration 10 +/- 2 years). BMD values expressed as the percentage of normal age- and sex-appropriate mean values, after weight adjustment, were as follows: lumbar spine 80 +/- 2%, femoral neck 83 +/- 2%, Ward's triangle 78 +/- 3% and trochanter 86 +/- 2%. All these values were significantly less than control (p less than 0.0001) and the decrement in BMD was more marked in Ward's triangle than at the other two femoral sites (p less than 0.05). In all regions BMD was unrelated to dose or duration of steroid treatment. It is concluded that there are reductions in the BMD of the lumbar spine and proximal femur in glucocorticoid-treated asthmatics, probably reflecting the mixed cortical/trabecular make-up of both regions.

Adult↗
PubMed1992

Relationships between upper-arm anthropometry and soft-tissue composition in postmenopausal women.

The value of upper-arm anthropometry as a measure of lean- and fat-tissue masses in 140 normal postmenopausal white women was assessed by studying the relationships of these measures to total and regional fat, and lean masses measured by dual-energy x-ray absorptiometry. Midarm circumference (MAC) was highly correlated with total and regional fat masses (r = 0.85-0.89, P less than 0.0001) but less so with lean-tissue masses (r = 0.26-0.34). Triceps-skinfold thickness was also correlated with fat masses (r = 0.70-0.74, P less than 0.0001) but not with lean mass (r = 0.02-0.09). The derived index, arm muscle area (AMA), was less closely correlated with fat mass than was MAC (r = 0.59-0.61), but its correlation with lean-tissue mass was comparable to that for MAC (r = 0.37-0.43). Multiple-regression analysis confirmed that the anthropometric indices were more closely correlated with fat than with lean mass. It is concluded that all these indices are useful measures of fat mass but that none, including AMA, is a specific index of lean-tissue mass in normal postmenopausal women.

Adipose Tissue↗
PubMed1992

Normal bone mineral density following cure of Cushing's syndrome.

OBJECTIVE: Both endogenous and exogenous glucocorticoid excess are well established as causes of osteoporosis. However, there are few data describing bone mineral density in these subjects following the restoration of normal steroid levels. The present study addresses this issue. DESIGN: A cross-sectional assessment of bone mineral density in patients cured of Cushing's syndrome, and comparison of each with four normal subjects matched by age, sex, weight, menopausal status and race, was used. PATIENTS: Seventeen adults cured of Cushing's syndrome 8.6 +/- 1.6 years (mean +/- SEM) took part. MEASUREMENTS: The bone mineral density of the lumbar spine and proximal femur was measured by dual energy X-ray absorptiometry. RESULTS: Bone mineral densities, relative to control, were 100 +/- 16, 98 +/- 14, 97 +/- 19 and 98 +/- 16% (mean +/- SD), in the lumbar spine, femoral neck, Ward's triangle and trochanteric regions, respectively. There was a positive relationship between bone density and time since cure (r = 0.24-0.59, in the four regions). In contrast, bone density was significantly reduced in five subjects with active Cushing's disease when similarly matched (BMD = 87 +/- 4, 83 +/- 4, 75 +/- 6 and 82 +/- 6%, in the respective regions; 0.01 less than P less than 0.05). CONCLUSIONS: Bone density is reduced in subjects with Cushing's syndrome but not in those having undergone cure some years previously. This implies that steroid-induced osteoporosis is substantially reversible, though long-term prospective studies will be necessary to establish this definitively.

Adrenalectomy↗
PubMed1992

Post-pregnancy osteoporosis associated with hypercalcaemia.

A lactating 31-year-old woman who developed four vertebral fractures 1-2 months after the delivery of her first child is described. She was hypercalcaemic (serum calcium up to 2.99 mmol/l), and urinary excretion of both calcium and hydroxyproline (an index of bone resorption) were markedly elevated. Serum levels of parathyroid hormone and 1,25-dihydroxyvitamin D were suppressed, but parathyroid hormone-related peptide was above normal. There was severe axial osteopenia, as assessed by dual-energy X-ray absorptiometry. The biochemical abnormalities were reversed within 2 weeks of weaning, with the exception of the parathyroid hormone-related peptide concentration, which declined more gradually. This appears to be the first description of the abnormalities in calcium metabolism and bone density from early in the course of post-pregnancy osteoporosis and it indicates that this condition is associated with high levels of osteolysis which return to normal after weaning. This rapid reversal of the metabolic abnormalities accounts for the inconclusive nature of previous descriptions of post-pregnancy osteoporosis, in which investigations were more delayed. It also emphasizes the importance of weaning in the management of women presenting with this condition. A possible aetiological role for parathyroid hormone-related peptide is discussed.

