Identification of two molecular-mass forms of phenylalanine hydroxylase that segregate independently in rats. Specific association of each form with certain rat strains

The nature of the different molecular-mass forms of phenylalanine hydroxylase in rat livers was examined by immunoprecipitation of the enzyme from crude liver extracts that had been radiolabelled by reductive methylation. The two forms of the enzyme were resolved by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and detected by fluorography. Segregation of the two forms of the enzyme was demonstrated in Sprague-Dawley rats, as would be expected if the two forms were products of allelic genes. In addition, hooded and albino Wistar rat livers contained only the slower-migrating form and Lewis rat livers contained only the faster-migrating form, and hence we suggest that the forms be referred to as W (for Wistar) and L (for Lewis). Peptide mapping showed that the W and L forms are closely related, and the difference between them appears to reside at one or other end of the polypeptide chain. The kidney contained the same forms as the liver in one-tenth the quantity, providing further evidence that the same phenylalanine hydroxylase gene is expressed in liver and kidney.     

Isolation and characterization of dihydropteridine reductase from human liver.

Dihydropteridine reductase (EC 1.6.99.7) was purified from human liver obtained at autopsy by a three-step chromatographic procedure with the use of (1) a naphthoquinone affinity adsorbent, (2) DEAE-Sephadex and (3) CM-Sephadex. The enzyme was typically purified 1000-fold with a yield of 25%. It gave a single band on non-denaturing and sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, and showed one spot on two-dimensional gel electrophoresis. The molecular weight of the enzyme was determined to be 50000 by sedimentation-equilibrium analysis and 47500 by gel filtration. On sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, a single subunit with mol.wt. 26000 was observed. A complex of dihydropteridine reductase with NADH was observed on gel electrophoresis. The isoelectric point of the enzyme was estimated to be pH 7.0. Amino acid analysis showed a residue composition similar to that seen for the sheep and bovine liver enzymes. The enzyme showed anomalous migration in polyacrylamide-gel electrophoresis. A Ferguson plot indicated that this behaviour is due to a low net charge/size ratio of the enzyme under the electrophoretic conditions used. The kinetic properties of the enzyme with tetrahydrobiopterin, 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetrahydropteridine, NADH and NADPH are compared, and the effects of pH, temperature and a number of different compounds on catalytic activity are presented.     

Allosteric characteristics of GTP cyclohydrolase I from Escherichia coli.

The kinetic and regulatory properties of GTP cyclohydrolase I were investigated using an improved enzyme assay and direct determination of the product, dihydroneopterin triphosphate. The enzyme was purified from Escherichia coli to absolute homogeneity as demonstrated by N-terminal sequencing of up to 50 amino acid residues. A 30-residue internal fragment showed 42% similarity with rat liver GTP cyclohydrolase I. The enzyme did not obey Michaelis-Menten kinetics or show a sigmoid reaction curve. The substrate saturation kinetics were found to be slow with low response to minor changes in GTP concentrations. GTP cyclohydrolase I has a relatively high apparent Km. The values are slightly different for enzyme purified by GTP-agarose (100 microM) and UTP-agarose (110 microM). Low turnover numbers of 12/min and 19/min were calculated for the respective enzyme preparations. GTP-cyclohydrolase-I activity was modulated in Vmax by K+, divalent cations, UTP and tetrahydrobiopterin. Divalent cations, such as Mg2+, had an activating effect with an optimum at 8 mM Mg2+. A different catalytic function and formation of a new, unidentified product by GTP cyclohydrolase I was observed in the presence of Ca2+. In the presence of 1 mM EDTA and Mg2+, GTP-cyclohydrolase-I activity was strongly inhibited by chelate complexes. UTP proved not to be a competitive inhibitor, but a positive modulator. The inhibition by chelate complexes was totally abolished by UTP. Tetrahydrobiopterin showed an inhibitory effect, with 50% inhibition at 100 microM tetrahydrobiopterin. UTP was able to reduce the inhibition by tetrahydrobiopterin. Using monoclonal antibody 1F11 (related to the GTP-binding site), and monoclonal antibody NS7 (mimicking tetrahydrobiopterin), different binding sites were demonstrated for GTP and tetrahydrobiopterin on each enzyme subunit. Western-blot competition analysis revealed a UTP-binding site different from the binding sites of GTP and tetrahydrobiopterin. Based on the kinetic behaviour and the kind of modulations observed we defined GTP cyclohydrolase I as an M-class allosteric enzyme.     

