Determination of the secondary structures of proteins by circular dichroism and optical rotatory dispersion

The secondary structure of staphylococcal enterotoxins A, B and C

The circular dichroism (CD) of staphylococcal enterotoxins A, B and C was measured. The CD of enterotoxins B and C were almost identical from 250 to 320 nm, but differed from the CD of enterotoxin A. The spectrum of enterotoxin A in this wavelength region contained the same bands with respect to both location and sign, but with significant differences in intensity. The CD spectra of enterotoxins B and C were also much more alike from 190 to 250 nm. Although all three enterotoxins had a major negative extremum at 215--218 nm, its magnitude was equal in enterotoxins B and C, but was substantially decreased in enterotoxin A. The secondary structure of the enterotoxins contained little alpha-helix as analyzed with CD models. A secondary structure of entertoxin B compured from a scheme based on a joint prediction histogram of five separate methods, placed 29 residues in alpha-helices, 71 in beta-pleated sheets, 88 in beta-turns and 55 in aperiodic conformation.

Hydrophobic interaction between the monomer of mitochondrial malate dehydrogenase and phospholipid membranes

Porcine mitochondrial malate dehydrogenase (EC 1.1.1.37) dissociates into subunits on dilution. The enzyme monomer caused large increases in the surface pressure of monolayers of 1:1 phosphatidylserine/phosphatidylcholine at air/water and oil/water interfaces. The monomer increased the permeability of phospholipid vesicles to 22Na+. Both effects were significantly greater than the corresponding effects of ribonuclease A, cytochrome c and the dimeric form of malate dehydrogenase. Changes in the circular-dichroism spectra of the enzyme indicated that conformational changes may be associated with dimer formation or when monomer interacts with lysophosphatidyl-choline. Similar interactions to those described may occur in situ when mitochondrial malate dehydrogenase is transported to the mitochondrial matrix from its site of synthesis on cytosolic ribosomes.

Modification of rat hemopexin properties upon heme binding

Rat hemopexin and its complex with heme were found to have the same Stokes' radius, 3.9 nm, as determined by gel filtration. Therefore no polymerisation occurs as a result of heme binding. The conformational parameters calculated from circular dichroism spectra indicate that hemopexin and its complex consist of 20% beta sheet and mainly of disordered structure. No change of the secondary structure is therefore observed upon heme binding. Hemopexin reveals five bands by analytical electrofocusing with pI ranging from 5.5 to 5.95. This microheterogeneity is not due to sialic acid differences between the variants. Upon heme binding the pI of the variants decrease to lower values (from 4.8 to 5.25). This decrease in the pI value of hemopexin is thought to modify the tertiary structure through charge effects and may allow the binding of the heme-hemopexin complex to the hepatocytes.

Basis of thermostability in pig heart lactate dehydrogenase treated with O-methylisourea

Acetamidination of pig heart lactate dehydrogenase (L-lactate:NAD+ oxidoreductase, EC 1.1.1.27) with ethyl acetimidate resulted in an increase of thermostability, and covalent bridge formation between pairs of lysine residues is observed. Guanidination with O-methylisourea of the enzyme also increases the thermostability, but such a bridge seems not to be formed. Increased thermostability of guanidinated enzyme is considered to be due to the shift of the pK values of the lysine residues from 10.5 to 12.5 after guanidination. Modification experiments with carbodiimide reveals that the enzyme contains 4.6 pairs of neighboring lysine and carboxyl residues per subunit, and amide bonding between 3.2 pairs results in an increase of thermostability. Guanidination of 4.6 Lys/subunit of the enzyme yields an enzyme derivative with considerably increased thermostability. Salt bridge formation between the 4.6 pairs of neighboring carboxyl and guanidinated lysine residues per subunit might make a major contribution to the increased thermostability of the guanidinated enzyme.

