Diacetyl (acetoin) reductase from Aerobacter aerogenes. Kinetic mechanism and regulation by acetate of the reversible reduction of acetoin to 2,3-butanediol

Study of the role of anaerobic metabolism in succinate production by Enterobacter aerogenes

Succinate is a core biochemical building block; optimizing succinate production from biomass by microbial fermentation is a focus of basic and applied biotechnology research. Lowering pH in anaerobic succinate fermentation culture is a cost-effective and environmentally friendly approach to reducing the use of sub-raw materials such as alkali, which are needed for neutralization. To evaluate the potential of bacteria-based succinate fermentation under weak acidic (pH <6.2) and anaerobic conditions, we characterized the anaerobic metabolism of Enterobacter aerogenes AJ110637, which rapidly assimilates glucose at pH 5.0. Based on the profile of anaerobic products, we constructed single-gene knockout mutants to eliminate the main anaerobic metabolic pathways involved in NADH re-oxidation. These single-gene knockout studies showed that the ethanol synthesis pathway serves as the dominant NADH re-oxidation pathway in this organism. To generate a metabolically engineered strain for succinate production, we eliminated ethanol formation and introduced a heterogeneous carboxylation enzyme, yielding E. aerogenes strain ΔadhE/PCK. The strain produced succinate from glucose with a 60.5% yield (grams of succinate produced per gram of glucose consumed) at pH <6.2 and anaerobic conditions. Thus, we showed the potential of bacteria-based succinate fermentation under weak acidic conditions.

Identification of acetoin reductases involved in 2,3-butanediol pathway in Klebsiella oxytoca

The acetoin reductase (AR) of Klebsiella oxytoca is responsible for converting acetoin into 2,3-butanediol (2,3-BDO) during sugar fermentation. Deleting the AR encoding gene (budC) in the 2,3-BDO operon does not block production of 2,3-BDO, as another similar gene exists in addition to budC called diacetyl/acetoin reductase (dar) which shares 53% identity with budC. In the present study, both budC and dar of K. oxytoca were independently cloned and expressed in Escherichia coli along with budA (acetolactate decarboxylase) and budB (acetolactate synthase), which are responsible for converting pyruvate into acetoin. The recombinant E. coli expressing budABC and budAB-dar produced 2,3-BDO from glucose but E. coli expressing only budAB did not and produced acetoin alone. This demonstrates that Dar functions similar to BudC. Mutants of budC, dar, and both genes together were developed in K. oxytoca ΔldhA (lactate dehydrogenase). K. oxytoca ΔldhA ΔbudC Δdar, deficient in both AR genes, showed reduced 2,3-BDO concentration when compared to K. oxytoca ΔldhA and K. oxytoca ΔldhA ΔbudC by 84% and 69%, respectively. Interestingly, K. oxytoca ΔldhA Δdar resulted in a significant reduction in the reversible conversion of 2,3-BDO into acetoin than that of K. oxytoca ΔldhA, which was observed in a glucose depleted fermentation culture. In addition, we observed that Dar played a key role in dissimilation of 2,3-BDO in media containing 2,3-BDO alone.

Improved production of 2,3-butanediol in Bacillus amyloliquefaciens by over-expression of glyceraldehyde-3-phosphate dehydrogenase and 2,3-butanediol dehydrogenase

BACKGROUND

Previously, a safe strain, Bacillus amyloliquefaciens B10-127 was identified as an excellent candidate for industrial-scale microbial fermentation of 2,3-butanediol (2,3-BD). However, B. amyloliquefaciens fermentation yields large quantities of acetoin, lactate and succinate as by-products, and the 2,3-BD yield remains prohibitively low for commercial production.

