The tyrosine-dependent oxidation of tetrahydropterins by lysolecithin-activated rat liver phenylalanine hydroxylase

In the presence of tyrosine, phenylalanine hydroxylase, which has been activated with lysolecithin, catalyzes the oxidation of tetrahydrobiopterin at a rate 10-20% that of the parallel reaction with phenylalanine. Unlike the reaction with phenylalanine, there is no net concomitant hydroxylation of tyrosine, although the amino acid is still a necessary component. Tyrosine appears to form an abortive complex with the activated enzyme, the pterin cofactor and molecular oxygen. The Km for tetrahydrobiopterin is identical for the reactions with phenylalanine and tyrosine, whereas the Km for tyrosine is approximately 3 1/2 times greater than the Km for phenylalanine. The tyrosine-dependent oxidation of tetrahydrobiopterin proceeds at both pH 6.8 and 8.2 and shows a similar dependence on the pH as that of the physiological reaction. Tetrahydrobiopterin can be replaced by the artificial cofactor, 6-methyltetrahydropterin, in the tyrosine-dependent oxidation at both pH 6.8 and 8.2. As in the parallel reaction with phenylalanine, both the Km for the cofactor and the Km for the aromatic amino acid increase with this substitution.     

Hydroxylation of 4-methylphenylalanine by rat liver phenylalanine hydroxylase

Rat liver phenylalanine hydroxylase that has been activated with lysolecithin catalyzes the hydroxylation of 4-methylphenylalanine in the presence of a pterin cofactor. Two products, 4-hydroxymethylphenylalanine and 3-methyltyrosine, can be detected. The total amount of amino acids hydroxylated is equal to the amount of tetrahydropterin oxidized. Isotopic labeling studies with 18O2 and H2(18)O show that the hydroxyl groups of both products are derived from molecular oxygen and not from water. Results obtained with 2H-labeled substrates support the conclusion that these products are formed via different mechanistic pathways. Our previous investigations on substrate analogs, as well as the present results, indicate that a highly reactive oxygen-containing intermediate, such as an enzyme-bound iron-oxo compound, must be the hydroxylating species. Our present results could stimulate further discussion of the possibility that the reaction mechanism for the "NIH-shift" of the methyl group may not involve the spontaneous opening of an epoxide intermediate.     

Purification of rat liver phenylalanine hydroxylase by affinity chromatography

Rat liver phenylalanine hydroxylase has been purified to homogeneity on a totally synthesized affinity matrix. The affinity matrix consisted of a succinylated diaminodipropylamine arm linked to Sepharose-4B, to which the cofactor, 6,7-dimethyl-5,6,7,8-tetrahydropterin, was covalently linked. The pure enzyme was eluted with buffered 50% ethylene glycol, 1 m KCl in one step after the 50% ammonium sulfate fraction of the rat liver homogenate was applied to the affinity column. Specific activities ranging from 1.4 to 3.0 units/mg of protein were obtained. The enzyme has been shown to be homogeneous by: (i) discontinuous gel electrophoresis, and (ii) sodium dodecyl sulfate gel electrophoresis. The subunit molecular weight was determined by the same technique and was calculated to be between 51,000 and 55,000.     

Glucocorticoid stimulation of tetrahydrobiopterin levels and phenylalanine hydroxylase activity in rat hepatoma cells

Incubation of H4-II-E-C3 rat hepatoma cells with either hydrocortisone or dexamethasone resulted in 3- to 5-fold increases in the levels of both phenylalanine hydroxylase and its essential cofactor, tetrahydrobiopterin. Maximum elevation of phenylalanine hydroxylase was noted after 24 h of incubation, whereas significant increases in tetrahydrobiopterin were found only after 48 h exposure of the cells to glucocorticoids. Removal of hormone from the culture medium resulted in rapid loss of cell tetrahydrobiopterin, but a much slower decline in the level of phenylalanine hydroxylase. Thus, although the levels of both phenylalanine hydroxylase and tetrahydrobiopterin in rat hepatoma cells are regulated by glucocorticoids, this regulation is apparently not strictly coordinated. Nevertheless, control of cellular tetrahydrobiopterin levels may be an important regulator of hepatic phenylalanine catabolism since significant increases in the ability of intact rat liver cells to hydroxylate phenylalanine were observed only after 48 h exposure to glucocorticoids, in correlation with increases in cell tetrahydrobiopterin content.     

Evidence for phosgene formation during liver microsomal oxidation of chloroform

When aerobically incubated with liver microsomes and NADPH, chloroform produces a stable adduct with cysteineas a nucleophilic trapping agent. The adduct was identified by thin layer chromatography, gas-liquid chromatography and combined gas chromatography-mass spectrometry as the reaction product of cysteine with phosgene.     

In vitro activation of rat liver phenylalanine hydroxylase by phosphorylation.

Essentially pure phenylalanine hydroxylase from rat liver can be activated between 2.5- and 3.0-fold by treatment with Mg2+, ATP, protein kinase, and cyclic AMP. The activation is seen when the hydroxylase is assayed in the presence of tetrahydrobiopterin, but not in the presence of 2-amino-4-hydroxy-6,7-dimethyltetrahydropteridine. In the presence of [gamma-32P]ATP, activation is accompanied by incorporation of 32P into the protein to the extent of 0.7 mol/mol of hydroxylase subunit (Mr = 50,000). Cehmical analysis of the untreated enzyme shows that it already contains about 0.3 mol of Pi/mol of hydroxylase. These results suggest that the activity of the hydroxylase may be regulated by phosphorylation.     

