Purification and characterization of peroxisomal apo and holo alanine:glyoxylate aminotransferase from bird liver.

Alanine:glyoxylate aminotransferase has been reported to be present as the apo enzyme in the peroxisomes and as the holo enzyme in the mitochondria in chick (white leghorn) embryonic liver. However, surprisingly, birds were found to be classified into two groups on the basis of intraperoxisomal forms of liver alanine:glyoxylate aminotransferase. In the peroxisomes, the enzyme was present as the holo form in group 1 (pigeon, sparrow, Java sparrow, Australian budgerigar, canary, goose, and duck), and as the apo form in group 2 (white leghorn, bantam, pheasant, and Japanese mannikin). In the mitochondria, the enzyme was present as the holo form in both groups. The peroxisomal holo enzyme was purified from pigeon liver, and the peroxisomal apo enzyme from chicken (white leghorn) liver. The pigeon holo enzyme was composed of two identical subunits with a molecular weight of about 45,000, whereas the chicken apo enzyme was a single peptide with the same molecular weight as the subunit of the pigeon enzyme. The peroxisomal holo enzyme of pigeon liver was not immunologically cross-reactive with the peroxisomal apo enzyme of chicken liver, the mitochondrial holo enzymes from pigeon and chicken liver, and mammalian alanine:glyoxylate aminotransferases 1 and 2. The mitochondrial holo enzymes from both pigeon and chicken liver had molecular weights of about 200,000 with four identical subunits and were cross-reactive with mammalian alanine:glyoxylate aminotransferase 2 but not with mammalian alanine:glyoxylate aminotransferase 1.     

Purification and partial characterization of chicken liver acetyl coenzyme A: arylamine N-acetyltransferase

Arylamine acetyltransferase (EC 2.3.1.5) was purified 120-fold from chicken liver. The enzyme showed a rise in activity from pH 6.5 to 7.7 followed by a constant activity to about pH 8.6. The relative molecular weight of the enzyme was about 34,000. The apparent Km for acetyl-CoA was 13 microM with 4-nitroaniline as acetyl-acceptor. CoA was a noncompetitive inhibitor relative to acetyl-CoA with apparent Ki value of 110 microM. With 4-methylaniline as substrate, arylamine acetyltransferase activity in pigeon liver was about 8 times greater than in chicken liver, and about 40 times greater than in rabbit.     

Purification and properties of purine nucleoside phosphorylases from bird liver

Chicken and pigeon liver PNPases differ in their isoelectric points (5.40 and 5.15), in their molecular weights (125,000 +/- 5,000; 78,000 +/- 5,000, determined on Sephadex G-200) and in their subunit molecular weight (62,000 +/- 10%; 75,000 +/- 10%, determined by sodium dodecil sulfate-polyacrylamide gel electrophoresis). The related molecular weights show a dimeric structure for the chicken liver enzyme and a monomeric structure for the pigeon liver enzyme. Activation energies are similar but differ in delta H values. Both PNPases are irreversibly inactivated by p-chloromercuribenzoate and 5,5'-dithiobis-(2-nitrobenzoic acid) when incubated with these reagents; inactivation can be reverted totally or partially by dithiothreitol and 2-mercaptoethanol.     

NAD+ kinase--a review

NAD+ kinase catalyzes the only (known) biochemical reaction leading to the production of NADP+ from NAD+. Most evidence indicates it is found in the cytoplasm, but reports of its presence in (other) cell bodies can not be discounted. Viewed as a protein, our knowledge of NADK composition and architecture is rudimentary. Though recognized as a large multimeric protein, no agreement is evident for the molecular weight (Mr = approximately 4-65 X 10(4] of the native protein. Is calmodulin an integral subunit of (some, all) NAD+ kinases (analogous to phosphorylase kinase in skeletal muscle)? Or is it an external modulator? Consensus is evident that a subunit of molecular weight 30-35 X 10(3) is a component of the mammalian and yeast kinase. In one case (rabbit liver) two types of subunits are reported to give rise to oligomers differing in molecular weight and catalytic activities. Viewed as an enzyme it is not known why such a complex aggregate is needed for what might otherwise appear to a routine phosphorylation reaction. Rapid equilibrium random (for pigeon liver and C. utilis preparations) and ping-pong (for A. vinelandii kinase) mechanisms have been proposed for the reaction, with multiple reactant binding sites indicated for the random cases. From the perspective of enzyme modulation, the demonstration that green plant and sea urchin egg kinases are targets for calmodulin regulation by intracellular Ca2+ links NADP+ production in these sources to the multi-level discriminatory control functions inherent to this Ca2+-protein complex. Significant questions arise from the results of various investigators considered in this review. These queries offer fertile ground for the selective design of key experiments directed to a better understanding of NAD+ kinase function and pyridine nucleotide biochemistry.     

