Light-dependent quenching of chlorophyll fluorescence in pea chloroplasts induced by adenosine 5′-triphosphate

Addition of ATP to chloroplasts causes a reversible 25–30% decrease in chlorophyll fluorescence. This quenching is light-dependent, uncoupler insensitive but inhibited by DCMU and electron acceptors and has a half-time of 3 minutes. Electron donors to Photosystem I can not overcome the inhibitory effect of DCMU, suggesting that light activation depends on the reduced state of plastoquinone. Fluorescence emission spectra recorded at ?196°C indicate that ATP treatment increases the amount of excitation energy transferred to Photosystem I. Examination of fluorescence induction curves indicate that ATP treatment decreases both the initial (F_o) and variable (F_v) fluorescence such that the ratio of F_v to the maximum (F_m) yield is unchanged. The initial sigmoidal phase of induction is slowed down by ATP treatment and is quenched 3-fold more than the exponential slow phase, the rate of which is unchanged. A plot of F_v against area above the induction curve was identical plus or minus ATP. Thus ATP treatment can alter quantal distribution between Photosystems II and I without altering Photosystem II-Photosystem II interaction. The effect of ATP strongly resembles in its properties the phosphorylation of the light-harvesting complex by a light activated, ATP-dependent protein kinase found in chloroplast membranes and could be the basis of physiological mechanisms which contribute to slow fluorescence quenching in vivo and regulate excitation energy distribution between Photosystem I and II. It is suggested that the sensor for this regulation is the redox state of plastoquinone.     

Linear and circular dichroism of membranes from Rhodopseudomonas capsulata

Absorption, linear dichroism and circular dichroism spectra of Rhodopseudomonas capsulata (wild-type-St. Louis strain, mutant Y5 and mutant Ala⁺) are particularly sensitive to the nature of the light-harvesting bacteriochlorophyll-carotenoid-protein complexes. Evidence for exciton-type interactions is seen near 855 nm in the membranes from the wild-type and from mutant Y5, as well as in an isolated B-800 + 850 light-harvesting complex from mutant Y5. The strong circular dichroism that reflects these interactions is attenuated more than 10-fold in membranes from the Ala⁺ mutant, which lacks both B-800 + 850 and colored carotenoids and contains only the B-875 light-harvesting complex. These results lead to the conclusion that these two light-harvesting complexes have significantly different chromophore arrangements or local environments.     

Monoclonal antibodies against human low density lipoprotein. Stoichiometric binding studies using Fab fragments

Because of its physical properties, apolipoprotein B (apo B) has remained poorly characterized. In an attempt to elucidate apo B structure, the Fab fragments of 3 different monoclonal anti-human apo B antibodies were tested in a quantitative assay for their binding to human low density lipoprotein (LDL). In each case the assay gave a linear Scatchard plot with a maximum of 1 Fab fragment bound to a single LDL particle. This result favors an LDL model containing a single large Mr apo B protein, which is not composed of multiple, identical, small Mr subunits.     

Secondary structure in ribonuclease. II. Relations between folding kinetics and secondary structure elements

The kinetics of regain of 2′-CMP binding are monitored during renaturation of RNAase S. Experiments were performed by mixing equimolar amounts of S-peptide with S-protein. The S-protein fragment was incubated initially (i.e. before mixing with S-peptide) at pH 6.2 or 1.7 and various guanidine hydrochloride (GuHCl) concentrations. Three well-resolved phases are observed. The fastest phase is second-order. The reciprocal half-time increases linearly with fragment concentration and is independent of initial conditions for the S-protein fragment. An apparent on rate of k_on = 2 × 10⁵m^?1s^?1 is measured in 0.5 m-GuHCl (pH 6.2) and 20 ° C. Identical association kinetics are observed by changes in tyrosine absorbance. The fraction of native RNAase S formed in this second-order reaction strictly equals the fraction of S-protein molecules with intact β-sheet in initial conditions. The relation holds for different pH values, GuHCl concentrations and temperatures. The fraction of apparent helical content of S-protein in initial conditions may also vary but this is not reflected by the association reaction. We interpret this to mean that the β-sheet but not the α-helices must be preformed in initial conditions in order to generate the high-affinity peptide binding site of S-protein. Furthermore, it is concluded that the S-protein moiety β-sheet forms or unfolds in a single one-step reaction. 2′-CMP binding reports, additionally, two slower phases of renaturation. These are produced by S-protein molecules that have their β-sheet unfolded in initial conditions. It is observed that a unique dependence of these two folding rates exists for RNAase A, RNAase S and S-protein as function of t_m, the temperature of half-completion of thermal denaturation as measured by unfolding of the β-sheet in the respective compound in final conditions. The t_m value varies with changing pH, with GuHCl concentration and (for RNAase S) with changing fragment concentration. The findings are interpreted to argue in favor of a sequential mechanism of folding, where the stability of a structural precursor determines the rate of folding.     

