Stereospecificity of Sodium Borohydride Reduction [PDF]

Vol. 254, No. 12. Issue of dune 25, pp. 5053-5057, 1979. Printed LIZ USA. Stereospecificity of Sodium Borohydride. Reduc

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THE ~JOURNAI. OF B~LOCKAL CHEMIST Vol. 254, No. 12. Issue of dune 25, pp. 5053-5057, Printed LIZ USA.

Stereospecificity Decarboxylase

1979

of Sodium Borohydride Reduction from Streptococcus faecaZis* (Received

John

C. Vederas,

From

the Department

I. David of Chemistry,

Reingold,

for publication,

and H. William

University

of Alberta,

l

Bacterial tyrosine decarboxylase (L-tyrosine carboxy-lyase; EC 4.1.1.25) is a pyridoxal phosphate-dependent enzyme which catalyzes the conversion of L-tyrosine and L-3,4-dihydroxyphenylalanine to tyramine and dopamine, respectively (l-4). During the formation of tyramine, a solvent proton replaces the carboxyl group with retention of configuration (5). Dunathan has proposed that most pyridoxal phosphate enzymes bind a single side of the cofactor. substrate complex thereby forcing all reactions to occur on the opposite side with retention of configuration (6, 7). Furthermore, the type of reaction which vitamin Bs enzymes catalyze is governed by conformational orientation of the breaking bond nearly perpendicular to the planar 7~ system of the cofactor; this is a situation in which maximal orbital overlap is achieved. In certain decarboxylases, the conformation about the o( carbonnitrogen bond of the amino acid moiety is at least partially controlled by binding of a distal group in the fully extended side chain (8-10). Making the reasonable assumption that steric effects force the aldimine double bond to be trans (6, ll), two of the remaining conformations of interest are those at C-4 to C-4’ and at C-5 to C-5’ in the cofactor. Considerable evidence indicates that rotational changes occur about one of these two bonds during binding of amino acids to the holoenzyme of aspartate aminotransferase (12-17). In the present work we use the stereochemistry of sodium boro[“H]hydride reduction of tyrosine decarboxylase from Streptococcus faecalis to determine the exposed face of the C-4’ double bond in the holoenzyme and in substrate complexes. It was recently shown that tryptophanase, an enzyme which promotes ai,/? eliminations and /3 replacements, is re* This work was supported by the Natural Sciences and Engineering Research Council Canada through Grant A0845 and by the University of Alberta General Research Fund. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “aduertiseme&” in accordance with 18 USC. Section 1734 solely to indicate this fact.

October

30, 1978, and in revised

Alberta,

Canada

form,

January

11, 1979)

Sellers

Edmonton,

T6G 2G2

duced from the si face at C-4’ of a cofactor.inhibitor complex (18). Arigoni and co-workers have established the same stereochemistry for reduction of aspartate aminotransferase complexes (15). It seemed likely that if conformational changes of the type observed with aspartate aminotransferase occurred in tyrosine decarboxylase, they could be detected by this method. In addition, the amino acid residue involved in binding of pyridoxal phosphate can be isolated and identified. Since bacterial decarboxylases show considerable homology at the active site (1, 19), it was expected that the cofactor would be bound to an t-amino group of lysine. EXPERIMENTAL

