Hyperphenylalaninemia and 7-pterin excretion associated with mutations in 4a-hydroxy-tetrahydrobiopterin dehydratase/DCoH: analysis of enzyme activity in intestinal biopsies.

Hyperphenylalaninemia, which can cause neurological disorders and mental retardation, results from a mutation in phenylalanine hydroxylase or an enzyme required for biosynthesis or regeneration of its cofactor, tetrahydrobiopterin. The hyperphenylalaninemia variant primapterinuria is characterized by the excretion of 7-biopterin (primapterin). This disorder is thought to be due to a deficiency of 4a-hydroxy-tetrahydrobiopterin dehydratase (pterin-4a-carbinolamine dehydratase), but a lack of tissue activity has not been directly demonstrated. The five mutations so far recognized in patients with primapterinuria are associated with either a single amino acid change or a premature stop codon. Only C81R has been successfully expressed in soluble form, and was found to have 40% of normal activity. Tissues which could be obtained by minimally invasive procedures were analyzed for dehydratase activity. None was detected in normal human white cells or fibroblasts. However, activity was found in intestine of rat, dog, pig, and particularly humans where it was only eight times lower than in liver. Distribution along the length and across the wall of small intestine was relatively uniform. Moreover, the dehydratases from human liver and intestinal mucosa have identical kinetic properties. A biopsy of duodenal mucosa from a patient with homozygous E96K dehydratase had activity of 55 nmol. min(-1)g(-1) mucosa compared to 329 +/- 32 nmol. min(-1)g(-1) tissue in controls (n = 12). The sixfold lower tissue activity of the E96K mutant alone may not be sufficient to account for the biochemical symptoms of primapterinuria in this patient. However, accumulation of a 4a-hydroxy-tetrahydrobiopterin degradation product (a side-chain cyclic adduct), which has been observed in vitro and appears to be a dehydratase inhibitor, may further exacerbate the problem.

Mutations in the pterin-4alpha-carbinolamine dehydratase (PCBD) gene cause a benign form of hyperphenylalaninemia.

Four patients with primapterinuria, postulated to be due to pterin-4alpha-carbinolamine dehydratase (PCD) deficiency, were diagnosed by biochemical and DNA analysis. All four patients presented in the neonatal period with hyperphenylalaninemia, and elevated neopterin and decreased biopterin levels in the urine. These symptoms are common to 6-pyruvoyltetrahydropterin synthase deficiency and thus there is a danger of misdiagnosis. In addition, all four patients had elevated urinary excretion of primapterin (7-biopterin), the only persistent biochemical abnormality. Analysis of fibroblast DNA from the patients identified the following mutations in the PCBD gene: one patient homozygous for the missense mutation E96K and one homozygous for the nonsense mutation Q97X, both in exon 4; one compound heterozygote with the mutations E96K and Q97X; and one patient with two different homozygous mutations: E26X in exon 2 and R87Q in exon 4. In two families, the parents were investigated and found to be obligate heterozygotes for particular mutations. One sibling was found to be unaffected. These results further substantiate the idea that primapterinuria is associated with mutations in the PCBD gene.

Stereospecificity and catalytic function of histidine residues in 4a-hydroxy-tetrahydropterin dehydratase/DCoH.