Adult↗
PubMed1992

Determinants of total body and regional bone mineral density in normal postmenopausal women--a key role for fat mass.

In order to determine which clinical, anthropometric, dietary, and biochemical variables are independent predictors of total and regional bone mineral density (BMD) in normal postmenopausal women, a cross-sectional study of 140 normal postmenopausal women has been carried out. Subjects were white, aged 45-71 yr (mean 58 yr), and had no history of disorders or medication use likely to influence bone or calcium metabolism. Multiple regression analysis was used to derive models for total and regional BMD in terms of the other variables measured. The analysis indicated that total body BMD was positively related to fat mass (P less than 0.0001), serum estrone (P = 0.0095) and age at menopause (P = 0.0165), and negatively related to age (P less than 0.0001), 24-h urine calcium (P = 0.0002), sex hormone-binding globulin (P = 0.0003), and serum alkaline phosphatase activity (P = 0.0029) (R2 = 0.61). Similar relationships were found in the subregions of the total body scans and in the lumbar spine and proximal femur, with insulin-like growth factor-1, parity, and age at menarche also being related to BMD at at least two of these sites. Lean body mass was not an independent correlate of BMD at any site once fat mass was taken into account. Muscle strength, physical activity, alcohol intake, and dietary intakes of calcium, sodium and protein did not emerge as significant predictors of BMD in this homogeneous group of postmenopausal women. We conclude that total body fat is the most significant predictor of BMD throughout the skeleton and this relationship is not explicable in terms of either estrone production in fat tissue or the dependence of skeletal load-bearing on fat mass. The mechanism underlying this relationship is an important question to be addressed in bone biology.

Aged↗
PubMed1992

Fat mass is an important determinant of whole body bone density in premenopausal women but not in men.

We recently reported that total body fat mass is the principal determinant of bone density in normal postmenopausal women. We have now reexamined the relationships among these variables and lean mass in 68 healthy premenopausal women and 51 men. Areal bone density (BMD), fat mass, and lean mass were measured in total body scans by dual-energy, x-ray absorptiometry. In women, BMD was correlated with weight (r = 0.69), fat mass (r = 0.60), and lean mass (r = 0.55). In men, the respective correlations were 0.56, 0.26 (NS), and 0.51. Multiple regression analysis confirmed a codependence of female BMD on fat and lean masses, whereas male BMD was related only to lean mass. Because BMD is an areal not volumetric density, it is dependent on body size. The analysis was therefore repeated using BMD/height as an index of "true" density. Correlations with fat mass were little changed but those with lean mass were reduced (women) or eliminated (men). By multiple regression, female BMD/height was related to fat mass alone, and in men there was a borderline effect of fat (P = 0.05) but none of lean mass. As a second method to exclude a scale artifact, fat mass was expressed as percent body weight. It was related to BMD (r = 0.48) only in women. It is concluded that bone density is closely related to fat mass in premenopausal women, but less so in men. In both sexes, apparent relationships between BMD and lean mass are artifacts attributable to the use of areal density (which is dependent on body size) as a surrogate for volumetric density. The mechanism of this fat-bone density relationship is an important question to be addressed in bone biology.

Adipose Tissue↗
PubMed1979

Mössbauer spectra of photosystem-I-reaction centres from the blue-green alga Chlorogloea fritschii.

Substantial amounts of iron have been shown by Mössbauer spectroscopy to be present in Photosystem-I preparations from the blue-green alga Chlorogloeta fritschii. Changes in the spectra on chemical reduction provide evidence that some of this iron is very similar to that found in the 4Fe-4S centres of ferredoxins. Such reduced samples also show e.p.r. signals consistent with maximum reduction of iron-sulphur centres A and B of Photosystem I. An unchanged component in the spectra indicates, assuming all centres A and B are reduced, the presence of another iron-containing species.

Cell Membrane↗
PubMed1979

Electron-paramagnetic-resonance measurements of the electron-transfer components of the reaction centre of Rhodopseudomonas viridis. Oxidation--reduction potentials and interactions of the electron acceptors.