Studies on the primary structure of rat liver phenylalanine hydroxylase

The primary structure of phenylalanine hydroxylase purified from rat liver was investigated with high speed gel filtration chromatography, cyanogen bromide cleavage and end group analyses of polypeptides derived from the enzyme. On gel filtration in the presence of 6M guanidine hydrochloride, the enzyme gave a single peak corresponding to a molecular weight of 52,000. In the same system the enzyme that had been cleaved with cyanogen bromide gave two peptides (CB1, Mr = 32,800 and CB2, Mr = 20,400). Sequence studies showed that the alignment of these two peptides was CB1 - CB2. Furthermore, in experiments using 32P phosphorylated enzyme, the site of phosphorylation by cAMP-dependent protein kinase was found to be located on the CB1 peptide. The NH2-terminus of this enzyme, which was found to be blocked, was shown to be N-acetylalanine. By both carboxypeptidase A digestion and hydrazinolysis, the carboxyl terminus was identified as serine. These data indicate that the phenylalanine hydroxylase molecule from rat liver is composed of subunits which are homogenous or, at least, very similar in their primary structure.     

Pteridine cofactor of phenylalanine hydroxylase in fetal rat liver

1. Pteridine cofactor of phenylalanine hydroxylase (EC 1.14.16.1) and dihydropteridine reductase (EC 1.6.99.7) in the phenylalanine hydroxylating system have been studied in the fetal rat liver. 2. Activities of pteridine cofactor and dihydropteridine reductase were measured as about 6 and 50%, respectively, of the levels of adult liver in the liver from fetuses on 20 days of gestation, at this stage the activity of phenylalanine hydroxylase was almost negligible in the liver. 3. Development of the activity of sepiapterin reductase (EC 1.1.1.153), an enzyme involved in the biosynthesis of pteridine cofactor, was studied in rat liver during fetal (20-22 days of gestation), neonatal and adult stages comparing with the activity of dihydrofolate reductase (EC 1.5.1.3). Activities of the enzymes were about 80 and 50%, respectively, of the adult levels at 20 days of gestation. 4. Some characteristics of sepiapterin reductase and dihydropteridine reductase of fetal liver were reported.     

On the phosphate content of rat liver phenylalanine hydroxylase purified by hydrophobic chromatography

Rat liver phenylalanine hydroxylase purified by hydrophobic chromatography has been found to contain significantly less protein-bound phosphate than enzyme purified by more conventional procedures. Studies with purified hydroxylase of defined phosphate content suggest that phosphorylated species of phenylalanine hydroxylase possess a higher affinity for the hydrophobic matrix than does the non-phosphorylated form. This selectivity may account for the lower phosphate content in phenylalanine hydroxylase purified by hydrophobic chromatography.     

The quantitative determination of phenylalanine hydroxylase in rat tissues. Its developmental formation in liver

A sensitive method was developed for determining the phenylalanine hydroxylase activity of crude tissue preparations in the presence of optimum concentrations of the 6,7-dimethyl-5,6,7,8-tetrahydropterin cofactor (with ascorbate or dithiothreitol to maintain its reduced state) and substrate. Tissue distribution studies showed that, in addition to the liver, the kidney also contains significant phenylalanine hydroxylase activity, one-sixth (in rats) or half (in mice) as much per g as does the liver. The liver and the kidney enzyme have similar kinetic properties; both were located in the soluble phase and were inhibited by the nucleo-mitochondrial fraction. Phenylalanine hydroxylase, like most rat liver enzymes concerned with amino acid catabolism, develops late. On the 20th day of gestation, the liver (and the kidney) is devoid of phenylalanine hydroxylase and at birth contains 20% of the adult activity. During the second postnatal week of development, when the phenylalanine hydroxylase activity was about 40% of the adult value, an injection of cortisol doubled this value. Cortisol had no significant effect on phenylalanine hydroxylase in adult liver or on phenylalanine hydroxylase in kidney at any age.     

Evidence for peroxisomal hydroxylase activity in rat liver

This study presents evidence for the first time that rat liver peroxisomes contain a hydroxylase capable of converting 3 alpha, 7 alpha, 12 alpha,- trihydroxy-5 beta-cholestane to a cholestanetetrol. Furthermore, this hydroxylase differs from both the mitochondrial and microsomal enzymes in its response to various co-factors. Highly purified peroxisomal, mitochondrial, and microsomal fractions from cholestryamine-treated rats were incubated with [22(23)-3H]3 alpha,7 alpha,12 alpha,-trihydroxy-5 beta-cholestane under a variety of conditions. The products were acidified, extracted, and subjected to thin-layer chromatography to determine the amount of cholestanetetrol produced. The identification of the 25- and 26-hydroxylated products from the incubations with the microsomes was confirmed by gas chromatography-mass spectrometry. Peroxisomal fractions incubated with a NADPH-generating system, Mg2+, and ATP showed a rate of 40 pmol/min/mg conversion of 3 alpha,7 alpha,12 alpha,-trihydroxy-5 beta-cholestane to a cholestanetetrol. Co-factor studies indicated that both the peroxisomal and mitochondrial hydroxylase activities were dependent on NADPH, Mg2+, and ATP (with different concentration requirements) whereas the microsomal hydroxylase(s) required only NADPH. An abstract of this work has been published (1).