Conformational effects of coenzyme binding to porcine lactic dehydrogenase

Changes induced on addition of the coenzyme, NADH or NAD+, to porcine lactic dehydrogenase isoenzymes H4 and M4 have been studied by hydrodynamic and spectroscopic methods. As shown by ultracentrifugal analysis, the native subunit structure remains unchanged on holoenzyme formation; an approximately 5% increase of the sedimentation coefficient, parallelled by a slight decrease of the partial specific volume (less than 1%) indicate a significant change in the native tertiary and/or quaternary structure of the enzymes, corroborating earlier calorimetric data (Hinz and Jaenicke, 1975). The binding constant for the enzyme from skeletal muscle (M4) and NADH are found to be in agreement with KD-values obtained from equilibrium dialysis, as well as spectroscopic and thermal titration experiments (8 muM). Far UV circular dichroism measurements do not show significant changes on ligand binding, indicating unchanged helicity or compensatory conformational effects. In the near UV, ligand binding is reflected by an extrinsic Cotton effect around 340 nm; in the range of aromatic absorption no changes are detectable. The experimental results suggest that there are gross structural changes on coenzyme binding to lactic dehydrogenase which do not affect the intrinsic spectral properties normally applied to analyze transconformation reactions in protein molecules.

Physicochemical studies of the protein-lipid interactions in melittin-containing micelles

Complexes of melittin with detergents and phospholipids have been characterized by fluorescence, circular dichroism, ultracentrifugation, quasi-elastic light scattering and 1H nuclear magnetic resonance (NMR) experiments. By ultracentrifugation and quasi-elastic light-scattering measurements it is shown that melittin forms stoichiometrically well-defined complexes with dodecylphosphocholine micelles consisting of one melittin molecule and approximately forty detergent molecules. Evidence from fluorescence, circular dichroism and 1H nuclear magnetic resonance experiments indicates that the conformation of melittin bound to micelles of various detergents or of diheptanoyl phosphatidylcholine is largely independent of the type of lipid and furthermore appears to be quite closely related to the conformation of melittin bound to phosphatidylcholine bilayers. 1H NMR is used to investigate the conformation of micelle-bound melittin in more detail and to compare certain aspects of the melittin conformation in the micelles with the spatial structures of monomeric and self-aggregated tetrameric melittin in aqueous solution. The experience gained with this system demonstrates that high resolution NMR of complexes of membrane proteins with micelles provides a viable method for conformational studies of membrane proteins.

Conformational studies on muscular parvalbumins cooperative binding of calcium (II) to parvalbumins

1H NMR and ORD were used to characterize the respective variations of tertiary structure and secondary structure of parvalbumins with calcium content ((Pa(O), without calcium and PaCa2 calcium saturated) and temperature. It has been observed that the tertiary structure can be lost without significant variation of the helical content. Cooperative binding of calcium to Pa(O) has been shown by NMR spectroscopy under low ionic strength conditions and at neutral pH. The present study shows that the calcium binding affinity of parvalbumin is dependent on the tertiary structure. Calcium binding and calcium release functions of parvalbumins in the muscle may be controlled by their tertiary structure.

Circular dichroism of lipoxygenase-1 from soybeans

The circular dichroism spectra of the three forms of lipoxygenase-1 from soybeans show characteristic differences in the region between 300 and 600 nm. Native lipoxygenase-1 only shows a negative dichroic band around 330 nm. Yellow lipoxygenase-1, obtained by addition of an equimolar amount of 13-F-hydroperoxylinoleic acid to the native enzyme, shows a positive Cotton effect at 425 nm, while the negative band band at 330 nm has increased in intensity. The blue enzyme, representing a complex of yellow enzyme with 13-L-hydroperoxylinoleic acid exhibits a negative dichroic band at 580 nm and positive bands at 410 and 391 nm. The near-ultraviolet CD spectra of the three forms of lipoxygenase are very similar, showing several well resolved positive dichroic bands at 0 degrees C. Using the method of Chen et al. (Chen, Y.-H., Yang, J.T. and Martinez, H.M. (1972) Biochemistry 11, 4120--4131) the contents of alpha-helix, beta- and unordered form of native lipoxygenase-1 were estimated to be 34, 27 and 39% respectively.

Secondary structure prediction of anterior pituitary hormones. Lack of correlation between predicted values and circular dichroism data

The secondary structures of human somatotropin, human choriomammotropin, ovine and porcine prolactin, human, ovine and porcine beta-lipotropin, human and ovine lutropin, human thyrotropin, human corticotropin, alpha-melanotropin and human beta-melanotropin have been predicted by the method of Chou & Fasman. Predicted contents of alpha-helix and beta-sheet do not correspond well with values estimated from circular dichroism spectra.