PROCEDURES

Materials-Commercially available chemicals were of reagent grade or highest purity available and were used without further purification. Crude tyrosine decarboxylase apoenzyme from Strepto coccus faecalis was obtained from Sigma Chemical Co. as an acetonedried powder. The enzyme was purified as described by Epps (2) and assayed by the method of Sundaresan and Coursin (3). Aspartate aminotransferase from porcine heart was obtained from Sigma and converted to the apoenzyme by published procedures (20, 21). All other enzymes were purchased from Sigma and used without further purification. Sodium boro[“H]hydride (422 mCi/mmol) was obtained from Amersham Corp. Instrumentation-Nuclear magnetic resonance spectra were measured on Varian HA-100 and Bruker HFX-SO/Nicolet 1085 spectrometers. Infrared spectra were determined on a Nicolet 7199 Fourier Transform Interferometer. Ultraviolet spectra and kinetic measurements were obtained on a Unicam SP1800 spectrophotometer. Mass spectra were determined on AEI MS50 and MS9 instruments using electron impact or chemical ionization (NH.]). Scintillation counting was done in Aquasol or Biofluor (New England Nuclear) on a Beckman LSlOOC instrument. Counting efficiency was determined by addition of a known quantity of [“Hltoluene standard (New England Nuclear). Thin layer or paper chromatograms of radioactive materials were scanned on a Packard 7201 radiochromatogram scanner. Chromatography-Paper chromatography employed the descending technique on Whatman No. 3MM paper which had been washed with 1 M citric acid followed by water. Precoated silica gel (F254) plates from E. Merck (0.25 mm thickness) were used for thin layer chromatography. The following solvent systems were used: System A, ethanol/water, 7:3; System B, ethanol/concentrated NH,OH, 85: 15; System C, methanol; System D, l-butanol/acetic acid/water, 4:l: 1; System E, pyridine/methanol/water, 1:5:20. Visualization methods included UV, ninhydrin spray, iodine vapor, and phosphomolybdic acid spray. N-Pyridoxyl Amino Acids and Derivatives-Inactive a-N-pyridoxyl-L-amino acids were prepared by a modification of reported procedures (15, 22). In a typical preparation, a mixture of L-amino acid (2.0 mmol) and pyridoxal hydrochloride (2.0 mmol) in methanol (40 ml) at 4°C was treated with solid sodium hydroxide (10 mmol) and stirred under nitrogen atmosphere until solution was complete. Solid sodium borohydride was added until the solution was decolorized. The mixture was warmed to 25”C, stirred 1 h, brought to pH 6 with concentrated HCl, and concentrated in uacuo. Recrystallization of the residue from water or water/methanol afforded the a-N-pyridoxyl-z-amino acids in yields of 65 to 95%. The mother liquors could be purified by chromatography on AG50-H+ with a linear HCl gradient (1 N versus 5 N) to give total yields of >95’%. N-Pyridoxyltyra-

5053

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Sodium boro[3H]hydride reduction of tyrosine decarboxylase from Streptococcus faecalis followed by complete hydrolysis of the enzyme produces t+[3H]pyridoxyllysine. Degradation of this material to [4’-3H]pyridoxamine and stereochemical analysis with apoaspartate aminotransferase shows that the re side at C-4’ of the cofactor is exposed to solvent at pH 5.5 and 7.0. After binding of L-tyrosine at pH 5.5 or tyramine at pH 7.0 to the holoenzyme, sodium boro[3H]hydride reduction proceeds from the si face at C-4’ of the substrate cofactor complex. This indicates one of two conformational changes occurs upon binding of substrate; either rotation about the C-4 to C-4’ bond in the cofactor or rotation about the axis through the C-5 and C-5’ bond.

of Tyrosine

Tyrosine

5054

Decarboxylase

TABLE

I

Reductions of tyrosine decarboxylase with sodium boro[3HJhydride in the presence ofpyridoralphosphate (PLP) followed by degradation to [4’-‘HJpyridoxamine and stereochemical analysis with apoaspartate aminotransferase. All analyses Reduction

were

run in duplicate [ ‘H]Pyridoxconditions amine isokited

and are +2%. [“H]Pyzidoxamine after exchange

dpmhg

pH pH pH pH pH

5.5, PLP 7.0, PLP 5.5, tyrosine, PLP 7.0, tyramine, PLP 5.0, u-CH:l-tyrosine, PLP No enzyme” ‘See

“Experimental

&m/w

sH lost

Predominant attack at C-4'

%,

22,500 38,300 18,500 13,700 9,240

17,600 37,700 4,760 3,190 6,100

22 2 72 77 34

42,700

21,900

49

re re si si re

Procedures.”