Purification and properties of phosphofructokinase 2/fructose 2,6-bisphosphatase from chicken liver and from pigeon muscle

Phosphofructokinase 2 and fructose 2,6-bisphosphatase extracted from either chicken liver or pigeon muscle co-purified up to homogeneity. The two homogeneous proteins were found to be dimers of relative molecular mass (Mr) close to 110,000 with subunits of Mr 54,000 for the chicken liver enzyme and 53,000 for the pigeon muscle enzyme. The latter also contained a minor constituent of Mr 54,000. Incubation of the chicken liver enzyme with the catalytic subunit of cyclic-AMP-dependent protein kinase in the presence of [gamma-32P]ATP resulted in the incorporation of about 0.8 mol phosphate/mol enzyme. Under similar conditions, the pigeon muscle enzyme was phosphorylated to an extent of only 0.05 mol phosphate/mol enzyme and all the incorporated phosphate was found in the minor 54,000-Mr constituent. The maximal activity of the native avian liver phosphofructokinase 2 was little affected by changes of pH between 6 and 10. Its phosphorylation by cyclic-AMP-dependent protein kinase resulted in a more than 90% inactivation at pH values below 7.5 and in no or little change in activity at pH 10. Intermediary values of inactivation were observed at pH values between 8 and 10. Muscle phosphofructokinase 2 had little activity at pH below 7 and was maximally active at pH 10. Its partial phosphorylation resulted in a further 25% decrease of its already low activity measured at pH 7.1 and in a negligible inactivation at pH 8.5. Phosphoenolpyruvate and citrate inhibited phosphofructokinase 2 from both origins non-competitively. The muscle enzyme and the phosphorylated liver enzyme displayed much more affinity for these inhibitors than the native liver enzyme. Fructose 2,6-bisphosphatase from both sources had about the same specific activity but only the chicken liver enzyme was activated about twofold upon incubation with ATP and cyclic-AMP-dependent protein kinase. All enzyme forms were inhibited by fructose 6-phosphate and this inhibition was released by inorganic phosphate and by glycerol 3-phosphate. Both liver and muscle fructose 2,6-bisphosphatases formed a 32P-labeled enzyme intermediate when incubated in the presence of fructose 2,6-[2-32P]bisphosphate.     

Chicken liver amidophosphoribosyltransferase. Ligand-induced alterations in molecular properties.

A homogeneous amidophosphoribosyltransferase (EC 2.4.2.14) preparation, which was sensitive to purine nucleotide inhibitors, was obtained from chicken liver. From the result of sodium dodecyl sulfate polyacrylamide gel electrophoresis, the subunit weight was estimated to be approximately 58 000. In Tris-HCl buffer, the predominant form of the enzyme had an S20,w of 6.5, Strokes radius of 40 A, and estimated molecular weight of 110 000. Incubation with 5-phosphoribosyl 1-pyrophosphate or Pi resulted in an increase in the S20,w to 9.1--9.5, Strokes radius 50 A, and estimated molecular weight to 200 000. Incubation of the large form with AMP led to a decrease in the molecular wight of the enzyme. It is concluded that chicken liver amidophosphoribosyltransferase is an allosteric protein whose activity is regulated by a series of conformational changes induced by a number of ligands.     

Sulfite oxidase from Merluccius productus

Sulfite oxidase (sulfite:oxygen oxidoreductase, EC 1.8.3.1) was purified 482-fold from liver of the Pacific hake Merluccius productus. The molecular weight of the enzyme was found to be 120 000 by gel exclusion chromatography on Sephadex G-100. Electrophoretic analysis on sodium dodecyl sulfate (SDS)-polyacrylamide gel revealed that the enzyme was composed of two subunits whose molecular weight was estimated to be 60 000. The pH optimum of the enzyme was 8.7; Ks for sulfite, 2.5 x 10(-5) M; and that for cytochrome c, 3.6 x 10(-7) M. The enzyme elicited an EPR signal at g = 1.97 characteristic of pentavalent molybdenum. Colorimetric analysis also disclosed that the enzyme contained 2 mol each of heme and molybdenum per mol of protein. This fish liver homogenate in isotonic sucrose solution was fractionated by differential centrifugation into nuclei, mitochondria, microsomes and supernatant (100 000 X g). The major portion of sulfite oxidase activity was found in mitochondria. The sulfite oxidase activity was markedly high in liver and kidney, as compared with that in heart, spleen, muscle, gill and eye.     