Circular dichroism spectra of purified cytochromes P-450 from rabbit liver microsomes

Circular dichroism (CD) spectra were measured for cytochromes P-450 (P-450) purified from phenobarbital- and 3-methylcholanthrene-induced rabbit liver microsomes. No striking difference in alpha-helix content was seen between phenobarbital-induced P-450 (PB P-450) (50%), phenobarbital-induced P-448 (PB P-448) (40%) and 3-methylcholanthrene-induced P-448 (MC P-448) (45--50%) in terms of ultraviolet CD spectra. Strong negative CD spectra associated with 3-methylcholanthrene transitions for MC P-448 in the near-ultraviolet region (250--310 nm) and weaker negative CD spectra associated with Soret transitions for PBP-448 ([theta] = 50 000) and MCP-448 ([theta] = 160 000), indicated that structures of these preparations are strikingly different from each other. Reduction of P-450 and P-448 led to a remarkable decrease of the Soret CD trough, suggesting that reduction was accompanied by a striking conformational change in the vicinity of the heme. Since CO complexes of reduced P-450 and P-448 showed a CD trough and an S-shaped CD, respectively, associated with the absorption peak at 450 nm, the heme vicinities are remarkably different from each other. The CD spectra in the visible region are also discussed. It was noticed that P-420, the denatured form of P-450, exhibited no CD spectra in the Soret and visible regions.

Molecular weight and structural studies on cephalopod rhodopsin

The protein moiety of squid (Watasenia scintillans) rhodopsin has been shown to have a molecular weight of 46 800 by means of amino acid analysis. This value was comparable to the value (51 000) obtained from SDS-polyacrylamide gel electrophoresis. After the squid eyes were incubated at 10 degrees C for 8 days, the rhodopsin showed a molecular weight of 39 000 on electrophoresis. The smaller molecular weight was ascertained by amino acid analysis of the rhodopsin; and may result from autolysis by the lysosomal enzyme. The rhodopsin in rhabdomeric membranes and in detergent solution was treated with chymotrypsin, papain or subtilisin. These enzymes first produced the 39 000 dalton rhodopsin and then cleaved this into the 25 000 and 14 000 dalton peptides without bleaching. The rhodopsin was attacked by proteases and readily lost an approx. 12 000 dalton peptide portion. This portion included the COOH-terminal and was rich in glutamic acid, proline, glycine, alanine and tyrosine residues.

Salt and temperature-dependent conformation changes in spectrin from human erythrocyte membranes

ORD and CD measurements of spectrin, in both the dimer and tetramer association state, indicate a high proportion of alpha-helix in this protein. At temperatures below 27 degrees C and in 0.1 M NaCl, the tetramer has an apparent helix content of 73% and the dimer, 68%. The conformation of both states is dependent on salt concentration and temperature. Low ionic strength solutions of spectrin display lowered sedimentation coefficients and a decreased apparent helix content, indicating perhaps a slight refolding and expansion of the molecule. In addition, spectrin in low ionic strength solutions undergoes a broad temperature-dependent transition spread from 20 to 50 degrees C, while in the presence of salt the transition is sharp and centered on 49 degrees C. The temperature-dependent changes in low ionic strength solutions appear to parallel the dissociation of tetramer to dimer.

The effects of chemical modification on the refolding transition of alpha-chymotrypsin

The role of several active site residues of alpha-chymotrypsin in the prototypical refolding transition between active and inactive forms of this enzyme is examined using chemical modification. Oxidation of Met-192 to the sulfoxide results in a derivative which remains entirely in an active state from pH 6 to 9. The derivative becomes inactive only at high pH with pKa = 10.3, delta H0 = 9.5 kcal and delta S0 = -15 eu., indicating the sulfoxide group supplies about 2.1 kcal of active state stabilization relative to the unoxidized methionine side chain. The refolding transition of N-methyl-His-57-alpha-chymotrypsin, in which a nitrogen of the "charge relay" histidine is methylated, displays one ionization process with an apparent pKa of 9.45. The absence of an additional ionization process with a pKa near 7 provides evidence that one of the ionizations in the six state mechanism which describes this transition in alpha-chymotrypsin is linked to the charge relay system. We also demonstrate, using alpha-chymotrypsin, Met-192-sulfoxide-alpha-chymotrypsin and N-methyl-His-57-alpha-chymotrypsin, that the 230 nm circular dichroism band is a quantitative probe of the active-inactive equilibrium, although the chromophore or chromophores responsible for this and another very large negative band at 202 nm have not been identified. Circular dichroism was used to observe the active-inactive equilibrium in methan sulfonyl-alpha-chymotrypsin and phenylmethane sulfonyl-alpha-chymotrypsin. The enhanced stability of the active state of these derivatives relative to alpha-chymotrypsin can be rationalized in terms of steric effects in the substrate side chain binding site.