Surface-enhanced resonance raman scattering spectroscopy of bacterial photosynthetic membranes: orientation of the carotenoids of Rhodobacter sphaeroides 2.4.1

Surface-enhanced resonance Raman scattering (SERRS) spectra were obtained from carotenoids, in the all-trans configuration, located on the antenna complexes of Rhodobacter sphaeroides 2.4.1 membranes. Since resonance Raman (RR) spectra are barely detectable at the concentration that SERRS signals saturate, SERRS represents a very sensitive means of detecting pigments in biological systems. Prominent SERRS spectra of sphaeroidenone were detected in chromatophores (cytoplasmic side out) but not in spheroplast-derived vesicles (periplasmic side out), demonstrating that the carotenoid is asymmetrically located on the cytoplasmic side of the cell membrane. Comparison of peak frequencies from SERRS and RR spectral data suggests that the carotenoids are oriented into the membrane with the methoxy end of the isoprenoid chains located closest to the cytoplasmic side of the intracytoplasmic membrane. This work not only shows that SERRS spectroscopy can provide information on the location of a chromophore in a biological membrane but also for the first time demonstrates that SERRS data can be used to ascertain the orientation of a chromophore within the membrane. This observation greatly increases the potential of this technique for structural analysis of intact membranes at the molecular level.     

Structural similarities among enzyme pterin binding sites as demonstrated by a monoclonal anti-idiotypic antibody

BALB/c mice were immunized with a synthetic co-factor of the aromatic amino acid hydroxylases, 6,7-dimethyl-5,6,7,8-tetrahydropterin, conjugated to albumin. Hybridoma cell lines isolated from the immunized mice secreted monoclonal antibodies reacting specifically with the pterin molecule and monoclonal antibodies which were found to bind phenylalanine hydroxylase. Several lines of evidence were consistent with the anti-phenylalanine hydroxylase antibodies being anti-idiotype antibodies mimicking the pterin molecule and binding to the pterin binding site of phenylalanine hydroxylase. (a) An anti-idiotype monoclonal antibody, NS7, when reimmunized into mice produced anti-pterin antibodies consistent with NS7 being an internal image anti-idiotypic antibody. (b) NS7 antibody was prevented from binding to phenylalanine hydroxylase when a competitive inhibitor of phenylalanine hydroxylase enzyme activity, 6,7-dimethyl-7,8-dihydropterin, was bound to phenylalanine hydroxylase. (c) NS7 antibody was shown to bind to a wide range of pterin-requiring enzymes: phenylalanine, tyrosine and tryptophan hydroxylases, dihydropteridine reductase, dihydrofolate reductase, and sepiapterin reductase. Thus the NS7 antibody has successfully mimicked a common portion of the pterin cofactors utilized by these enzymes and demonstrated structure homology in their pterin binding sites despite their diverse function and little amino acid sequence homology except among the three aromatic amino acid hydroxylases.     

Genetics of the mammalian phenylalanine hydroxylase system. II. Immunological and two-dimensional gel electrophoretic studies of phenylalanine hydroxylase in cultured normal and mutant rat hepatoma cells

We have examined 11 previously described cultured rat hepatoma mutants with absent or reduced phenylalanine hydroxylase activity (Choo and Cotton, 1977). Immunological and electrophoretic methods failed to detect any structurally altered protein in these mutants. In nine independently isolated revertants from four different mutants, wild-type protein was regained (or accentuated). This evidence suggests that the mutation involved in these mutants is most likely to be regulatory in nature. These studies have provided three reasons for believing that in cultured rat hepatoma cells one gene codes for a single polypeptide chain, a number of which combine to form the active phenylalanine hydroxylase multimer: (1) Analysis of the purified protein by two-dimensional electrophoresis revealed only a single polypeptide chain. (2) This polypeptide was diminished or undetectable in crude extracts of 11 independently isolated mutants with absent of reduced activity. (3) In none of these 11 mutants was the polypeptide we have designated to be phenylalanine hydroxylase present at normal levels, as would be expected if the mutation were at another locus responsible for a possible second subunit.     