Three conserved histidines have been shown to be important for the enzymatic activity of 4a-hydroxy-tetrahydropterin dehydratase, a bifunctional enzyme which is involved in regeneration of tetrahydrobiopterin and is also a cofactor (DCoH) for the transcription factor HNF-1alpha. The 4a isomer dependent kinetics of the mutants of rat/human enzyme, H61A, H62A, and H79A, and the effect of diethylpyrocarbonate (DEPC) have been investigated to elucidate the dehydratase mechanism. At pH 6.5 wild-type enzyme is inactivated by DEPC after derivatization of one histidine, shown to be H61 by comparison to H61A. H79 is also derivatized by DEPC at pH 7.0 and above, whereas H62 does not react at any pH. Dehydratase activity of H61A with 4a(R)-hydroxy-6(S)-methyl-tetrahydropterin was not detectable. In contrast, although Km for the enantiomeric 4a(S)-hydroxy-6(R)-methyl-tetrahydropterin was 65-fold higher than with wild-type, kcat was 86% of wild-type. H79A gave complementary results: activity with 4a(S)-hydroxy-6(R)-methyl-tetrahydropterin was undetectable, but 4a(R)-hydroxy-6(S)-methyl-tetrahydropterin had almost normal Km and 75% of wild-type kcat. Replacing H62 with alanine decreased kcat/Km 80- and 60-fold, and kcat to 24% and 89% of wild-type for the 4a(R),6(S)- and 4a(S),6(R)- isomers, respectively. Near neutral pH nonenzymatic dehydration catalyzed by solvated proton had a rate constant of 1.55 x 10(5) M-1 sec-1. A break in the rate versus pH curve at 5.95 was tentatively assigned to protonation of the carbinolamine guanidinium system. The free acid of acetic acid and the imidazolium ion showed general acid catalysis of 18.5 and 1.5 M-1 sec-1, respectively, in dehydrating the neutral carbinolamine. Compared to the later value, dehydratase effective molarity is 11 M. These results are consistent with a dehydratase mechanism in which H61 and H79 act as general acid catalysts for the stereospecific elimination of the 4a(R)- and 4a(S)-hydroxyl groups, respectively. The role of H62 is primarily binding substrate, with an additional component of base catalysis.

Activity of the bifunctional protein 4a-hydroxy-tetrahydropterin dehydratase/DCoH during human fetal development: correlation with dihydropteridine reductase activity and tetrahydrobiopterin levels.

The dehydratase activity of the bifunctional protein, 4a-hydroxy-tetrahydropterin dehydratase/DCoH was measured in liver during early human fetal development to determine whether it appeared in concert with the other components of the phenylalanine hydroxylating system. Catalytic activity of the dehydratase is detectable as early as 6.7 weeks and increases linearly with time, reaching 31% of the adult value by 17.3 weeks of gestational age. Close correlation was found with the development of dihydropteridine reductase, which increased linearly to 37% of the adult value at 17.3 weeks of gestation. From 8-18 weeks of gestation tetrahydrobiopterin in fetal liver was 0.86 microM (33% of adult value). The ratio of 7-biopterin to 6-biopterin was more than 8-fold higher in fetal than in adult liver during this time. The co-development of 4a-hydroxytetrahydropterin dehydratase with dihydropteridine reductase strongly supports a physiologically significant role for the dehydratase in tetrahydrobiopterin regeneration. In addition, the results have lead to a hypothesis for the transient nature of the hyperphenylalaninemia observed in a variant form of PKU in which levels of 7-biopterin are elevated.

Catalytic characterization of 4a-hydroxytetrahydropterin dehydratase.

The cofactor product of the aromatic amino acid hydroxylases, 4a-hydroxy-6(R)-tetrahydrobiopterin, requires dehydration before tetrahydrobiopterin can be regenerated by dihydropteridine reductase. Carbinolamine dehydration occurs nonenzymatically, but the reaction is also catalyzed by 4a-hydroxytetrahydropterin dehydratase. This enzyme has the identical amino acid sequence to DCoH, the dimerization cofactor of the transcription regulator, HNF-1 alpha. The catalytic activity of rat liver dehydratase was characterized using a new assay employing chemically synthesized 4a-hydroxytetrahydropterins. The enzyme shows little sensitivity to the structure or configuration of the 6-substituent of its substrate, with Km's for 6(S)-methyl, 6(R)-methyl, 6(S)-propyl, and 6(R)-L-erythro-dihydroxypropyl all between 1.5 and 6 microM. Turnover numbers at 37 degrees C are 50-90 s-1 at pH 7.4 and 2.5-3-fold lower at pH 8.4. Both 4a(R)- and 4a(S)-hydroxytetrahydropterins are good substrates. The quinoid dihydropterin products are strong inhibitors of the dehydratase with KI's about one half of their respective Km's, but no inhibition was observed with 7,8-dihydropterins or tetrahydropterins. The enzyme contains no metals and no phosphorus. Reaction mechanisms which involve either acid and/or base catalysis are discussed. 4a-Hydroxy-6(R)-tetrahydrobiopterin was determined not to be a product inhibitor of phenylalanine hydroxylase. It is concluded that the dehydratase (which was found to be 6 microM in rat liver) is essential in vivo to prevent rearrangement of 4a-hydroxy-6(R)-tetrahydrobiopterin and to maintain the supply of tetrahydrobiopterin cofactor for the hydroxylases under conditions where the nonenzymatic rate would be inadequate.