Subunits of fatty acid synthetase complexes. Comparative study of enzyme activities and properties of the half-molecular weight nonidentical subunits of fatty acid synthetase complexes obtained from rat, human, and chicken liver and yeast.

Rat, human, and chicken liver and yeast fatty acid synthetase complexes were dissociated into half-molecular weight nonidentical subunits of molecular weight 225,000–250,000 under the same conditions as used previously for the pigeon liver fatty acid synthetase complex [Lornitzo, F. A., Qureshi, A. A., and Porter, J. W. (1975) J. Biol. Chem.250, 4520–4529]. The separation of the half-molecular weight nonidentical subunits I and II of each fatty acid synthetase was then achieved by affinity chromatography on Sepharose ?-aminocaproyl pantetheine. The separations required, as with the pigeon liver fatty acid synthetase, a careful control of temperature, ionic strength, pH, and column flow rate for success, along with the freezing of the enzyme at ?20 °C prior to the dissociation of the complex and the loading of the subunits onto the column. The separated subunit I (reductase) from each fatty acid synthetase contained β-ketoacyl and crotonyl thioester reductases. Subunit II (transacylase) contained acetyl- and malonyl-coenzyme A: pantetheine transacylases. Each subunit of each complex also contained activities for the partial reactions, β-hydroxyacyl thioester dehydrase (crotonase), and palmitoyl-CoA deacylase. The specific activities of a given partial reaction did not vary in most cases more than twofold from one fatty acid synthetase species to another. The rat and human liver fatty acid synthetases required a much higher ionic strength for stability of their complexes and for the reconstitution of their overall synthetase activity from subunits I and II than did the pigeon liver enzyme. On reconstitution by dialysis in high ionic strength potassium phosphate buffer of subunits I and II of each complex, 65–85% of the control fatty acid synthetase activity was recovered. The rat and human liver fatty acid synthetases cross-reacted on immunoprecipitation with antisera. Similarly, chicken and pigeon liver fatty acid synthetases crossreacted with their antisera. There was, however, no cross-reaction between the mammalian and avian liver fatty acid synthetases and the yeast fatty acid synthetase did not cross-react with any of the liver fatty acid synthetase antisera.     

Bovine brain purine-nucleoside phosphorylase purification, characterization, and catalytic mechanism.

Bovine brain purine-nucleoside phosphorylase (purine-nucleoside:orthophosphate ribosyltransferase, EC 2.4.2.1) was purified to homogeneity at a specific activity of 78 mumol min-1 mg of protein-1. A molecular weight of 78 000-80 000 was calculated for the native enzyme by fel filtration on Sephadex. Gel electrophoresis in the presence of sodium dodecyl sulfate indicated subunits of molecular weight of 38 000. Chemical and kinetic studies strongly implicated histidine and cysteine as catalytic groups at the active site of the enzyme. The pKa's determined for ionizable groups at the active site of the free enzyme were 5.8 and 8.2. Enzyme completely inactivated by p-chloromercuribenzoate was partially reactivated enzyme. A strong susceptibility to photooxidation in presence of methylene blue was observed. Photoinactivation was pH dependent, implicating histidine as the susceptible group at the active site. A rapid loss of catalytic activity upon incubation at 55 degrees C suggested heat lability. An activation energy of 9.6 kcal/mol was calculated. The nature of the catalytic mechanism of the enzyme was investigated, and initial velocity studies showed linear converging patterns of double-reciprocal plots of the data, consistent with a sequential catalytic mechanism. The product inhibition pattern was at variance with both the ordered Bi-Bi and random mechanisms. The observed competition between purine and nucleoside, and between inorganic orthophosphate and ribose 1-phosphate for this ordered mechanism, suggest a Theorell-Chance mechanism. Michaelis constants determined for substrates of the enzyme were 4.35 X 10(-5) M for guanosine, 3.00 X 10(-5) M for guanine, and 2.15 X 10(-2) M for inorganic orthophosphate.     

