Aggregation of the 636 nm emitting monomeric protochlorophyllide form into flash-photoactive, oligomeric 644 and 655 nm emitting forms in vitro

Artificial formation of flash-photoactive oligomeric protochlorophyllide complexes was found in etiolated pea (Pisum sativum L. cv. Zsuzsi) epicotyl homogenates containing glycerol (40% v/v) and sucrose (40% m/v). The 77 K fluorescence emission spectra indicated that the ratio of the 644 and 655 nm emitting forms to the 636 nm form increased during 3 to 5-day incubation in the dark at −14 °C. Electron micrographs showed the presence of well-organized prolamellar bodies in the homogenates. The same phenomena were found when the homogenates were frozen into liquid nitrogen and thawed to room temperature in several cycles. Similar treatments of intact epicotyl pieces caused significant membrane destructions. In homogenates, the in vitro produced 644 and 655 nm emitting protochlorophyllide forms were flash-photoactive; the extent of phototransformation increased compared to that in native epicotyls. The newly appeared 692 nm chlorophyllide band showed a blue shift (similar to the Shibata shift in leaves), however this process took place only partially due to the effect of the isolation medium.These results prove that the in vitro accumulated 644 and 655 nm protochlorophyllide forms were produced from the flash-photoactive 636 nm emitting monomeric NADPH:protochlorophyllide oxidoreductase units via aggregation, in connection with structure stabilization properties of glycerol and sucrose.     

Functional and structural alterations induced by copper in xanthine oxidase

Xanthine oxidase （XO）, a key enzyme in purine metabolism, produces reactive oxygen species causing vascular injuries and chronic heart failure. Here, copper＇s ability to alter XO activity and structure was investigated in vitro after pre-incubation of the enzyme with increasing Cue＋ concentrations for various periods of time. The enzymatic activity was measured by following XO-catalyzed xanthine oxidation to uric acid under steady-state kinetics conditions. Structural alterations were assessed by electronic absorption, fluorescence, and circular dichroism spectroscopy. Results showed that Cu^2＋ either stimulated or inhibited XO activity, depending on metal concentration and preincubation length, the latter also determining the inhibition type. Cu^2＋-xo complex formation was characterized by modifications in XO electronic absorption bands, intrinsic fluorescence, and α-helical and β-sheet content. Apparent dissociation constant values implied high- and low-affinity Cu^2＋ binding sites in the vicinity of the enzyme＇s reactive centers. Data indicated that Cu^2＋ binding to high-affinity sites caused alterations around XO molybdenum and flavin adenine dinucleotide centers, changes in secondary structure, and moderate activity inhibition; binding to low affinity sites caused alterations around all XO reactive centers including FeS, changes in tertiary structure as reflected by alterations in spectral properties, and drastic activity inhibition. Stimulation was attributed to transient stabilization of XO optimal conformation. Results also emphasized the potential role of copper in the regulation of XO activity stemming from its binding properties.     

New vistas for P-aminobenzoate participation in the biosynthesis of dihydro folate: a tentative model of the tetrahydro folate multienzyme complex.

Figures 6a and 6b and Table 2 show the united pattern of possible pathways of THFA biosynthesis with the substrates and the enzymes involved. With substrate selection ten different individual enzyme activities can be distinguished, but A2′ is identical with A2, and their apparent molecular weight is 28,000 daltons ±7%, and similarly c1–4 are the same enzymes with an apparent molecular weight of 40,000 daltons ±5% (Tóth-Martinez et al., 1974a). The identity of these enzymes has preliminary been shown, and construction of the THFA-MEC model was partly based on these findings.So, no distinction can be made among functioning MEC-es. The different pathways, mentioned in the introductory part of this paper, can be a product of the separated study of the individual enzymic steps of DHFA (THFA) biosynthesis. By all means it is an important approach to understand the dynamics of the integrated process what we tentatively suggest in this paper for further elucidation.     