Role of C6 chirality of tetrahydropterin cofactor in catalysis and regulation of tyrosine and phenylalanine hydroxylases.

The chiral specificities of bovine striatal tyrosine hydroxylase (TH) (unphosphorylated and phosphorylated by cAMP-dependent protein kinase) and rat liver phenylalanine hydroxylase (PH) were examined at physiological pH using the pure C6 stereoisomers of 6-methyl- and 6-propyl-5,6,7,8-tetrahydropterin (6-methyl-PH4 and 6-propyl-PH4) and (6R)- and (6S)-tetrahydrobiopterin (BH4). Both PH and phosphorylated TH have substantially higher Vmax values with the unnatural (6R)-propyl-PH4 than the natural (6S)-propyl-PH4 (approximately 6- and 11-fold, respectively). However, the Km's are also higher such that Vmax/Km is almost unaffected by C6 chirality. Unphosphorylated TH has equal Km values for both isomers of 6-propyl-PH4, but has about a 6 times greater Vmax with the unnatural isomer, making it the fastest cofactor yet for this form of the enzyme. With the shorter 6-methyl group, chiral differences are still recognized by phosphorylated TH but hardly at all by PH. Inhibition of both PH and TH by amino acid substrate which occurs with (6R)-BH4 as cofactor is also observed with (6S)-propyl-PH4 but not with (6S)-BH4, (6R)-propyl-PH4, or (6R)- or (6R,S)-methyl-PH4. The Km for (6S)-BH4 with phosphorylated TH is nearly 3 times higher than with (6R)-BH4, but Vmax is unchanged. With unphosphorylated TH, (6S)-BH4 produces very low decelerating rates, which was shown not to be due to irreversible inactivation of the enzyme. The Km for (6R)-BH4 with either hydroxylase is 10 times higher than for the equivalently configured (6S)-propyl-PH4. Comparison of these two cofactors reveals that the 1' and 2' side-chain hydroxyl groups of the natural cofactor promote different regulatory functions in PH than in TH.

Changes in the cofactor binding domain of bovine striatal tyrosine hydroxylase at physiological pH upon cAMP-dependent phosphorylation mapped with tetrahydrobiopterin analogues.

The structure of the cofactor binding domain of tyrosine hydroxylase (TH) was examined at physiological pH by determining kinetic parameters of (R)-tetrahydrobiopterin [(R)-BH4] and a series of tetrahydropterin (PH4) derivatives (6-R1-6-R2-PH4: R1 = H and R2 = methyl, hydroxymethyl, ethyl, methoxymethyl, phenyl, and cyclohexyl; R1 = methyl and R2 = methyl, ethyl, propyl, phenyl, and benzyl). A minimally purified TH preparation that was not specifically phosphorylated (designated as "unphosphorylated") was compared with enzyme phosphorylated with cAMP-dependent protein kinase. The Km for tyrosine with most tetrahydropterin analogues ranged between 20 and 60 microM with little decrease upon phosphorylation. Two exceptions were an unusually low Km of 7 microM with 6-ethyl-PH4 and a high Km of 120 microM with 6-phenyl-6-methyl-PH4, both with phosphorylated TH. Tyrosine substrate inhibition was elicited only with (R)-BH4 and 6-hydroxymethyl-PH4. With unphosphorylated TH (with the exception of 6-benzyl-6-methyl-PH4, Km = 4 mM) an inverse correlation between cofactor Km and side-chain hydrophobicity was observed ranging from a high with (R)-BH4 (5 mM) to a low with 6-cyclohexyl-PH4 (0.3 mM). An 8-fold span of Vmax was seen overall. Phosphorylation caused a 0.6-4-fold increase in Vmax and a 35-2000-fold decrease in Km for cofactor, ranging from a high of 60 microM with 6-methyl-PH4 to a low of 0.6 microM with 6-cyclohexyl-PH4. A correlation of the size of the hydrocarbon component of the side chain with affinity is strongly evident with phosphorylated TH, but in contrast to unphosphorylated enzyme, the hydroxyl groups in hydroxymethyl-PH4 (20 microM) and (R)-BH4 (3 microM) decrease Km in comparison to that of 6-methyl-PH4. Although 6,6-disubstituted analogues were found with affinities near that of (R)-BH4 (e.g., 6-propyl-6-methyl-PH4, 4 microM), they were frequently more loosely associated with phosphorylated TH than their monosubstituted counterparts (6-phenyl-PH4, 0.8 microM; cf. 6-phenyl-6-methyl-PH4, 8 microM). A model of the cofactor side-chain binding domain is proposed in which a limited region of nonpolar protein residue(s) capable of van der Waals contact with the hydrocarbon backbone of the (R)-BH4 dihydroxypropyl group is opposite to a recognition site for hydroxyl(s). Although interaction with either the hydrophilic or hydrophobic regions of unphosphorylated tyrosine hydroxylase is possible, phosphorylation by cAMP-dependent protein kinase appears to optimize the simultaneous operation of both forces.