Leghaemoglobin from Trifolium subterraneum. Purification and characterization

Leghaemoglobin from the subclover, Trifolium subterraneum cultivar Woogenellup, has been fractionated into at least four electrophoretically distinct components using the ion-exchange chromatographic procedure described by Appleby et al. (Appebly, C.A., Nicola, N.A., Hurrell, J.G.R. and Leach, S.J. (1975) Biochemistry 14, 4444--4450) for soybean leghaemoglobins. Unlike those of soybean, the subclover leghaemoglobins showed no evidence of autoxidation under identical isolation procedures, implying that these proteins have an unusually stable ferrous oxidation state. Circular dichroism in the far-ultraviolet (200--240 nm) indicated a high helicity (approx. 70%) as has been reported for other species of leghaemoglobins. However, circular dichroism in the near-ultraviolet region (240--300 nm) indicated that the haem-protein interactions may be considerably different in the subclover leghaemoglobins and this may explain their atypical resistance to autoxidation and the absence of nicotinate binding in these proteins.

Alcohol dehydrogenase from Rhizopus javanicus

Alcohol dehydrogenase of Rhizopus javanicus was purified, and its physical and chemical characteristics were determined. The intact enzyme was shown to have a molecular weight of approximately 60,000. Since the smallest apparent subunit was 14,000, the enzyme was presumed to be composed of four subunits. The crude mycelial extract contained multiple forms of the enzyme, which were separated by ion-exchange chromatography.

A highly reactive tyrosine residue as part of the indole and benzodiazepine binding site of human serum albumin

The interaction of L-tryptophan and four benzodiazepine derivatives with tyrosine-modified human serum albumin was investigated by equilibrium dialysis and circular dichroism measurements. Out of the 18 tyrosine residues of the human serum albumin molecule, only 9 could be modified with tetranitromethane. At least up to a degree of modification of 5, the conformation of human serum albumin was not changed and no crosslinking and fractionation has been found, as revealed from circular dichroism measurements in the far ultraviolet range and from SDS polyacrylamide electrophoresis. The modification of only 2 out of the 9 accessible tyrosine residues of human serum albumin strongly affects the binding of L-tryptophan and diazepam to their common, stereospecific bindining site. This was evidently shown by a reduction of the association constants by more than 90% and by a large reduction of the extrinsic Cotton effects of four benzodiazepines bound to human serum albumin. The numbers of binding sites remained unchanged. Strong evidence was presented that only one tyrosine residue, which reacts faster with tetranitromethane than all others, is mainly involved in the specific indole and benzodiazepine binding site of human serum albumin. The location of this highly reactive tyrosine residue and that of the specific indole and benzodiazepine binding site within the human serum albumin primary structure is discussed.

Isolation and physical characterization of bovine lens crystallins

The soluble proteins from bovine lens homogenate were separated on Sepharose CL-6B (2 X 200 cm) in 0.05 M tris-NaHSO3 pH 8.2 buffer containing 20 mM EDTA. Five sharp and defined fractions (HM alpha, alpha, beta H, beta L, gamma) were obtained. Each crystallin fraction was further purified by rechromatography on the same column. Each protein fraction was pure as judged by ultracentrifugation and SDS-gel electrophoresis. The molecular weights of the five fractions were 3.04 x 10(6), 5.83 x 10(5), 1.58 x 10(5) , 4.59 x 10(4), 2.14 x 10(4) as determined from sedimentation coefficient and intrinsic viscosity data by Scheraga-Mandelkern equation, which was in close agreement with that obtained by gel filtration. The polypeptide composition of crystallins as determined by SDS-gel electrophoresis revealed one band for high molecular weight alpha (HM alpha) and alpha, three for beta H, two for beta L and one for gamma. The gross CD patterns of crystallins were about the same in the peptide region (200 nm similar to or approximately 250 nm) with a minimum centered at about 217 nm, indicative of a beta-sheet structure in all crystallins. The [theta] values at 217 nm ranged from --1700 to --3700 degrees cm2 per decimole. The CD spectra of these crystallins in the aromatic region (250 nm similar to or approximately 300 nm) were different, reflecting the different contributions of aromatic amino acids to the tertiary structure of crystallins.