Tyrosine decarboxylase from Streptococcus faecalis is isolated primarily as the apoenzyme (2), and is used extensively in determinations of pyridoxal phosphate (3) and tyrosine (25). Incubation with pyridoxal phosphate led to formation of the active holoenzyme which could be reduced with sodium

boro[“H]hydride. Digestion of the reduced enzyme with a protease followed by complete hydrolysis with hydrochloric acid afforded tritiated l -pyridoxyllysine. The phosphate is cleaved during hydrolysis. This product was identified by extensive co-chromatography with authentic material and by isotopic dilution followed by purification to constant specific activity. Thus, in analogy to other decarboxylases (1, 19), tyrosine decarboxylase binds pyridoxal phosphate at the Eamino group of a lysine residue. The reduction proceeded at the pH optimum of the enzyme, pH 5.5 (a), or at pH 7, but the yield of pyridoxyllysine was drastically diminished at the higher pH. Oxidative degradation of the [3H]pyridoxyllysine with sodium hypochlorite (15, 18) produced [4’-“Hlpyridoxamine. The stereochemistry of the label was determined by incubation with apoaspartate aminotransferase, which is known to exchange only the pro-4’S hydrogen (7, 24, 26). Since most of the label was retained (Table I), reduction occurred primarily from the re face at C-4’ of the imine double bond in the holoenzyme. Under identical assay conditions, apoaspartate aminotransferase exchanged 50 ? 1% of the label of nonstereospecifically tritiated pyridoxamine. If an excess of the natural substrate, L-tyrosine, was incubated with tyrosine decarboxylase holoenzyme at pH 5.5, reduction with sodium boro[“H]hydride followed by hydrolysis produced tritiated pyridoxyltyrosine. No tritiated pyridoxyltyramine could be detected under these conditions. Degradation of the pyridoxyltyrosine with sodium hypochlorite yielded [4’-“Hlpyridoxamine which was stereochemically analyzed as before. Since most of the tritium was exchanged (Table I), this pyridoxamine was labeled primarily at the pro S heterotopic hydrogen. In contrast to the holoenzyme reduction, the substrate.cofactor complex on the enzyme surface was attacked mostly from the si face at C-4’ of the imine double bond. If an excess of tyramine was incubated with tyrosine decarboxylase and pyridoxal phosphate at pH 5.5, reduction and hydrolysis did not give detectable amounts of pyridoxyltyramine. The major product was pyridoxyllysine obtained from reduction of the holoenzyme. At pH 7, a reasonable quantity of tritiated pyridoxyltyramine could be produced by reduction with sodium boro[“H]hydride. Stereochemical analysis of the [4’-3H]pyridoxamine isolated after degradation demonstrated that most of the reduction at C-4’ had occurred from the si face, just as with the tyrosine. cofactor complex at pH 5.5.’ Sodium boro[“H]hydride reduction of tyrosine decarboxyl-

’ One unit of tyrosine decarboxylase is the amount of enzyme which catalyzes the decarboxylation of 1.0 pmol of tyrosine/min under the reported assay conditions (3).

’ Preliminary results show that tyrosine exchanges one of the hydrogens adjacent pH 7. See also Ref. 27.

to [ ‘Hlpyridoxamine previously pyridoxamine transferase

by methods exactly analogous to those published

(15,

18). The stereochemistry of the tritium at C-4’ of was determined by incubation with aspartate aminoapoenzyme according to Dunathan et al. (24). RESULTS