METHODOLOGY/PRINCIPAL FINDINGS

In the 2,3-butanediol metabolic pathway, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) catalyzes the conversion of 3-phosphate glyceraldehyde to 1,3-bisphosphoglycerate, with concomitant reduction of NAD(+) to NADH. In the same pathway, 2,3-BD dehydrogenase (BDH) catalyzes the conversion of acetoin to 2,3-BD with concomitant oxidation of NADH to NAD(+). In this study, to improve 2,3-BD production, we first over-produced NAD(+)-dependent GAPDH and NADH-dependent BDH in B. amyloliquefaciens. Excess GAPDH reduced the fermentation time, increased the 2,3-BD yield by 12.7%, and decreased the acetoin titer by 44.3%. However, the process also enhanced lactate and succinate production. Excess BDH increased the 2,3-BD yield by 16.6% while decreasing acetoin, lactate and succinate production, but prolonged the fermentation time. When BDH and GAPDH were co-overproduced in B. amyloliquefaciens, the fermentation time was reduced. Furthermore, in the NADH-dependent pathways, the molar yield of 2,3-BD was increased by 22.7%, while those of acetoin, lactate and succinate were reduced by 80.8%, 33.3% and 39.5%, relative to the parent strain. In fed-batch fermentations, the 2,3-BD concentration was maximized at 132.9 g/l after 45 h, with a productivity of 2.95 g/l·h.

CONCLUSIONS/SIGNIFICANCE

Co-overexpression of bdh and gapA genes proved an effective method for enhancing 2,3-BD production and inhibiting the accumulation of unwanted by-products (acetoin, lactate and succinate). To our knowledge, we have attained the highest 2,3-BD fermentation yield thus far reported for safe microorganisms.

A 2,3-butanediol dehydrogenase from Paenibacillus polymyxa ZJ-9 for mainly producing R,R-2,3-butanediol: purification, characterization and cloning

A 2,3-butanediol dehydrogenase (BDH) from Paenibacillus polymyxa ZJ-9 was purified to homogeneity via fractional ammonium sulfate precipitation, followed by two steps of anion-exchange chromatography using DEAE-Sepharose and Source 15Q, obtaining a 35-fold increase in specific activity and 34.9% yield. The molecular weights of the purified BDH subunit and holoenzyme were 44.5 and 90.0 kDa, respectively, as detected via SDS-PAGE and gel filtration chromatography. These results were significantly different from those of other reported BDHs. Substrate specificity experiments showed that the enzyme could function preferentially as a reductase rather than as a dehydrogenase, and was mainly responsible for the reduction of R-acetoin to R,R-2,3-butanediol. Gene cloning, sequencing, and expression experiments further demonstrate that this enzyme was a new type of BDH.

Properties of 2,3-Butanediol Dehydrogenases from Lactococcus lactis subsp. lactis in Relation to Citrate Fermentation

Two 2,3-butanediol dehydrogenases (enzymes 1 and 2; molecular weight of each, 170,000) have been partially purified from Lactococcus lactis subsp. lactis (Streptococcus diacetylactis) D10 and shown to have reductase activity with either diacetyl or acetoin as the substrate. However, the reductase activity with 10 mM diacetyl was far greater for both enzymes (7.0- and 4.7-fold for enzymes 1 and 2, respectively) than with 10 mM acetoin as the substrate. In contrast, when acetoin and diacetyl were present together, acetoin was the preferred substrate for both enzymes, with enzyme 1 showing the more marked preference for acetoin. meso-2,3-Butanediol was the only isomeric product, with enzyme 1 independent of the substrate combinations. For enzyme 2, both the meso and optical isomers of 2,3-butanediol were formed with acetoin as the substrate, but only the optical isomers were produced with diacetyl as the substrate. With batch cultures of strain D10 at or near the point of citrate exhaustion, the main isomers of 2,3-butanediol present were the optical forms. If the pH was sufficiently high (>pH 5), acetoin reduction occurred over time and was followed by diacetyl reduction, and meso-2,3-butanediol became the predominant isomer. Interconversion of the optical isomers into the meso isomer did occur. The properties of 2,3-butanediol dehydrogenases are consistent with diacetyl and acetoin removal and the appearance of the isomers of 2,3-butanediol.

Characterization of two aldo-keto reductases from Gluconobacter oxydans 621H capable of regio- and stereoselective alpha-ketocarbonyl reduction