Oxidation-reduction potentiometry was carried out on Rhodopseudomonas viridis chromatophores. Measurements of e.p.r. signals of the semiquinone-iron type at g=1.82 have revealed a more complex situation than previously reported. The presence of three different components is indicated. The midpoint potential (E(m)) of the primary acceptor quinone/semiquinone couple was found to be approx. -165mV at pH10, with a pK being reached at around pH7.5. The primary acceptor also accepts a second electron with an E(m) of -525mV, but this redox transition exhibits a hysteresis effect. Interaction effects indicate the presence of another component with E(m) values at pH10 of approx. -165mV (pK reached at around pH7.5) for single reduction and -350mV (pK at pH10 or greater) for double reduction. It is suggested that this component is the secondary acceptor. Another semiquinone-iron-type component which gives a g=1.82 signal is also present. This component is distinguishable from the primary acceptor by its e.p.r. spectrum, which shows a double peak at g=1.82 and a g(x) line at g=1.76. This component has E(m) values at pH10 for single and double reduction of -15mV and approx. -150mV respectively. Both of these E(m) values are pH-dependent. The presence of an interaction between this component and the photoreduced primary acceptor indicates the close proximity of these components. However, the midpoint potential of this component indicates a function as a secondary electron-transport component rather than an electron acceptor in the reaction centre. The dependence of the bacteriopheophytin intermediate (I) doublet e.p.r. signal on the presence of the semiquinone-iron form of the primary acceptor is demonstrated. The midpoint potential of the I/I(-) couple is estimated to be lower than -600mV.

Bacterial Chromatophores↗
PubMed1979

A surgical approach to primary hyperparathyroidism, 1971-1976.

Forty-two patients with primary hyperparathyroidism were seen in Auckland Hospital between 1971-1976. These patients were reviewed as to age, sex, mode of presentation, pathology and location of lesions. All were treated surgically. There were two deaths due to severe hypercalcaemia. The value of parathyroid hormone assay and neck vein catheterisation is assessed. The practical aspects of neck exploration for hyperparathyroidism are outlined. The two problems of the treatment of chemical hyperparathyroidism and the recognition and treatment of parathyroid hyperplasia are discussed.

Adenoma↗
PubMed1978

Variable chlorophyll a fluorescence from P-700 enriched photosystem I particles dependent on the redox state of the reaction centre.

1. Photosystem I particles enriched in P-700 prepared by Triton X-100 treatment of chloroplasts show a light-induced increase in fluorescence yield of more than 100% in the presence of dithionite but not in its absence. 2. Steady state light maintains the P-700, of these particles, in the oxidised state when ascorbate is present but in the presence of dithionite only a transient oxidation occurs. 3 EPR data show that, in these particles, the primary electron acceptor (X) is maintained in the reduced state by light at room temperature only when the dithionite is also present. In contrast, the secondary electron acceptors are reduced in the dark by dithionite. 4. Fluorescence emission and excitation spectra and fluorescence lifetime measurements for the constant and variable fluorescence indicate a heterogeneity of the chlorophyll in these particles. 5. It is concluded that the variable fluorescence comes from those chlorophylls which can transfer their energy to the reaction centre and that the states PX and P+X are more effective quenchers of chlorophyll fluorescence than PX-, where P is P-700.

Chlorophyll↗
PubMed1978

The role of the membrane-bound iron-sulphur centres A and B in the photosystem I reaction centre of spinach chloroplasts.

Photosystem I particles prepared from spinach chloroplast using Triton X-100 were frozen in the dark with the bound iron-sulphur Centre A reduced. Illumination at cryogenic temperatures of such samples demonstrated the photoreduction of the second bound iron-sulphur Centre B. Due to electron spin-electron spin interaction between these two bound iron-sulphur centres, it was not possible to quantify amounts of Centre B relative to the other components of the Photosystem I reaction centre by simulating the line-shape of its EPR spectrum. However, by deleting the free radical signal I from the EPR spectra of reduced Centre A alone or both Centres A plus B reduced, it was possible to double integrate these spectra to demonstrate that Centre B is present in the Photosystem I reaction centre in amounts comparable to those of Centre A and thus also signal I (P-700) and X. Oxidation-reduction potential titrations confirmed that Centre A had Em congruent to -550 mV, Centre B had Em congruent to -585 mV. These results, and those presented for the photoreduction of Centre B, place Centre B before Centre A in the sequence of electron transport in Photosystem I particles at cryogenic temperatures. When both A and B are reduced, P-700 photooxidation is reversible at low temperature and coupled to the reduction of the component X. The change from irreversible to reversible P-700 photooxidation and the photoreduction of X showed the same potential dependence as the reduction of Centre B with Em congruent to -585 mV, substantiating the identification of X as the primary electron acceptor of Photosystem I.

Chloroplasts↗