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A F Miller

Publications and source records attributed to A F Miller.

29 records · Page 2Linked to original sources
PubMed1993

The effect of acacia gum and a water-soluble dietary fiber mixture on blood lipids in humans.

Water-soluble dietary fibers (WSDF) are generally thought to lower cholesterol. This study compared the cholesterol-lowering effects of a medium viscosity WSDF mixture (psyllium, pectin, guar gum and locust bean gum) with an equal amount of WSDF from acacia gum, which has a lower viscosity. Hypercholesterolemic males (n = 13) and females (n = 16) were randomly assigned to one of two WSDF treatments provided in a low-calorie powder form for mixing into beverages (< 4 kcal/serving). Subjects were instructed to mix powders into their usual beverages and to consume them three times daily (5 g WSDF/serving) for 4 weeks while consuming their typical fat-modified diets. Exercise and body weights were also held constant. The WSDF mixture yielded a 10% decrease in plasma total cholesterol (from 251 +/- 20 to 225 +/- 19 mg/dL; p < 0.01), and a 14% reduction in low-density lipoprotein cholesterol (from 167 +/- 14 to 144 +/- 14 mg/dL; p < 0.001). No significant changes in plasma high-density lipoprotein cholesterol, very-low-density lipoprotein cholesterol or triglycerides were observed. In contrast, the acacia gum-treated group showed no change in any plasma lipid parameters. The WSDF treatments did not produce significant changes in mean dietary intakes within or between treatment groups. These data support previous findings that a diet rich in select WSDF can be a useful cholesterol-lowering adjunct to a fat-modified diet, but that caution should be exercised in ascribing cholesterol-lowering efficacy to dietary fibers based solely on their WSDF classification. Finally, WSDF viscosity is a potential cholesterol-lowering factor to be explored further.

Adult↗
PubMed1992

NMR studies of the conformational change in human N-p21ras produced by replacement of bound GDP with the GTP analog GTP gamma S.

1H-Detected 15N-edited NMR in solution was used to study the conformational differences between the GDP- and GTP gamma S-bound forms of human N-p21ras. The amide protons of 15N-labeled glycine and isoleucine were observed. Resonances were assigned to residues of particular interest, glycines-60 and -75 and isoleucines-21 and -36, by incorporating various 13C-labeled amino acids in addition to [15N]glycine and [15N]iosleucine and by replacing Mg2+ by Co2+. When GTP gamma S replaced GDP in the active site of p21ras, only 5 of the 14 glycine amide resonances show major shifts, indicating that the conformational effects are fairly localized. Responsive glycines-10, -12, -13, and -15 are in the active site. Gly-75, located at the far end of a conformationally-active loop and helix, also responds to substitution of GTP gamma S for GDP, while Gly-77 does not, supporting a role for Gly-75 as a swivel point for the conformational change. The amide proton resonances of isoleucines-36 and -21 and a third unidentified isoleucine also undergo major shifts upon replacement of GDP by GTP gamma S. Thus, the effector-binding loop containing Ile-36 is confirmed to be involved in the conformational change, and the alpha-helix containing Ile-21 is also shown to be affected.

Carbon Isotopes↗
PubMed1992

The dependence on iron availability of allocation of iron to nitrogenase components in Klebsiella pneumoniae and Escherichia coli.

Nitrogenase contains approximately 38 iron ions/complete unit. Therefore, we sought to identify steps and genes involved in nitrogenase production that are responsive to iron availability. We have characterized nitrogenase production in Klebsiella pneumoniae grown in a range of different iron concentrations. We find significant accumulation (50-75%) and normal synthesis rates of the structural polypeptides, even under conditions in which the observed nitrogenase activities are only 14-28% of those observed in iron-sufficient conditions. Thus, maturation instead of synthesis of the structural polypeptides is primarily responsible for the iron dependence of nitrogenase activity. We have also used a binary plasmid system in Escherichia coli to investigate the contributions of various nitrogen fixation (nif) genes to the iron dependence of nitrogenase production. At least one of the nif genes DKTYENXUSVW can modulate synthesis of the structural polypeptide NIF H in response to iron availability. We speculate that an iron-deficient complex of the product(s) of at least one of these genes may repress structural polypeptide synthesis in iron-depleted K. pneumoniae. Such a system would compensate for the inactivity of NIF L in iron-depleted cultures and ensure balanced production of the structural polypeptides of nitrogenase in accordance with the iron available for their maturation.

Amino Acid Sequence↗
PubMed1991

A guide to electron paramagnetic resonance spectroscopy of Photosystem II membranes.

This guide is intended to aid in the detection and identification of paramagnetic species in Photosystem II membranes, by electron paramagnetic resonance spectroscopy. The spectral features and occurrence of each of the electron paramagnetic resonance signals from Photosystem II are discussed, in relation to the nature of the moiety giving rise to the signal and the role of that species in photosynthetic electron transport. Examples of most of the signals discussed are shown. The electron paramagnetic resonance signals produced by the cytochrome b6f and Photosystem I complexes, as well as the signals from other common contaminants, are also reviewed. Furthermore, references to seminal experiments on bacterial reaction centers are included. By reviewing both the spectroscopic and biochemical bases for the electron paramagnetic resonance signals of the cofactors that mediate photosynthetic electron transport, this paper provides an introduction to the use and interpretation of electron paramagnetic resonance spectroscopy in the study of Photosystem II.

Cell Membrane↗
PubMed1990

Electron-transfer events leading to reconstitution of oxygen-evolution activity in manganese-depleted photosystem II membranes.