decarboxylase to nitrogen

holoenzyme in tyramine at

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mine was synthesized analogously. In the preparation of l -N-pyridoxyl-L-lysine, it was advantageous to use a-N-carbobenzoxy-L-lysine in the above procedure (23). The protecting group on the a-nitrogen was subsequently removed by reflux in 6 N HCl or hydrogenation (15). The pyridoxyl derivatives were homogeneous by TLC in several systems, possessed sharp melting points, gave correct microanalyses, and had spectral properties (‘H NMR, ‘,‘C NMR, IR, UV, mass spectra) consistent with the proposed structures. Preparation of Nonstereospecifically labeled [4’-“HJPyridoxamine-Pyridoxamine tritiated at C-4’ to an equal extent in both R and S configurations was prepared by two methods. This material was synthesized by exchange with [3H]H~0 as reported (24). An alternative procedure involved reduction of a mixture of L-tyrosine and pyridoxal hydrochloride with sodium boro[ ‘HIhydride as described above to produce tritiated pyridoxyltyrosine. This material was degraded to [4’-‘Hlpyridoxamine by methods exactly analogous to those published previously (15, 18). Identification of e-Pyridoxyllysine after Reduction of Tyrosine Decarboxylase Holoenzyme-A mixture of purified tyrosine decarboxylase apoenzyme (200 units,’ 60 units/mg of protein) and pyridoxal phosphate (0.25 mM) in 20 ml of sodium phosphate (0.13 M)/Citi-ate (0.043 M) buffer (pH 5.5) was incubated at 37OC for 1 h. The enzyme was reduced with a solution of 50 mCi of sodium boro[“H]hydride and 10 mg of sodium boro[‘H]hydride in 1.0 ml of water. After 1 h, the mixture was dialyzed, heated 5 min in boiling water, cooled, brought to pH 8.5 with concentrated NaOH, and incubated with 53 mg of protease from Streptomyces griseus (5 units/mg) at 37°C for 12 h. The mixture was treated with 25 ml of concentrated HCI and refluxed 12 h under nitrogen atmosphere before being cooled and concentrated In UCKUO. The residue was chromatographed on 200 ml of AG50-H’ with a linear gradient of 1 N HCl and 5 N HCl (1 liter each). Thin layer chromatography of the column fractions in Systems A and B, and paper chromatography in Systems A + E against a reference of inactive l -pyridoxyl-L-lysine allowed identification of t-[,‘H]pyridoxyllysine in the eluate. Fractions containing the desired substance were combined, diluted with 70 mg of inactive e-pyridoxyl-L-lysine, and rechromatographed repeatedly on AG50-H+ with a linear gradient of 2 N and 5 N HCl. Final purification was achieved by preparative paper chromatography in System A to yield 1.53 PCi of t-r’H]pyridoxyllysine. No impurities or significant change in specific activity could be observed by recrystallization, paper chromatography in Systems B, C, D, or E , or thin layer chromatography in System B. Reduction of Substrates Bound to Tyrosine Decarboxylase Holoenzyme-In a typical experiment, tyrosine decarboxylase (200 units,’ 60 units/mg of protein) was incubated 1 h at 37’C in 20 ml of 0.1 M potassium or sodium phosphate buffer, pH 5.5, which contained pyridoxal phosphate (0.25 mM). The amino acid (0.10 mmol) was added in 10 ml of the same buffer without pyridoxal phosphate. After 5 to 10 min, the mixture was reduced with 50 mCi of sodium boro[ ‘HIhydride and the pyridoxyl amino acid was obtained by degradation of the enzyme as described above. Purification was effected by dilution with 100 mg of inactive pyridoxyl amino acid, repeated chromatography on AG50-H’ with linear HCl gradients (1 N versus 5 N), and recrystallization. Purity was checked by paper chromatography in Systems A and B, and by thin layer chromatography in System B. Experiments with tyramine were exactly analogous, but the reductions had to be done at pH 7.0 to obtain reasonable amounts of [:‘H]pyridoxyltyramine. Stereochemical Analysis of [3HJPyridoxyl Derivatives--In order to analyze the stereochemistry of reduction at C-4’ of the cofactor. substrate complexes, the tritiated pyridoxyl derivatives were degraded

Reduction

Tyrosine

Decarboxylase

i?*

ATTACK

i-

@Jo

80

4’.s

.t-R -

4

DEGRADATION

+

T

H

HO

HO

14,-S)

PYRIDOXAMINE

(4,-R)

PYRIDOXAMINE

AMINOTRANSFERASE

4

‘H20 ‘H

‘H

HO

HO

3H-

toss

3H-RETENTION

1. Stereochemical outcome of sodium boro[“H]h&ide reduction of enzyme-bound pyridoxal phosphate imines, degradation to [“Hlpyridoxamine, and analysis wit,h apoaspartate aminotransferase. FIG.