Genetics of the mammalian phenylalanine hydroxylase system. Studies of human liver phenylalanine hydroxylase subunit structure and of mutations in phenylketonuria.

Phenylalanine hydroxylase was purified from crude extracts of human livers which show enzyme activity by usine two different methods: (a) affinity chromatography and (b) immunoprecipitation with an antiserum against highly purified monkey liver phenylalanine hydroxylase. Purified human liver phenylalanine hydroxylase has an estimated mol. wt. of 275 000, and subunit mol. wts. of approx. 50 000 and 49 000. These two molecular-weight forms are designated H and L subunits. On two-dimensional polyacrylamide gel under dissociating conditions, enzyme purified by the two methods revealed at least six subunit species, which were resolved into two size classes. Two of these species have a molecular weight corresponding to that of the H subunit, whereas the other four have a molecular weight corresponding to that of the L subunit. This evidence indicates that active phenylalanine hydroxylase purified from human liver is composed of a mixture of sununits which are different in charge and size. None of the subunit species could be detected in crude extracts of livers from two patients with classical phenylketonuria by either the affinity or the immunoprecipitation method. However, they were present in liver from a patient with malignant hyperphenylalaninaemia with normal activity of dihydropteridine reductase.     

Staphylococcal nuclease reviewed: A prototypic study in contemporary enzymology

Summary This is the third in a series of four articles in which the chemical, enzymological and crystallographic work on Ribonucleate (deoxribonucleate)-3-nucleotidohydrolase, EC 3.1.4.4 (staphylococcal nuclease, micrococcal nuclease) will be reviewed and correlated. This article describes the structure of the nuclease and of a nuclease-inhibitor complex as determined by x-ray crystallography. The crystal structures are correlated with some of the known chemical and enzymological properties of the enzyme, and the three areas combined to propose a mechanism of action.This article is the third in a series of four devoted to the stapholoccal nuclease. Reviews concerning its isolation and enzymology (1) as well as the features of its ligand binding site (2) have appeared in previous issues. Work from this laboratory has been supported by grants from the National Institute of General Medical Sciences, NIH and from the Robert A. Welch Foundation to F. A. Cotton and E. E. Hazen, Jr.     

Haemorrheological response to plasma exchange in Raynaud's syndrome.

Eight patients with Raynaud''s syndrome were treated by weekly plasma exchange for four weeks using a Haemonetics Model 30 Blood Processor. The mean whole-blood viscosity at a shear rate of 0.77/s was significantly lower after treatment, and the mean index of red-cell deformability was significantly improved. In four patients studied serially the mean percentage fall in whole-blood viscosity after a single plasma exchange was 49% at 0.77/s but only 14% at 91/s. All patients noticed symptomatic improvement including healing of ischaemic digital ulcers. In six patients the number of digital arterial segments containing detectable blood flow was measured by directional Doppler; in all six the number increased. It is concluded that plasma exchange is an effective means of haemorrheological treatment and may be beneficial in patients with digital ischaemia.     

Affinity chromatography of phenylalanine hydroxylase. The structure of a pteridine adsorbent

1. Four independent methods have established that the structure of a previously reported pteridine affinity adsorbent, 6,7-dimethyl-5,6,7,8-tetrahydropterin--CH-Sepharose, is 5(CH-Sepharosyl)-6,7-dimethyl-5,6,7,8-tetrahydropterin. 2. A novel reaction, the carbodiimide-promoted coupling of a carboxyl group to N-5 of a tetrahydropterin, is described. 3. Two novel adsorbents, 5-formyl-tetrahydrofolate--AH-Sepharose and 5-methyl-tetrahydrofolate--AH-Sepharose, are described which may be useful not only in the study of phenylalanine hydroxylase but also in the study of folate-metabolizing enzymes.