Ferredoxin from Selenomonas ruminantium

Ferredoxin from the strict rumen anaerobe Selenomonas ruminantium has been purified to homogeneity and characterized with respect to its molecular weight and amino acid composition. The molecular weight of ferredoxin was 9,880. The A₃₈₀/A₂₈₀ absorbance ratio of the pure ferredoxin was 0.54 with a molar extinction coefficient of 31,000 M^-1 cm^-1 at 380 nm. Ferredoxin was reduced by cell-free extracts in the presence of hydrogen gas or pyruvate and acetyl coenzyme A.Abbreviations [Fe_x/S_y] denotes an iron sulfur cluster containing x iron and y sulfur atoms     

Sensitivity of chondrocytes of growing cartilage to reactive oxygen species

Vascular invasion of calcified cartilage, during endochondral ossification, is initiated and sustained by invasive cells (endothelial cells and macrophages) which degrade the tissue by releasing lytic enzymes. Concurrently, reactive oxygen species (ROS) are also released by these cells and we hypothesize that ROS also contribute to the degradation of the tissue. As a preliminary approach to this problem, the antioxidant activities and the effect of ROS on hypertrophic cartilage and chondrocytes (HCs) were investigated. Compared to resting or articular chondrocytes, HCs exhibited higher catalase but lower SOD specific activities and lower PHGPx concentration, thus revealing a defence activity specific against H₂O₂. Moreover, dose-dependent depletion of ATP occurred after few minutes of exposure to ROS, and a long-term treatment (16 h incubation with ROS) promoted the release of LDH activity and a significant variation of the poly- to mono-unsaturated fatty acid ratio. Finally, the incubation of HCs with low ROS doses induced the release of sedimentable alkaline phosphatase activity (matrix vesicles). How the obtained results fit the in vivo occurring events is discussed.     

The osmochemistry of electron-transfer complexes

Detailed molecular mechanisms of electron transfer-driven translocation of ions and of the generation of electric fields across biological membranes are beginning to emerge. The ideas inherent in the early formulations of the chemiosmotic hypothesis have provided the framework for this understanding and have also been seminal in promoting many of the experimental approaches which have been successfully used. This article is an attempt to review present understanding of the structures and mechanisms of several osmoenzymes of central importance and to identify and define the underlying features which might be of general relevance to the study of chemiosmotic devices.     

Evidence for two independent pathways of electron transfer in mitochondrial NADH:Q oxidoreductase. II. Kinetics of reoxidation of the reduced enzyme

The pre-steady-state kinetics of reoxidation of NADH:Q oxidoreductase present in submitochondrial particles has been studied by the freeze-quench method. It was found that at pH 8 only 50% of the Fe-S clusters 2 and 4 and 75% of the clusters 3 were rapidly reoxidised after transient and complete reduction by a pulse of NADH in the presence of excess NADPH. Thus, NADPH keeps 50% of the clusters 2 and 4 and 25% of the clusters 3 permanently reduced at this pH. Since NADH oxidation is nearly optimal at this pH, whereas NADPH oxidation is virtually absent, it was concluded that these permanently reduced clusters were not involved in the NADH oxidation activity. Incomplete reoxidation of the clusters 2, 3 and 4 after a pulse of NADH was also found in the absence of NADPH, both at pH 6.5 and at pH 8. A pulse of NADPH given at pH 6.5, where NADPH oxidation by oxygen is nearly optimal, caused a slow reduction of 50% of clusters 2 and 4 and 30% of the clusters 3, which persisted for a period of at least 15 s. It was concluded that these clusters were not involved in the oxidation of NADPH by oxygen, as catalysed by the particles. As a working hypothesis a dimeric model for NAD(P)H:Q oxidoreductase is proposed, consisting of two different protomers. One of the protomers, containing FMN and the Fe-S clusters 1–4 in stoichiometric amounts, only reacts with NADH, and its oxidation by ubiquinone is rapid at pH but slow at pH 6.5. The other protomer, containing FMN and the clusters 2, 3 and 4, reacts with both NADH and NADPH and has a pH optimum at 6–6.5 for the reaction with ubiquinone.     

Purification and properties of cinnamyl alcohol dehydrogenase from higher plants involved in lignin biosynthesis

The purification and characterization of an NADP(H) specific cinnamyl alcohol oxidoreductase is reported. The enzyme, which has been purified 600 ×, shows an absolute specificity for the cinnamyl moiety. The reaction is readily reversible but is strongly inhibited by aldehyde substrates. The enzyme belongs to class A of NAD(P) specific oxidoreductases. The distribution of this activity throughout a wide variety of taxonomically different plant groups as well as plant parts has revealed a possible correlation with lignin biosynthesis.