Two cytosolic NADPH-dependent carbonyl reductases from Gluconobacter oxydans 621H, Gox0644 and Gox1615, were heterologously produced in Escherichia coli. The recombinant proteins were purified to homogeneity and characterized. Gox0644 and Gox1615 were dimers with native molecular masses of 66.1 and 74.5 kDa, respectively. The enzymes displayed broad substrate specificities and reduced alpha-ketocarbonyls at the keto moiety most proximal to the terminus of the alkyl chain to produce alpha-hydroxy carbonyls, as demonstrated by NMR. With respect to stereoselectivity, protein Gox0644 specifically reduced 2,3-pentanedione to 2R-hydroxy-pentane-3-one, whereas Gox1615 produced 2S-hydroxy-pentane-3-one. Both enzymes also reduced 1-phenyl-1,2-propanedione to 2-hydroxy-1-phenylpropane-1-one, which is a key intermediate in the production of numerous pharmaceuticals, such as antifungal azoles and antidepressants. Gox0644 displayed highest activities with 2,3-diones, alpha-ketoaldehydes, alpha-keto esters, and 2,5-diketogluconate. Gox1615 was less active with these substrates, but displayed a broader substrate spectrum reducing a variety of alpha-diketones and aldehydes.

Induction of some enzymes of citrate metabolism inLeuconostoc lactisand other heterofermentative lactic acid bacteria

Citrate stimulated growth, totally induced citrate lyase, partly induced acetolactate synthase activity and partly repressed both diacetyl and acetoin reductases inLeuconostoc lactisNCW1. Similar results were obtained with 2 other leuconostocs and a heterofermentative lactobacillus. In 2 of the 3 leuconostocs tested, diacetyl reductase and acetoin reductase were NADPH specific, while in the 2 heterofermentative lactobacilli, they were NADH specific.

Enhanced 2,3-butanediol production by addition of acetic acid in Paenibacillus polymyxa

By the addition of 150 mM acetate into a batch culture at an initial pH of 6.8, the production of 2,3-butanediol (BDL) by Paenibacillus polymyxa reached 248 mM, yielding 0.87 mol.mol(-1) glucose, where the ratio of acetate consumed to glucose consumed (A/C ratio) was calculated as 0.35 mol acetate mol(-1) glucose. Therefore, a fed-batch culture was carried out by feeding glucose and acetate at a ratio of 0.35 mol acetate mol(-1) glucose. In the fed-batch culture performed at pH 6.8, BDL production reached 637 mM, yielding 0.81 mol.mol(-1) glucose, although the A/C ratio was only 0.18 mol acetate mol(-1) glucose. By decreasing pH to 6.3 in the fed-batch culture, BDL production reached 566 mM, yielding 0.88 mol.mol(-1) glucose and the A/C ratio was 0.32 mol acetate mol(-1) glucose. The optical purity of BDL, which was expressed as enantiomeric excess, was retained at more than 98% of the (R, R)-stereoisomer at the end of culture, which was comparable to that without acetate addition.

Identification of human liver diacetyl reductases by nano-liquid chromatography/Fourier transform ion cyclotron resonance mass spectrometry

Several forms of diacetyl-reducing enzyme were found to exist in the human liver cytosol. Three (DAR-2, DAR-5, and DAR-7) of them were purified as a single band on SDS-PAGE by a combination of a few kinds of column chromatographies. The in-gel tryptic digests of the purified enzymes were analyzed by nano-liquid chromatography (LC)/Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS), which provided peptide masses at a ppm-level accuracy. The enzymes, DAR-2, DAR-5, and DAR-7, were identified as alcohol dehydrogenase beta subunit (ADH2), carbonyl reductase (CBR1), and aldehyde reductase (AKR1A1), respectively, by peptide mass fingerprinting. In addition, an alternating-scan acquisition of nano-LC/FT ICR mass spectra, i.e., switching of normal acquisition conditions and in-source fragmentation conditions scan by scan, provided sets of parent and fragment ion masses of many of the tryptic peptides in a single LC/MS run. The peptide sequence-tag information at the ppm-level accuracy was used to further confirm the protein identities. It was demonstrated that nano-LC/FT ICR MS can be used for rigorous protein identification at a subpicomole level as an alternative technique to nano-LC/MS/MS.