Pyrimidodiazepine, a ring-strained cofactor for phenylalanine hydroxylase.

Homologues of 6-methyl-7,8-dihydropterin (6-Me-7,8-PH2) and 6-methyl-5,6,7,8-tetrahydropterin (6-Me-PH4), expanded in the pyrazine ring, were synthesized to determine the effect of increased strain on the chemical and enzymatic properties of the pyrimidodiazepine series. 2-Amino-4-keto-6-methyl-7,8-dihydro-3H,9H-pyrimido[4,5-b] [1,4]diazepine (6-Me-7,8-PDH2) was found to be more unstable in neutral solution than 6-Me-7,8-PH2. Its decomposition appears to proceed by hydrolytic ring opening of the 5,6-imine bond, followed by autooxidation. 6-Me-7,8-PDH2 can be reduced, either chemically or by dihydrofolate reductase (Km = 0.16 mM), to the 5,6,7,8-tetrahydro form (6-Me-PDH4). This can be oxidized with halogen to quinoid dihydropyrimidodiazepine (quinoid 6-Me-PDH2), which is a substrate for dihydropteridine reductase (Km = 33 microM). Whereas quinoid 6-methyldihydropterin was found to tautomerize to 6-Me-7,8-PH2 in 95% yield in 0.1 M tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl), pH 7.4, quinoid 6-Me-PDH2 gives only 53% 6-Me-7,8-PDH2, the remainder decomposing via an initial opening of the diazepine ring. Additional evidence for the extra strain in the pyrimidodiazepine system is the cyclization of quinoid 6-N-(2'-aminopropyl)divicine to quinoid 6-Me-PH2 in 57% yield in 0.1 M Tris-HCl, pH 7.4. By comparison, no quinoid 6-Me-PDH2 is formed from the homologue quinoid 6-N-(3'-aminobutyl)divicine. A small (2%) yield of 6-Me-PDH4 is found if the unstable C4a-carbinolamine intermediate is trapped by enzymatic dehydration and reduction. Although phenylalanine hydroxylase utilizes 6-Me-PDH4 (Km = 0.15 mM), the maximum velocity of tyrosine production is 20 times slower than that with 6-Me-PH4, indicating that a ring opening reaction is not a rate-limiting step in the hydroxylase pathway. Further, the maximum velocities of 2,5,6-triamino-4(3H)-pyrimidinone, 2,6-diamino-5-(methylamino)-4(3H)-pyrimidinone, and 2,6-diamino-5-(benzylamino)-4(3H)-pyrimidinone span a 35-fold range. These cofactors would theoretically form the same oxide of quinoid divicine if oxygen activation involves a carbonyl oxide intermediate. Thus, the limiting step is also not transfer of oxygen from this hypothetical intermediate to the phenylalanine substrate.

6,6-Dimethylpterins: stable quinoid dihydropterin substrate for dihydropteridine reductase and tetrahydropterin cofactor for phenylalanine hydroxylase.