tRNA(adenine-1-)-methyltransferase from Thermus thermophilus HB8

tRNA(adenine-1-)-methyltransferase (EC 2.1.1.36) was isolated from the extreme thermophile Thermus thermophilus strain HB8. The specific activity of the enzyme is about 50 000 and the yield of activity more than 20%. The method of isolation consists of five steps and is valid for isolation of mg quantities of the enzyme. The purified protein preparation is practically homogeneous in SDS-gel electrophoresis, the position of the protein band corresponds to a molecular weight of 25 000. By gel filtration on Sephadex G-100 the molecular weight of the native protein was found to be 70 000. These data allow to suggest a subunit structure of the enzyme. The enzyme is highly thermostable and is most active at 80 degrees C. The only activity of the enzyme is to methylate A58 in the T psi X loop of tRNA.     

Presence of (2'-5')oligoadenylate synthetase in avian erythrocytes

(2'-5')Oligoadenylate synthetase (2-5A synthetase) was found in avian erythrocyte lysates from chicken, goose, and pigeon, with high levels being observed in chicken erythrocytes. No activities, however, were detected in erythrocytes from human, sheep, mouse, turtle, frog, trout, or lamprey. In chicken erythrocyte lysate, about 70% of ATP was converted to 2-5A molecules during a 20-h incubation, in which the tri- and tetra-adenylate were the major products. The tri-, tetra-, penta-, and hepta-adenylate were synthesized sequentially, but the levels of the di-adenylate were low throughout the reaction. 2-5A synthetase was also seen in erythrocytes from specific pathogen-free chickens, suggesting that the enzyme was not produced as a result of microbial infections. 2-5A synthetases from avian erythrocytes of chicken and pigeon were found not only in cytoplasms, but also in nuclei. No enzyme activity, however, was detected in the nuclear fraction of goose erythrocytes. The molecular size of 2-5A synthetase in nuclei from chicken erythrocytes was 45,000-60,000 daltons, while cytoplasms contained an 85,000- to 120,000-dalton enzyme. In addition, the synthetase was present in several types of chicken tissue including liver, intestine, bone marrow, spleen, bursa, pancreas, and thymus, but not in brain, heart, or stomach.     

Carboxylesterases (EC 3.1.1). The molecular sizes of chicken and pig liver carboxylesterases.

The molecular size of pig liver carboxylesterase has been investigated under a variety of conditions of pH and ionic strength. From equilibrium and velocity sedimentation at pH 4.0 and pH 7.5, and from chromatography on Sephadex G-200,we conclude that the monomeric molecular weight is similar to 65,000 daltons and that the enzyme associates to form trimers. Association equilibrium constants for the monomer-trimer system were estimated to be 0.02 1-2 g-2 at pH 4 (concentration-dependent molecular weight data) and 2 times 10-5 1-2g-2 at pH 7.5 (frontal gel chromatographic results). These studies were aided by comparisons of the properties of the pig liver enzyme with those of chicken liver carboxylesterase, which is shown to exhibit the velocity and equilibrium sedimentation characteristics of a homogeneous protein with molecular weight similar to 65,000. Studies of pig and chicken liver carboxylesterases in 6 M guanidinium chloride, 0.1 M in beta-mercaptoethanol, support the proposition that the monomeric species of these enzymes have molecular weights of similar to 65,000. On polyacrylamide gel electrophoresis in SDS, there is no evidence for a major species of molecular weight less than similar to 65,000 for the pig enzyme, but ca. 50 percent of the chicken esterase is dissociated into two species of molecular weight similar to 30,000.     