Scale-down model to simulate spatial pH variations in large-scale bioreactors

For the first time a laboratory-scale two-compartment system was used to investigate the effects of pH fluctuations consequent to large scales of operation on microorganisms. pH fluctuations can develop in production-scale fermenters as a consequence of the combined effects of poor mixing and adding concentrated reagents at the liquid surface for control of the bulk pH. Bacillus subtilis was used as a model culture since in addition to its sensitivity to dissolved oxygen levels, the production of the metabolites, acetoin and 2,3-butanediol, is sensitive to pH values between 6.5 and 7.2. The scale-down model consisted of a stirred tank reactor (STR) and a recycle loop containing a plug flow reactor (PFR), with the pH in the stirred tank being maintained at 6.5 by addition of alkali in the loop. Different residence times in the loop simulated the exposure time of fluid elements to high values of pH in the vicinity of the addition point in large bioreactors and tracer experiments were performed to characterise the residence time distribution in it. Since the culture was sensitive to dissolved oxygen, for each experiment with pH control by adding base into the PFR, equivalent experiments were conducted with pH control by addition of base into the STR, thus ensuring that any dissolved oxygen effects were common to both types of experiments. The present study indicates that although biomass concentration remained unaffected by pH variations, product formation was influenced by residence times in the PFR of 60 sec or longer. These changes in metabolism are thought to be linked to both the sensitivity of the acetoin and 2,3-butanediol-forming enzymes to pH and to the inducing effects of dissociated acetate on the acetolactate synthase enzyme.

Properties of diacetyl (acetoin) reductase from Bacillus stearothermophilus

The cells of Bacillus stearothermophilus contain an NADH-dependent diacetyl (acetoin) reductase. The enzyme was easily purified to homogeneity, partially characterised, and found to be composed of two subunits with the same molecular weight. In the presence of NADH, it catalyses the stereospecific reduction of diacetyl first to (3S)-acetoin and then to (2S,3S)-butanediol; in the presence of NAD+, it catalyses the oxidation of (2S,3S)- and meso-butanediol, respectively, to (3S)-acetoin and to (3R)-acetoin, but is unable to oxidise these compounds to diacetyl. The enzyme is also able to catalyse redox reactions involving some endo-bicyclic octen- and heptenols and the related ketones, and its use is suggested also for the recycling of NAD+ and NADH in enzymatic redox reactions useful in organic syntheses.

Purification, characterization and some properties of diacetyl(acetoin) reductase from Enterobacter aerogenes

A new method, faster, milder and more efficient than the one previously described [Bryn, K., Hetland, O. & Stormer, F. C. (1971) Eur. J. Biochem, 18, 116-119], for purification of diacetyl(acetoin) reductase from Enterobacter aerogenes is proposed. The experiments carried out with the electrophoretically pure preparations obtained by this procedure show that the enzyme (a) produces L-glycols from the corresponding L-alpha-hydroxycarbonyls by reversible reduction of their oxo groups and also reduces the oxo group of uncharged alpha-dicarbonyls converting them into L-alpha-hydroxycarbonyls, and (b) is specific for NAD. This is a new enzyme for which we suggest the systematic name of L-glycol: NAD+ oxidoreductase and the recommended name of L-glycol dehydrogenase(NAD). The molecular mass, pI, affinity for substrates and pH profiles of this enzyme are also described.

Purification and characterization of a (R)-2,3-butanediol dehydrogenase from Saccharomyces cerevisiae

A NAD-dependent (R)-2,3-butanediol dehydrogenase (EC 1.1.1.4), selectively catalyzing the oxidation at the (R)-center of 2,3-butanediol irrespective of the absolute configuration of the other carbinol center, was isolated from cell extracts of the yeast Saccharomyces cerevisiae. Purification was achieved by means of streptomycin sulfate treatment, Sephadex G-25 filtration, DEAE-Sepharose CL-6B chromatography, affinity chromatography on Matrex Gel Blue A and Superose 6 prep grade chromatography leading to a 70-fold enrichment of the specific activity with 44% yield. Analysis of chiral products was carried out by gas chromatographic methods via pre-chromatographic derivatization and resolution of corresponding diastereomeric derivatives. The enzyme was capable to reduce irreversibly diacetyl (2,3-butanediol) to (R)-acetoin (3-hydroxy-2-butanone) and in a subsequent reaction reversibly to (R,R)-2,3-butanediol using NADH as coenzyme. 1-Hydroxy-2-ketones and C5-acyloins were also accepted as substrates, whereas the enzyme was inactive towards the reduction of acetone and dihydroxyacetone. The relative molecular mass (Mr) of the enzyme was estimated as 140,000 by means of gel filtration. On SDS-polyacrylamide gel the protein decomposed into 4 (identical) subunits of Mr 35,000. Optimum pH was 6.7 for the reduction of acetoin to 2,3-butanediol and 7.2 for the reverse reaction.