ase holoenzyme in the presence of excess a-methyl-L-tyrosine at pH 5.5 gave tritiated pyridoxyl-cr-methyl+tyrosine after hydrolysis. Degradation and stereochemical analysis of C-4’ surprisingly showed a slight preference for reduction from the re face. The reduction is very slow and proceeds poorly relative to the others. DISCUSSION

Our study shows that like other pyridoxal phosphate enzymes, tyrosine decarboxylase from Streptococcus faecalis binds the cofactor as an imine at the l -aminoyroup of a lysine residue. It is likely that there is considerable structural homology with other bacterial decarboxylases at the active site (19) and that the mechanistic considerations described below may be applicable to other related enzymes. Since ultraviolet studies show that the cofactor imine double bond is generally in conjugation with the pyridine ring (l), a reducing agent such as boro[“H]hydride can approach C-4’ from one of two sides of this planar system. Stereochemical analysis of the reduced carbon demonstrates that primarily the re face at C4’ is exposed to solvent in the enzyme. cofactor complex. Entry of a substrate amino acid into the active site results in transimination (28), presumably by attack of the amino group from the re face. According to Dunathan’s hypothesis (6, 7), the new substrate-cofactor aldimine will conformationally orient the (Y carbon-nitrogen bond of the amino acid moiety so as to place the bond between the carboxyl carbon and the a: carbon nearly perpendicular to the plane of the conjugated pyridine. In such a conformation, maximal orbital overlap is achieved between the breaking u bond and the extended r system. Recent model studies have shown that the rate of racemization and hydrogen exchange at the (Y carbon of amino acidpyridoxal Schiff’s bases is determined by the proportion of

5055

conformer having the C,-H,, bond orthogonal to the ~7system (29). Binding of a distal group in the amino acid side chain determines which of two possible C,-N conformations having the C,-COOH bond orthogonal in decarboxylases is actually present in the active site. This has been demonstrated for a number of decarboxylation reactions (&lo), and is in accord with the substrate specificity observed with tyrosine decarboxylase (1,2,4). Since loss of CO, occurs from the same side as rotonation of the resulting planar quinoid intermediate (5); Y only one face of the enzyme-bound complex is readily accessible at the cr carbon. We suggest that the same side of the substrate. cofactor complex is exposed at C-4‘. Sodium boro[“H]hydride reductions of either tyrosine or tyramine complexes with pyridoxal phosphate in the active site of tyrosine decarboxylase proceed primarily from the si face at C-4’. In the absence of substrates, the opposite side (re) at C4’ of the cofactor is exposed to solvent than in their presence; the re side is exposed in the holoenzyme. Lack of complete stereospecificity may be due to reduction of some Schiff’s base complex outside of the active site, or to some conformational mobility within the site. Several experiments using substoichiometric amounts of pyridoxal phosphate produced no significant increase in stereospecificity of reduction. Our assumption that the same side of the substrate. cofactor complex is exposed at C-4’ as at C, is supported by the abortive transamination (32) and normal decarboxylation (31) reactions of glutamate decarboxylase which occur with protonation from the si face at C-4’ and retention at C,,, respectively. Two types of conformational changes can explain the difference in stereochemistry of reduction of the enzyme. cofactor complex relative to the enzyme.cofactor.substrate complexes. In the first model (11, 13, 14), the imine nitrogen is always on the same side of the C-4 to C-4’ bond as the phenolic OH (cisoid conformation), and rotation about an axis through the C-5 to C-5’ bond during binding of substrate exposes a new side of both the double bond and the pyridine ring. Model studies and calculations show that in the absence of enzyme, pyridoxal Schiffs bases prefer a cisoid conformation (11, 33). A second possibility is that the pyridine ring keeps the same side exposed while a rotation about the C-4 to C-4’ bond during substrate binding exposes a new face of the imine bond. This requires a transoid to cisoid reorientation on going from the enzyme.cofactor complex to the enzyme.cofactor.substrate complex. Such conformational changes have been proposed by Metzler and co-workers for aspartate aminotransferase (12, 17) on the basis of absorption spectra and x-ray diffraction studies. Syncatalytic modifications of aspartate aminotransferases have also indicated conformational changes during substrate binding (16,34). Arigoni and co-workers have shown that reduction of this holoenzyme occurs from the re side at C-4’, whereas after addition of substrate reduction proceeds from the si face (15). This is the same result we observe with tyrosine decarboxylase. After binding of an inhibitor, tryptophanase is also reduced from the si face at C-4’ (18). These results suggest a very close catalytic relationship between enzymes of widely varying function and provide strong support for Dunathan’s hypothesis (6, 7). At present, it is not possible to distinguish between the two possible conformational changes in tyrosine decarboxylase, but we favor rotation about the C-4 to C-4’ bond because of analogy with aspartate aminotransferase, and because of the relatively minor structural rearrangement which is necessary. Although tyrosine decarboxylase shows a sharp pH opti“Retention of configuration has also been observed decarboxylase (30) and glutamate decarboxylase (31).