The thermal depolymerization of porcine submaxillary mucin

The time dependence of the molecular weight, radius of gyration, and hydrodynamic size distribution for porcine submaxillary mucin (PSM) in solution have been studied using static and dynamic light scattering. The weight average molecular weight (Mw) of PSM in 6 M guanidine HCl, pH 7, is initially 3 X 10(6) and decreases with time in three phases: rapidly from 3-2 X 10(6), less rapidly from 2-0.9 X 10(6), and slowly below 0.9 X 10(6). The rates of decrease are much greater at pH 2. The energy of activation associated with each phase is 20 kcal/mol, which is similar to that reported for peptide bond cleavage at an aspartic acid residue. Addition of mercaptoethanol to PSM in 6 M guanidine HCl leads to a rapid decrease in Mw to 0.9 X 10(6), followed by a very slow further decrease. These results suggest that native PSM consists of subunits (Mw = 0.9 X 10(6] that are linked by disulfide bonds to form dimers (Mw = 2 X 10(6] and then higher aggregates. This cross-linking appears to occur at unglycosylated regions of the protein core, which are believed to be richer in aspartic acid than the rest of the molecule.     

Interactions of manganese(II) and small ligands with Busycon canaliculatum hemocyanin

Cyanide (5 X 10(-3) M) and thioacetamide (5 X 10(-3) M) increase the P50 values (P02 required for 50% oxygenation) of hemocyanin by 100%, respectively. Using an ion-exchange method involving 14CN-, we have found that cyanide forms a 1:1 complex with hemocyanin in the concentration range examined: Kf = 2.3 X Mw M-1 at room temperature, where Kf is association constant and Mw is molecular weight of hemocyanin. This strong binding of cyanide to hemocyanin is to be expected from the effect of this ion on the oxygenation of hemocyanin. The effects of manganese(II) ion and fluoride on the oxygenation of hemocyanin are found to be weak. The nmr measurements, however, suggest that manganese(II) ion does have some interactions with the active site of hemocyanin.     

Mitochondrial adenylate kinase from chicken liver. Purification characterization and its cell-free synthesis

Mitochondrial adenylate kinase has been purified 5400-fold from chicken liver extract in an overall yield of 36%. The purified enzyme has a specific activity of 810 U/mg, a molecular weight of 28 000, and the following amino acid composition: 21 aspartic acid or asparagine, 14 threonine, 17 serine, 27 glutamic acid or glutamine, 16 proline, 22 glycine, 22 alanine, 15 valine, 6 methionine, 11 isoleucine, 29 leucine, 5 tyrosine, 7 phenylalanine, 16 lysine, 7 histidine, 19 arginine, 3 half-cystine, and no tryptophan, totalling 257 residues. The purified enzyme has one disulfide bond and one sulfhydryl group. The disulfide bond is related to the active conformation of the enzyme, whereas the sulfhydryl group does not contribute to the enzyme activity. The sulfhydryl group is easily oxidized in the presence of Cu2+ resulting in the formation of dimer with about one half of the specific activity of the monomer. The enzyme is similar to porcine heart mitochondrial adenylate kinase in antigenicity but different from chicken cytosolic adenylate kinase. Mitochondrial adenylate kinase was synthesized in the mRNA-dependent rabbit reticulocyte lysate system programmed with total chicken liver RNA. The mobility in sodium dodecylsulfate gel electrophoresis of the product obtained in vitro was the same as that of the purified mitochondrial adenylate kinase. This evidence indicates that the mitochondrial adenylate kinase is synthesized as a polypeptide with a molecular weight indistinguishable from that of the mature protein.     

Two activated forms of protease-sensitive protein kinase isolated from rat liver plasma membrane

Rat liver plasma membrane contains a protease-activated kinase which corresponds to protein kinase C. When the solubilized enzyme was digested with trypsin in the absence of NaCl, a partially activated form with an approximate molecular weight of 8 X 10(4) was produced. However, another active form of molecular weight of 5 X 10(4) was obtained when the enzyme was digested in the presence of NaCl. The larger molecular weight form was converted to the smaller form by tryptic digestion in the presence of NaCl. These results suggest the existence of two protease-activated forms of protein kinase C.     

delta-Aminolevulinate synthase isozymes in the liver and erythroid cells of chicken

Antibodies raised against the purified chicken liver delta-aminolevulinate synthase showed a partial cross-reactivity with the chicken erythroid delta-aminolevulinate synthase. delta-Aminolevulinate synthase synthesized in vitro using polysomes from erythroid cells showed a subunit molecular weight of 55,000, whereas the enzyme synthesized in vitro using liver polysomes had a subunit molecular weight of 73,000. delta-Aminolevulinate synthase isolated from mitochondria of erythroid cells showed a molecular weight of 53,000, while the enzyme in liver mitochondria had a value of 65,000. These observations imply that the erythroid delta-aminolevulinate synthase differs from the hepatic enzyme.     