Kinetics of the diacetyl and 2,3-pentanedione reduction by diacetyl reductase (alpha-diketone reductase (NAD)) from Staphylococcus aureus

The kinetic mechanism of diacetyl and 2,3-pentanedione reduction by diacetyl reductase from Staphylococcus aureus was investigated. The shape of the primary double reciprocal plots, the product inhibition pattern, and the features of the inhibition by a substrate analogue (acetone) show that diacetyl is reduced via an Ordered Bi-Bi mechanism, and 2,3-pentanedione by an Ordered Bi-Bi or Theorell-Chance mechanism. NADH is the leading substrate in both reactions. Affinity constants for the coenzyme and the substrates and inhibition constants for NAD, acetoin, and acetone were also calculated. This enzyme has a high affinity for NADH; Km (31-50 microM) and Ks (20-27 microM) for this compound are around one-tenth of the NADH intracellular concentration. Therefore, it must operate in vivo saturated with the coenzyme. This condition is not adequate to play the role, formerly proposed for diacetyl reductases, of regulating the equilibrium between oxidized and reduced forms of pyridine-nucleotides.

End Products of Glucose Fermentation by Brochothrix thermosphacta

Anaerobically, Brochothrix thermosphacta fermented glucose primarily to l -lactate, acetate, formate, and ethanol. The ratio of these end products varied with growth conditions. Both the presence of acetate and formate and a pH below about 6 increased l -lactate production from glucose. Small amounts of butane-2,3-diol were also produced when the pH of the culture was low (≤5.5) or when acetate was added to the growth medium. Radioactive label from [1- 14 C]acetate was incorporated into ethanol and l -lactate, implying reversibility of pyruvate-formate lyase. In crude extracts, the following enzymes involved in pyruvate metabolism were demonstrated: lactate dehydrogenase, phosphotransacetylase, acetate kinase, acetaldehyde dehydrogenase (coenzyme A acetylating), ethanol dehydrogenase, pH 6 acetolactate-forming enzyme, and diacetyl (acetoin) reductase. The lactate dehydrogenase did not require fructose-1,6-disphosphate or Mn 2+ for activity.

Enhanced Production of 2,3-Butanediol by Klebsiella pneumoniae Grown on High Sugar Concentrations in the Presence of Acetic Acid

The bioconversion of sugars present in wood hemicellulose to 2,3-butanediol (hereafter referred to as butanediol) by Klebsiella pneumoniae grown on high initial concentrations (up to 10%) of sugars was investigated. Initial fermentation studies with a chemically defined medium suggested that sugar levels in excess of 2% could not be utlized even when a higher inoculum size (5 to 10%) was used. The addition of nutrient supplements, viz., yeast extract, urea, ammonium sulfate, and trace elements resulted in a 10 to 50% increase in butanediol yields, although sugar utilization remained incomplete. The concentration of end products normally found at the termination of fermentation was shown to be noninhibitory to growth and substrate utilization. Acetic acid was inhibitory at concentrations above 1%, although growth and butanediol yield were stimulated in cultures supplemented with lower levels of acetic acid. The efficient utilization of 4% substrate concentrations of d -glucose and d -xylose was achieved, resulting in butanediol yields of 19.6 and 22.0 g/liter, respectively.

Purification and some properties of L-glycol dehydrogenase from hen's muscle

1. An enzyme which catalyzes the NAD(P)H-linked reversible reduction of uncharged vicinal dicarbonyls and alpha-hydroxycarbonyls to L-(+)-glycols has been purified from hen's muscle. This enzyme has not been previously described. 2. According to the rules of the I.U.P.A.C.-I.U.B. Enzymes Commission, the systematic name of L-(+)-glycol:NAD(P) oxidoreductase and the trivial name of L-glycol dehydrogenase are proposed for the enzyme. 3. Three forms of this enzyme differing in pI have been isolated; two forms, which together represent about 90% of total recovered activity, and electrophoretically pure. 4. Molecular weight, pH profiles and affinity for substrates are also described.