with

lysine

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APO-ASPARTASE

4

Reduction

Tyrosine

5056 C-4

to C-4’

Decarboxylase

Reduction

ROTATION:

SUBSTRATE *

i

\Re

ATTACK /’

C-5

to

C-S’

AXIS

Si ATTACK

ROTATION:

ATTACK SUBSTRATE

,H

+

-

Lyr

I

FIG. 2. Two possible conformational for exposure of the si face at C-4’.

changes may occur upon binding of

mum at 5.5 (2), reductions of the holoenzyme still proceeded at pH 7.0. After addition of tyrosine, reduction could only be detected at the lower pH. In contrast, reductions in the presence of tyramine had to be done at pH 7.0 to obtain detectable amounts of the expected product.” This may be due to relatively poor binding of tyramine at the lower pH values. Interestingly, glutamate decarboxylase is resistant to reduction by sodium borohydride at pH 7.2 (35) because of addition of a sulfhydryl group to the imine double bond to form an aldamine (36). Although pH changes affected the yields of reduction drastically, the exposed side of the imine bond was unaltered in the range of pH 5.5 to 7.0, Amino acids with an a-methyl group are effective inhibitors of normal decarboxylation reactions, and also undergo some abortive transamination (32, 37, 38). When a-methyl-L-tyrosine was added to tyrosine decarboxylase holoenzyme, the borohydride reduction was very inefficient and surprisingly showed a slight preference for the re side at C-4’. This may be due to difficulty in attaining the correct conformational orientation because of steric interference of the methyl group. We are currently investigating the abortive transamination reactions of tyrosine decarboxylase and the stereochemistry of protonation at C-4’. Spectroscopic studies aimed at distinguishing between the two possible conformational changes are also in progress. It is conceivable that eventually a large number of pyridoxal phosphate enzymes of widely varying

function may be shown to undergo during substrate binding.

and can account

conformational

changes

REFERENCES 1. Boeker, E. A., and Snell, E. E. (1972) in The Enzymes (Bayer, P. D., ed) 3rd Ed, Vol. 6, pp. 217-253, Academic Press, New York 2. Epps, H. M. R. (1944) Biochem. J. 38,242 3. Sundaresan, P. R., and Coursin, D. B. (1970) in Methods Enzymol. 18A,509-512 4. Gale, E. F. (1946) Adu. Enzymol. 6, 1 5. Belleau, B., and Burba, J. (1960) J. Am. Chem. Sot. 82, 5751 6. Dunathan, H. C. (1971) Adu. Enzymol. 35, 79 7. Dunathan, H. C., and Voet, J. G. (1974) Proc. Natl. Acad. Sci. U. s. A. 71,3888 8. Bailey, G. B., Chotamangsa, O., and Vuttivej, K. (1970) Biochemistry 9, 3243 9. Fonda, M. L. (1972) Biochemistry 11, 1304 10. O’Leary, M. H., and Piazza, G. J. (1978) J. Am. Chem. Sot. 100, 632 11. Tumanyan, V. G., Mamaeva, 0. K., Bocharov, A. L., Ivanov, V. I., Karpeisky, M., and Yakovlev, G. I. (1974) Eur. J. Biochem. 50, 119 12. Fisher, T. L., and Metzler, D. E. (1969) J. Am. Chem. Sot. 91, 5323 13. Ivanov, V. I., and Karpeisky, M. Y. (1969) Adu. Enzymol. 32, 21 14. Braustein, A. E. (1973) in The Enzymes (Boyer, P. D., ed) 3rd Ed, Vol. 9, pp. 455-462, Academic Press, New York 15. Austermuhle-Bertola, E. (1973) Dissertation No. 5009 ETH ZUrich