Characterization of porins from the outer membrane of Salmonella typhimurium. 2. Physical properties of the functional oligomeric aggregates

We have purified to homogeneity, from mutant strains of Salmonella typhimurium, the small oligomers of porin that confer permeability channels to artificial vesicle membranes reconstituted from phospholipids and lipopolysaccharide. The molecular weights of the porin oligomers from the strains SH5551 and SH6017 appeared to be 130000 and 125000, respectively, and those of the monomers were 41000 and 37500, respectively, when determined by sedimentation equilibrium in the presence of dodecylsulfate. It was thus concluded that the functional porin oligomers consisted of three identical subunits. The Stokes' radius of the trimer . dodecylsulfate complex was around 5 nm. The trimer bound less dodecylsulfate than the monomer. The trimer . dodecylsulfate complex retained at room temperature the native conformation of porin, which is rich in beta-structure. When the trimers were dissociated further by various treatments, only the porin monomers were recovered in significant amounts, and the permeability-conferring activity was lost simultaneously. We propose, therefore, that the trimer is the minimal functional unit of porin that is capable of forming permeability channels in the outer membrane of Salmonella typhimurium.

Protease B from Saccharomyces cerevisiae. Purification and characterization

Protease B has been isolated from Saccharomyces cerevisiae and purified in six steps as follows: autolysis of the yeast cells, ammonium sulfate fractionation, activation of the proteolytic enzymes, chromatography on DEAE-cellulose, chromatography on CM-cellulose and finally, a second chromatography on DEAE-cellulose. The preparation was shown to be homogeneous on polyacrylamide gels in the absence as well as in the presence of sodium dodecylsulfate. Furthermore, the molecular weight (43,000 daltons) and the isoelectric point (5.45) were in good agreement with earlier published values. The amino acid composition is reported. The absence of disulfide bonds in protease B has to be outlined. The amino acid residues of the protein have been found to be folded nearly quantitatively (at least 80%) in a beta-conformation as deduced from a circular dichroism study. Finally, the tryptophan residues (5 mol/mol protein) are largely buried in the hydrophobic core of the enzyme.

Chemical modification of taipoxin and the consequences for phospholipase activity, pathophysiology, and inhibition of high-affinity choline uptake

Treatment of taipoxin with p-bromophenacyl bromide resulted in modification of single histidine residues in the alpha and beta subunits. The modification decreased the neurotoxicity (lethality) 350-fold, but the inhibitory action on high-affinity choline transport was reduced only threefold. The phospholipase activity and Ca2+-association constants for taipoxin and its subunits were determined. A model for the neurotoxicity of taipoxin indicates the alpha subunit as the ultimate cause of the disruption of synaptic transmission.

Ligand-induced conformational transitions and secondary structure composition of chicken liver pyruvate carboxylase

Apparent conformational transitions induced in chicken liver pyruvate carboxylase by substrates, KHCO(3) and MgATP, and the allosteric effector, acetyl-CoA, were studied by using the fluorescent probe, 8-anilinonaphthalene-1-sulphonic acid and c.d. Fluorescence measurements were made with both conventional and stopped-flow spectrophotometers. Additions of acetyl-CoA and/or ATP to the enzyme-probe solutions quenched fluorescence of the probe by the following cumulative amounts regardless of the sequence of additions: acetyl-CoA, 10-13%; ATP, 21-24%; acetyl-CoA plus ATP, about 35%. Additions of KHCO(3) had no effect on the fluorescence. The rates of quenching by acetyl-CoA and MgATP (in the presence of acetyl-CoA) were too rapid to measure by stopped-flow kinetic methods, but kinetics of the MgATP effect (in the absence of acetyl-CoA) indicate three unimolecular transitions after the association step. The negligible effect of the probe on enzyme catalytic activity, a preservation of the near-u.v. c.d. effect of MgATP and acetyl-CoA in the presence of the probe and no observable unimolecular transitions after binding of the probe to the enzyme indicate that the probe had no deleterious effect on the enzyme. In contrast with results with 8-anilinonaphthalene-1-sulphonic acid, fluorescence of the epsilon-derivative of acetyl-CoA or ATP [fluorescent analogues; Secrist, Barrio, Leonard & Weber (1972) Biochemistry11, 3499-3506] was not changed when either one was added to the enzyme. Secondary-structure composition of chicken liver pyruvate carboxylase estimated from the far-u.v. c.d. spectrum of the enzyme is 27% helix, 7% beta-pleated sheet and 66% other structural types.