Bacterial 2,3-butanediol dehydrogenases

Enterobacter aerogenes, Aeromonas hydrophila, Serratia marcescens and Staphylococcus aureus possessing L(+)-butanediol dehydrogenase produced mainly meso-butanediol and small amounts of optically active butanediol; Acetobacter suboxydans, Bacillus polymyxa and Erwinia carotovora containing D(-)-butanediol dehydrogenase produced more optically active butanediol than meso-butanediol. Resting and growing cells of these organisms oxidezed only one enantiomer of racemic butanediol. The D(-)-butanediol dehydrogenase from Bacillus polymyxa was partially purified (30-fold) with a specific activity of 24.5. Except NAD and NADH no other cofactors were required. Optimum pH-values for oxidation and reduction were pH 9 and pH 7, respectively. The optimum temperature was about 60 degrees C. The molecular weight was 100000 to 107000. The Km-values were 3.3 mM for D(-)butanediol, 6.25 mM for meso-butanediol, 0.53 mM for acetoin, 0.2 mM for NAD, 0.1 mM for NADH, 87 mM for diacetyl, 38 mM for 1,2-propanediol; 2,3-pentanedion was not a substrate for this enzyme. The L(+)butanediol dehydrogenase from Serratia marcescens was purified 57-fold (specific activity 22.3). Besides NAD or NADH no cofactors were required. The optimum value for oxidation was about pH9 and for reduction pH 4.5. The optimum temperature was 32-36 degrees C. The molecular weight was 100000 to 107000. The Km-values were 5 mM for meso-butanediol, 10 mM for racemic butanediol, 6.45 for acetoin, 1 mM for NAD, 0.25 mM for NADH, 2.08 mM for diacetyl, 16.7 mM for 2,3-pentanedion and 11.8 mM for 1,2-propanediol.

Physiological and biochemical role of the butanediol pathway in Aerobacter (Enterobacter) aerogenes

Aerobacter (Enterobacter) aerogenes wild type and three mutants deficient in the formation of acetoin and 2,3-butanediol were grown in a glucose minimal medium. Culture densities, pH, and diacetyl, acetoin, and 2,3-butanediol levels were recorded. The pH in wild-type cultures dropped from 7.0 to 5.8, remained constant while acetoin and 2,3-butanediol were formed, and increased to pH 6.5 after exhaustion of the carbon source. More 2,3-butanediol than acetoin was formed initially, but after glucose exhaustion reoxidation to acetoin occurred. The three mutants differed from the wild type in yielding acid cultures (pH below 4.5). The wild type and one of the mutants were grown exponentially under aerobic and anaerobic conditions with the pH fixed at 7.0, 5.8, and 5.0, respectively. Growth rates decreased with decreasing pH values. Aerobically, this effect was weak, and the two strains were affected to the same degree. Under anaerobic conditions, the growth rates were markedly inhibited at a low pH, and the mutant was slightly more affected than the wild type. Levels of alcohol dehydrogenase were low under all conditions, indicating that the enzyme plays no role during exponential growth. The levels of diacetyl (acetoin) reductase, lactate dehydrogenase, and phosphotransacetylase were independent of the pH during aerobic growth of the two strains. Under anaerobic conditions, the formation of diacetyl (acetoin) reductase was pH dependent, with much higher levels of the enzyme at pH 5.0 than at pH 7.0. Lactate dehydrogenase and phosphotransacetylase revealed the same pattern of pH-dependent formation in the mutant, but not in the wild type.

Purification and properties of a diacetyl reductase from Escherichia coli

A reduced nicotinamide adenine dinucleotide phosphate (NADPH)-dependent reductase with the ability to reduce diacetyl has been isolated from Escherichia coli and has been purified 800-fold to near homogeneity. The product of the reduction of diacetyl was shown to be acetoin. The enzyme proved to catalyze the oxidation of NADPH in the presence of both uncharged alpha- and beta-dicarbonyl compounds. Even monocarbonyl compounds showed slight activity with the enzyme. On the basis of its substrate specificity, it is suggested that the enzyme functions as a diacetyl reductase. In contrast to other diacetyl reductases, the one reported here is specific for NADPH and does not possess acetoin reductase activity. The pH optimum of this enzyme was found to be between 6 and 7. The maximal velocity for the NADPH-dependent reduction of diacetyl was determined to be 9.5 mumol per min per mg of protein and the K(m) values for diacetyl and NADPH were found to be 4.44 mM and 0.02 mM, respectively. The molecular weight was estimated by gel filtration on Sephadex G-100 to be approximately 10,000.