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Re

Tyrosine

Decarboxylase

16. Gehring, H., and Christen, P. (1978) J. Biol. Chem. 253, 3158 17. Metzler, C. M., Metzler, D. E., Martin, D. S., Newman, R., Arnone, A., and Rogers, P. (1978) J. Biol. Chem. 253, 5251 18. Vederas, J. C., Schleicher, E., Tsai, M.-D., and Floss, H. G. (1978) J. Biol. Chem. 253, 5350 19. Applebaum, D., Sabo, D. L., Fischer, E. H., and Morris, D. R. (1975) Biochemistry 14, 3675 20. Slebe, J. C., and Martinez-Carrion, M. (1977) J. Biol. Chem. 253, 2093 21. Martinez-Carrion, M., Tiemeier, D. C., and Peterson, D. L. (1970) Biochemistry 9, 2574 22. Heyl, D., Harris, S. A., and Folkers, K. (1948) J. Am. Chem. Sot. 70,3429 23. Forrey, A. W., Olsgaard, R. B., Nolan, C., and Fischer, E. H. (1971) Biochimie 53, 269 24. Dunathan, H. C., Davis, L., Kury, P. G., and Kaplan, M. (1968) Biochemistry 7,4532 25. Gale, E. F. (1974) in Methods of Enzymatic Analysis (Bergmeyer, H. U., ed) 2nd English Ed, Vol. 4, pp. 1662-1668, Verlag Chemie, Weinheim, Germany 26. Besmer, P., and Arigoni, D. (1969) Chimia 23, 190 27. Mandeles, S., Koppelman, R., and Hanke, M. E. (1954) J. Biol.

Reduction

5057

Chem. 209,327 28. Tobias, P. S., and Kallen, R. G. (1975) J. Am. Chem. Sot. 97, 6530 29. Tsai, M. D., Weintraub, H. J. R., Byrn, S. R., Chang, C., and Floss, H. G. (1978) Biochemistry 17, 3183 30. Leistner, E., and Spenser, I. D. (1975) J. Chem. Sot. Chem. Commun. 378 31. Yamada, H., and O’Leary, M. H. (1977) J. Am. Chem. Sot. 99, 1660 32. Sukhareva, B. S., Dunathan, H. C., and Braunstein, A. E. (1971) FEBS Lett. 15, 241 33. Tsai, M. D., Byrn, S. R., Chang, C., Floss, H. G., and Weintraub, J. R. (1978) Biochemistry 17, 3177 34. Pfister, K., Kagi, J. H. R., and Christen, P. (1978) Proc. N&Z. Acad. Sci. U. S. A. 75, 145 35. Anderson, J. A., and Chang, H. W. (1965) Arch. Biochem. Biophys. 110, 346 36. O’Leary, M. H., and Brummund, W., Jr. (1974) J. Biol. Chem. 249,3737 37. O’Leary, M. H., and Baughn, R. L. (1977) J. Biol. Chem. 252, 7168 38. O’Leary, M. H., and Herreid, R. M. (1978) Biochemistry 17, 1010

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Stereospecificity of sodium borohydride reduction of tyrosine decarboxylase from Streptococcus faecalis. J C Vederas, I D Reingold and H W Sellers J. Biol. Chem. 1979, 254:5053-5057.

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