Purification and characterization of an agglutinin in the red alga Agardhiella tenera

The red alga, Agardhiella tenera was found to contain a glycoprotein which agglutinates mouse leukemia cells, L5178Y but not L1210. It also agglutinates guinea pig and rabbit erythrocytes, and has weak activity against human A, B and O, mouse, horse and sheep erythrocytes and hamster and mouse lymphocytes. The agglutination was not inhibited by simple sugars. The major active component was purified and determined to be a beta-structure protein containing 2.7% glucose as sugar moiety. The molecular weight was estimated to be 12,000 by gel filtration and 13,000 by polyacrylamide gel electrophoresis, in the presence of sodium dodecylsulfate. Its isoelectric point was 6.1, and it contained high amounts of glycine, serine and threonine, but no half cystine or histidine. It had no subunit structure, and the C- and N-terminal amino acids were threonine and arginine, respectively.

Effect of phospholipase A2 on purified gastric vesicles

The phospholipid and fatty acid composition and role of phospholipids in enzyme and transport function of gastric (H+ + K+)-ATPase vesicles was studied using phospholipase A2 (bee venom). The composition (%) was phosphatidyl-choline (PC) 33%; sphingomyelin (sph) 25%; phosphatidylethanolamine (PE) 22%; phosphatidylserine (PS) 11%; and phosphatidylinositol (PI) 8%. The fatty acid composition showed a high degree of unsaturation. In both fresh and lyophilized preparations, even with prolonged incubation, only 50% of phospholipids were hydrolyzed, but the amount of PE and PS disappearing was increased following lyophilization. There was a marked decrease in K+-ATPase activity (75%) but essentially no loss of the associated K+ p-nitrophenyl phosphatase was found. ATPase activity could be largely restored by various phospholipids (PE greater than PC greater than PS). There was also an increase in Mg2+-ATPase activity, partially reversed in fresh preparations by the addition of phospholipids (PE greater than PS greater than PC). Proton transport activity of the preparation was rapidly inhibited, initially due to a large increase in the HCl permeability of the preparation. Associated with these enzymatic and functional changes, the ATP-induced conformational changes, as indicated by circular dichroism spectra were inhibited.

Characterization of glutathione reductase from porcine erythrocytes

Glutathione reductase (NAD(P)H: oxidized-glutathione oxidoreductase, EC 1.6.4.2) was purified to homogeneity from porcine erythrocytes by use of affinity chromatography on 2',5'-ADP-Sepharose 4-B. Analytical ultracentrifugation experiments were analysed to give the following physical parameters for the enzyme: s20,w = 5.7 S, D20,w = 50 microgram2/s, and Mw = 103 000 (protein concentration, 0.5 mg/ml). The frictional ratio was 1.37 and the Stokes radius was 4.3 nm. The enzyme molecule is a dimer composed of subunits of equal size each containing a FAD molecule. The amino acid compositions and circular dichroism spectra of the porcine and human enzymes indicated extensive structural similarities. The isoelectric point was at pH 6.85 (at 4 degrees C). The absorption spectrum of the oxidized enzyme had maxima at 377 and 462 nm. In vivo the enzyme appears to be partially reduced. At a physiological concentration of reduced glutathione the apparent Michaelis constants for glutathione disulfide and NADPH were higher than in the absence of reduced glutathione. At 0.15 M ionic strength the catalytic activity obtained with NADPH as reductant was optimal at pH 7 and more than 200 times higher than that obtained with NADH. S-sulfoglutathione and some mixed disulfides of glutathione were poor substrates with the exception of the mixed disulfide of coenzyme A and reduced glutathione. The purified enzyme displayed low transhydrogenase activity with oxidized pyridine nucleotide analogs and diaphorase activity with 2,6-dichlorophenolindophenol as acceptor substrates; both NADPH and NADH served as donors.