Purification and characterisation of leukotriene A4 hydrolase from rat neutrophils

Leukotriene A4 hydrolase was rapidly and extensively purified from rat neutrophils using anion exchange and gel filtration high-pressure liquid chromatography. The enzyme which converts the allylic epoxide leukotriene A4 to the 5,12-dihydroxyeicosatetraenoic acid leukotriene B4 was localized in the cytosolic fraction and exhibited an optimum activity at pH 7.8 and an apparent Km for leukotriene A4 between 2 X 10(-5) and 3 X 10(-5) M. The purified leukotriene A4 hydrolase was shown to have a molecular weight of 68 000 on sodium dodecylsulfate polyacrylamide gel electrophoresis and of 50 000 by gel filtration. The molecular weight and monomeric native form of this enzyme are unique characteristics which distinguish leukotriene A4 hydrolase from previously purified epoxide hydrolases.     

Pigeon liver amidophosphoribosyltransferase. Ligand-induced alterations in molecular and kinetic properties.

Amidophosphoribosyltransferase (EC 2.4.2.14) has been partially purified from pigeon liver, and ligand-induced alterations in molecular and kinetic properties have been studied. In Tris-HCl buffer the predominant form of the enzyme has an s20,w of 5.9 +/- 0.7, Stokes radium of 42 A, and estimated molecular weight of 102,000. Incubation with phosphoribosylpyrophosphate (PP-ribose-P) results in an increase in the s20,w to 7.9 +/- 0.6, Stokes radius to 53 A, and estimated molecular weight to 172,000. Incubation of this larger form with purine ribonucleotides leads to a decrease in the molecular weight of amidophosphoribosyltransferase that is proportional to the concentration of purine ribonucleotide. Purine ribonucleotides produce sigmoidal kinetics with respect to the substrate PP-ribose-P, with Hill coefficients of 1.4 to 1.6 and 1.8 to 2.0 in the presence of AMP and GMP, respectively. Incubation with 0.6 M KCl leads to sigmoidal kinetics. Hill coefficient of 1.8 and dissociation of the larger form of amidophosphoribosyltransferase. Inorganic phosphate has complex effects upon the enzyme. In 25 mM potassium phosphate buffer the enzyme aggregates to a large form with an s20,w of 8.3 +/- 0.2, Stokes radius of 53 A, and estimated molecular weight of 181,000. Inorganic phosphate and PP-ribose-P both stabilize the enzyme to storage in vitro at 4 degrees. However, inorganic phosphate is 4 times more effective than PP-ribose-P in preventing inactivation of the enzyme by sodium dodecyl sulfate. Inorganic phosphate produces sigmoidal kinetics with respect to PP-ribose-P, Hill coefficient of 1.5. The interaction coefficients for AMP and GMP are reduced from 1.8 to 1.2 and 2.2 to 1.4, respectively, in the presence of 25 mM potassium phosphate. It is concluded that pigeon liver amidophosphoribosyltransferase is a complex allosteric protein whose activity is regulated by a series of conformational changes induced by a number of ligands.     

Partial purification of 6-(D-erythro-1',2',3'-trihydroxypropyl)-7,8-dihydropterin triphosphate synthetase from chicken liver.

An enzyme that catalyzes the formation of 6-(D-erythro-1',2',3'-trihydroxypropyl)-7,8-dihydropterin triphosphate (D-erythrodihydroneopterin triphosphate) and formic acid from GTP has been purified about 3700-fold from homogenates of chicken liver. The molecular weight of the enzyme, D-erythrodihydroneopterin triphosphate synthetase (GTP cyclohydrolase), has been estimated to be 125,000 by gel filtration on Ultrogel AcA-34. The enzyme functions optimally between pH 8.0 and 9.2 and is considerably heat-stable. No cofactors or metal ions have been demonstrated to be required for activity; however, the reaction is strongly inhibited by Cu2+ and Hg2+. GTP is the most efficient substrate, with GDP being 1/17 as active and guanosine, GMP, and ATP being inactive. The Km for GTP has been found to be 14 micrometer. Although the overall reaction catalyzed by D-erythrodihydroneopterin triphosphate synthetase from chicken liver is identical with that from Escherichia coli GTP cyclohydrolase, immunological studies show no apparent homology between the two enzymes.