O2-evolution activity and the Mn complex can be reconstituted in photosystem II by a process called photoactivation. We have studied the elementary steps in photoactivation by using electron paramagnetic resonance spectroscopy to probe electron transport in Mn-depleted photosystem II membranes. The electron donation reactions in Mn-depleted photosystem II were found to be identical with those in untreated photosystem II, except that electron donation from the Mn complex was absent and could be replaced by slower electron donation from exogenous Mn2+. Mn2+ photooxidation by Mn-depleted photosystem II membranes correlates with reconstitution of O2-evolution activity. However, photooxidation of Mn2+ occurs in competition with photooxidation of the tyrosine residue YD, and cytochrome b-559. Thus, these two species are excluded from direct participation in the initial steps in the assembly of the Mn complex. Because photooxidation of Mn2+ is slower than photooxidation of the competing electron donors, cytochrome b-559 and chlorophyll, as well as recombination of the charge-separated states chlorophyll+QA- or YZ+QA-, these other reactions dominate in a single photochemical turnover reaction. This provides a molecular basis for both the low yield and low quantum yield of photoactivation. The first photochemical step in the assembly of the Mn complex results in photooxidation of one Mn2+ ion. Therefore, the first intermediate in assembly of the Mn complex contains Mn3+. On the basis of these results and previous kinetic studies [Miller, A.-F., & Brudvig, G. W. (1989) Biochemistry 28, 8181], we conclude that the second intermediate of Mn complex assembly contains Mn2+Mn3+, which is photooxidized to Mn3+2.

Cell Membrane↗
PubMed1989

Characterization of the multiple forms of cytochrome b559 in photosystem II.

Cytochrome b559 is an essential component of the photosystem II (PSII) protein complex. Its function, which has long been an unsolved puzzle, is likely to be related to the unique ability of PSII to oxidize water. We have used EPR spectroscopy and spectrophotometric redox titrations to probe the structure of cytochrome b559 in PSII samples that have been treated to remove specific components of the complex. The results of these experiments indicate that the low-temperature photooxidation of cytochrome b559 does not require the presence of the 17-, 23-, or 33-kDa extrinsic polypeptides or the Mn complex (the active site in water oxidation). We observe a shift in the g value of the EPR signal of cytochrome b559 upon warming a low-temperature photooxidized sample, which presumably reflects a change in conformation to accommodate the oxidized state. At least three redox forms of cytochrome b559 are observed. Untreated PSII membranes contain one high-potential (375 mV) and one intermediate-potential (230 mV) cytochrome b559 per PSII. Thylakoid membranes also appear to contain one high-potential and one intermediate-potential cytochrome b559 per PSII, although this measurement is more difficult due to interference from other cytochromes. Removal of the 17- and 23-kDa extrinsic polypeptides from PSII membranes shifts the composition to one intermediate-potential (170 mV) and one low-potential (5 mV) cytochrome b559. This large decrease in potential is accompanied by a very small g-value change (0.04 at gz), indicating that it is the environment and not the ligand field of the heme which changes significantly upon the removal of the 17- and 23-kDa polypeptides.

Chlorophyll↗
PubMed1989

Manganese and calcium requirements for reconstitution of oxygen-evolution activity in manganese-depleted photosystem II membranes.

The Mn complex of photosystem II and O2-evolution activity are reconstituted in Mn-depleted photosystem II membranes in a light-dependent process called photoactivation. Recovery of O2-evolution activity requires both Mn2+ and Ca2+ in the photoactivation medium. The Mn2+ and Ca2+ dependences of both the effective rate constant and yield of photoactivation have been determined. A comparison of these data with the predictions of mathematical models for photoactivation leads to the conclusion that photoactivation occurs in two stages. The first stage, photoligation of Mn, requires light and depends primarily on Mn2+. The second stage, binding of Ca2+, is required for expression of O2-evolution activity. This two-stage model affords an excellent fit to the data and provides dissociation constants and binding stoichiometries for Ca2+ and Mn2+. We conclude that one Mn2+ ion is bound and photooxidized in the rate-determining step(s) of photoactivation. On the basis of these results and data already in the literature, the molecular details of the elementary steps in photoactivation are discussed and a mechanism of photoactivation is proposed.

Calcium↗
PubMed1986

Effect of the 17- and 23-kilodalton polypeptides, calcium, and chloride on electron transfer in photosystem II.

Electron paramagnetic resonance (EPR) measurements were performed on photosystem II (PSII) membranes that were treated with 2 M NaCl to release the 17- and 23-kilodalton (kDa) polypeptides. By using 75 microM 3-(3,4-dichlorophenyl)-1,1-dimethylurea to limit the photosystem II samples to one stable charge separation in the temperature range of 77-273 K, we have quantitated the EPR signals of the several electron donors and acceptors of photosystem II. It was found that removal of the 17- and 23-kDa polypeptides caused low potential cytochrome b559 to become fully oxidized during the course of dark adaptation. Following illumination at 77-130 K, one chlorophyll molecule per reaction center was oxidized. Between 130 and 200 K, both a chlorophyll molecule and the S1 state were photooxidized and, together, accounted for one oxidation per reaction center. Above 200 K, the chlorophyll radical was unstable. Oxidation of the S1 state gave rise to the S2-state multiline EPR signal, which arises from the Mn site of the O2-evolving center. The yield of the S2-state multiline EPR signal in NaCl-washed PSII membranes was as high as 93% of the control, untreated PSII membranes, provided that both Ca2+ and Cl- were bound. Furthermore, the 55Mn nuclear hyperfine structure of the S2-state multiline EPR signal was unaltered upon depletion of the 17- and 23-kDa polypeptides. In NaCl-washed PSII samples where Ca2+ and/or Cl- were removed, however, the intensity of the S2-state multiline EPR signal decreased in parallel with the fraction of PSII lacking bound Ca2+ and Cl-.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium↗