The tautomeric structure of the cofactor product of aromatic amino acid hydroxylases, quinoid dihydrobiopterin, is still unknown. Characterization of this molecule, which is also the substrate for dihydropteridine reductase (EC 1.6.99.7), has been hindered by the rapid rearrangement of quinoid dihydropterins to 7,8-dihydropterins. This tautomerization can be prevented by disubstitution at the 6-position. A procedure is presented for the synthesis of 6,6-disubstituted pterins from a vicinal diamine and 2-amino-6-chloro-4(3H)-pyrimidinone. The method is illustrated with the specific synthesis of 6,6-dimethyltetrahydropterin (6,6-Me2PH4). 6,6-Me2PH4 is a cofactor for rat liver phenylalanine hydroxylase (EC 1.14.16.1), with enzyme kinetic parameters similar to those of its positional isomer, 6,7-dimethyltetrahydropterin. The resulting quinoid 6,6-dimethyldihydropterin (q-6,6-Me2PH2) is stable; the half-life in 0.1 M Tris-HCl, pH 7.4, at 27 and 37 degrees C is 4 and 1.25 h, respectively. q-6,6-Me2PH2, produced either by phenylalanine hydroxylase or by chemical oxidation of 6,6-Me2PH4, is a substrate for dihydropteridine reductase, with a Km of 0.4 mM and a maximum velocity double that of the natural isomer of quinoid dihydrobiopterin. In concentrations up to 0.4 mM q-6,6-Me2PH2 is not an inhibitor of phenylalanine hydroxylase, in contrast to 6-methyl-7,8-dihydropterin and 7,8-dihydrobiopterin which inhibit competitively, with Ki's of 0.2 mM and 0.05 mM, respectively. The stability of q-6,6-Me2PH2 has facilitated definitive determination of chemical and physical properties of a quinoid dihydropterin.

Incorporation of molecular oxygen into pyrimidine cofactors by phenylalanine hydroxylase.

The 5-amino substituents of two pyrimidine cofactors of rat liver phenylalanine hydroxylase, 2,5,6-triamino-4-pyrimidinone (TP) and 5-benzylamino-2,6-diamino-4-pyrimidinone (BDP), have been shown to be cleaved quantitatively by enzyme (Bailey, S. W., and Ayling, J. E. (1980) J. Biol. Chem. 255, 7774-7781). That the pyrimidine product of this process (when carried out in the presence of 2-mercaptoethanol) is 2,6-diamino-5-hydroxy-4-pyrimidinone (divicine) is further confirmed by mass spectrometry of an isolated t-butyldimethylsilyl derivative. The origin of the oxygens in this divicine was studied with enzyme reactions containing 18O2. Corrected for the loss in the controls, the divicine generated by phenylalanine hydroxylase from TP and BDP incorporated one atom of 18O with an efficiency of 98 +/- 5% and 100 +/- 3%, respectively, even though these reactions are partially uncoupled. The position of the isotope was unambiguously assigned to the 5-hydroxyl group by the simultaneous use of [18O] TP and 18O2, the divicine from which was found to be doubly labeled. o-Methylphenylalanine stimulates a rate of cofactor oxidation at least 10-fold greater than its own rate of hydroxylation. The majority of divicine isolated from phenylalanine hydroxylase incubations with o-methyl substrate analog was labeled with oxygen from 18O2. The demonstration, with phenylalanine hydroxylase, that one atom of molecular oxygen remains attached to position 5 of pyrimidine cofactor, provides the first strong evidence for activation of oxygen by aromatic amino acid monooxygenases via covalent addition to C4a of tetrahydrobiopterin.

Phenylalanine hydroxylase from human kidney.

In this report the presence, and level, of phenylalanine hydroxylase in the cortex of human kidney is established. The average activity found in 15 surgically removed kidneys was 47.2 plus or minus 11.2 mU/g wet weight of tissue. The average value, determined under the same experimental conditions, for two human liver biopsies was 217 mU/g tissue. Of five autopsy livers obtained 2.5-4 h postmortem, four contained no activity, and only 1-2 percent of normal was found in the fifth. Autopsy kidneys were similarly inactive. The presence of a highly active degradative enzyme could not be demonstrated in autopsy liver homogenates; it was established that the lack of activity was not due to an inhibitory component. A possible interpretation of this phenomenon is discussed. According to work published elsewhere [13] the kidney and liver enzymes appear to be similar. Thus, surgically removed kidneys provide an alternative source of human phenylalanine hydroxylase which can be used to study phenylketonuria.