Expression of thymidylate synthetase activity in Bacillus subtilis upon integration of a cloned gene from Escherichia coli

The gene from Escherichia coli encoding thymidylate synthetase was cloned in the plasmid pBR322. The resulting chimeric plasmid, pER2, was effective in transforming both E. coli and Bacillus subtilis to thymine prototrophy. Uncloned linear E. coli chromosomal DNA was unable to transform thymine-requiring strains of B. subtilis to thymine independence. Linearization of the chimeric plasmid, pER2, with restriction enzymes markedly diminished its ability to transform B. subtilis auxotrophs. The Thy+ transformants derived from the transformation of B. subtilis with pER2 DNA did not contain detectable extrachromosomal DNA as demonstrated by Southern hybridization patterns and centrifugation in CsCl gradients of DNA isolated from B. subtilis colonies transformed with the chimeric plasmid. We conclude that the DNA from the chimeric plasmid was integrated into the chromosome of B. subtilis, demonstrating that extensive homology is not required for the integration of foreign DNA. This is the first reported case of a gene from a Gram-negative bacterium functioning in a Gram-positive organism.     

Mononuclear and binuclear Co(III)-dioxygen adducts of bleomycin. Circular dichroism and electron paramagnetic resonance studies

Co(II) interacts with bleomycin in aqueous solution, in the presence of air, to give a short lived mononuclear superoxo-Co(III) complex (I) identified previously, by Sugiura, by electron paramagnetic resonance measurements. This complex rapidly releases O₂ to yield the dinuclear μ-peroxo-Co(III) complex (II), but is stabilized by the presence of DNA yielding a new superoxo long lived species (I′). The absorption and circular dichroism spectra of the three species (I,I′,II) have been characterized.     

Studies on the effects of the antitumor agent camptothecin and derivatives on DNA. Camptothecin potentiated cleavage of DNA by bleomycin in vitro

Addition of sodium camptothecin (2a, Fig. 1) in comparable low concentrations to the glycopeptide antitumor antibiotic bleomycin (BLM) leads to enhanced rates of single-strand scission of PM2-covalently closed circular DNA, whereas sodium camptothecin alone has no effect. A similar enhancement of DNA scission by sodium camptothecin is produced with the 1 : 1 bleomycin-iron complex alone or in conjunction with NADPH as an additional reductant. The interpretation that camptothecin may substitute for the reducing requirement of the antibiotic is supported by its oxidation at 37°C by the 1 : 1 bleomycin iron complex, by iron salts or more efficiently by hydrogen peroxide to the known hemiacetal (3, Fig. 1).Electrochemical studies of 2a, its analogues and selected model compounds established that the α-pyridone ring D is most susceptible to a one-electron reduction at a reversible potential of ?0.95 ± 0.01 V. The reduced camptothecin is a transient species readily capable of donating an electron. This process may by compatible with a coupled reduction of the sequestered Fe(III) in the glycopeptide antibiotic necessary for the expression of antibiotic and antitumor properties. The results may provide a mechanistic rationale for the observed potentiation of the antitumor activity of bleomycin by camptothecin in vivo.     

Anisakis simplex and Anisakis physeteris: physicochemical properties of larval and adult hemoglobins

To resolve the taxonomic relationship between two types of parasitic nematode larvae (Type I and II) and two species of parasitic nematode adults (Anisakis simplex and A. physeteris) of the aquatic ascarid genus Anisakis, collected in Japanese coastal water, a comparison was made of their hemoglobins' physicochemical properties. The larval hemoglobins were more similar to each other in electrophoretic pattern than to either adult, indeed there were few similarities whatsoever in these patterns of larval and adult hemoglobins. However, isoelectric points were 6.2 for the Type I larva and for A. simplex and 5.4 for the Type II larva and for A. physeteris. All samples showed identical patterns in spectrophotometric scanning. The circular dichroic spectra of the samples were also virtually identical, although slight differences were noted in the oxygenated hemoglobins; the Type II larva and A. physeteris exhibited a small positive peak at 575 nm but the Type I larva and A. simplex exhibited a much smaller peak (negative position). The sedimentation coefficients of the samples possessed essentially identical values (11.2–12.4). The molecular weights of the samples were estimated, roughly, to be in the range 33 to 43 × 10⁴ by thin-layer chromatography on Sephadex G-200. The evidence suggests that a relationship may exist between the Type I larva and A. simplex, and between the Type II larva and A. physeteris.     

Circular dichroism studies on the binding of type I substrates and reverse type I compounds to rabbit liver microsomal cytochrome P-450.

Liver microsomes from control, 3-methylcholanthrene-treated, and phenobarbital-treated New Zealand White rabbits were examined for differences detectable by circular dichroism (CD) spectroscopy. Addition of the Type I substrate cyclohexane to phenobarbital microsomes decreases the negative ellipticity at about 418 nm and concomitantly increases the negative ellipticity at about 395 nm. Cyclohexane added to microsomes from control or 3-methylcholanthrene-treated animals shows little or no CD changes in these wavelength regions. The effect by cyclohexane is completely reversed by the subsequent addition of butanol-1. Addition of benzo[a]pyrene to phenobarbital microsomes also decreases the negative ellipticity at about 418 nm, and this effect can be completely reversed with the subsequent addition of butanol-1. The ellipticity at about 395 nm is reversed in sign and is markedly increased by benzo[a]pyrene, however, and this effect is not changed with the subsequent addition of butanol-1. Restoring the cyclohexane- or benzo[a]pyrene-induced changes by the subsequent addition of alcohol is proportional to the aliphatic chain length, with 4 or more carbon atoms being maximally effective. Primary alcohols inhibit aryl hydrocarbon (benzo[a]pyrene) hydroxylase (EC 1.14.14.2) activity of phenobarbital microsomes, and the inhibitory effect is enhanced with increasing chain length of the alcohols; 4 or more carbon atoms being maximally effective. Stimulation of monooxygenase metabolism of cyclohexane or benzo[a]pyrene by NADPH results in restoration of the negative ellipticity band at about 418 nm, whereas the ellipticity peak at about 395 nm remains unchanged. More negative ellipticity at about 210 and 222 nm is found in phenobarbital microsomes than in control or 3-methylcholanthrene microsomes and cyclohexane addition in vitro increases these negative ellipticity peaks in phenobarbital microsomes but not in control or 3-methylcholanthrene microsomes.These results show that with CD studies one can detect directly both high spin (negative ellipticity peak at 385–395 nm) and low spin (negative ellipticity peak at about 418 nm) P-450 iron in liver microsomes from control, 3-methylcholanthrene-treated, or phenobarbital-treated rabbits. These data are consistent with a weak ligand such as oxygen, rather than a stronger ligand such as nitrogen, in the sixth position of 6-coordinated (low spin) ferric iron in P-450 in vivo. Type I substrates such as cyclohexane or benzo[a]pyrene, when bound to P-450, change low spin P-450 iron to the high spin state. Cyclohexane-bound high spin P-450 iron in vitro is more easily converted to low spin iron by butanol-1 than is benzo[a]pyrene-bound high spin P-450 iron. Liver microsomal proteins from phenobarbital-treated rabbits have a higher helical content than those from either control or 3-methylcholanthrene-treated rabbits. Cyclohexane addition in vitro increases this helical character only in phenobarbital microsomes, indicating that one or more forms of phenobarbital-induced P-450 apoproteins is (are) more specific for cyclohexane binding and metabolism than control or 3-methylcholanthrene-induced forms of P-450.