Development of phenylalanine hydroxylase activity in guinea pig liver

Effects of dietary glycerol on the expression of pterin carbinolamine dehydratase in the rat

Earlier studies have shown that the abundance of hepatic phenyl-alanine hydroxylase (PAH) diminishes to 60% of control values in rats fed with a diet composed of 40% (w/w) glycerol [Guerin, Walsh, Donlon and Kaufman (1998) Int. J. Biochem. Cell Biol. 30, 1047-1054]. In this experimental model, there are corresponding decreases in the hepatic concentrations of both the hydroxylase cofactor, tetrahydrobiopterin, and the nucleotide guanosine triphosphate. We now show that the cytoplasmic activities of hepatic pterin-4a-carbinolamine dehydratase (PCD) are also lower in these animals, by approx. 50% compared with control values. Immunoblotting confirmed a diminution of protein abundance in vivo. PCD also functions as a dimerization cofactor (DCoH) for the hepatocyte nuclear factor 1alpha (HNF1alpha) and the relative abundance of PCD/DCoH in the nucleus is also decreased. There is a small reduction in the mRNA levels for PAH and for PCD/DCoH in the glycerol-fed animals. In the kidney, there is also a diminution in the abundance of both PAH and PCD proteins. Hepatic GTP cyclohydrolase I activity was not altered and the abundance of hepatic HNF1alpha remained unchanged. HNF1alpha is required for the expression of PAH in the liver and our results support a role for PCD/DCoH, through its interaction with HNF1alpha, in regulating the expression of PAH.

The kidney is an important site for in vivo phenylalanine-to-tyrosine conversion in adult humans: A metabolic role of the kidney

Synthesis of Tyr in the human body occurs by hydroxylation of the indispensable amino acid Phe. Until now, it was believed that in humans, this process was restricted to the liver, but we provide compelling evidence of production of Tyr from Phe in the kidney. To determine whether the human kidney produces Tyr, we measured Tyr balance, the Tyr appearance rate, and the Phe-to-Tyr conversion in 12 healthy human subjects by using [(15)N]Phe and [(2)H(4)]Tyr as tracers. Renal plasma flow was measured by using paraaminohippurate, and sampling from the femoral artery and renal veins was performed. The results were compared with those obtained in 12 control subjects undergoing hepatic vein catheterization and infusion of identical tracers. In all 12 subjects, there was a net uptake of Phe by the kidney (2.2 +/- 1.2 micromol/min), whereas Tyr was released (5.3 +/- 1.5 micromol/min). In contrast, there was a net uptake of both Phe (9.5 +/- 1.2 micromol/min) and Tyr (14.3 +/- 1.3 micromol/min) by the splanchnic bed. Phe conversion to Tyr occurred at a rate of 5.2 +/- 1.2 micromol/min in kidney and 3.0 +/- 0.7 micromol/min in the splanchnic bed. The kidney contributed a substantial amount of Tyr to the systemic circulation where the splanchnic bed was a net remover of Tyr. Our results demonstrate that the kidney is the major donor of Tyr to the systemic circulation by its conversion of Phe to Tyr. This observation may have important clinical implications for patients with both renal and hepatic disease, who may be at risk of Phe overloading and Tyr deficiency, and it should be considered when parenteral or enteral nutrients are administered rich in Phe and low in Tyr.

Developmental pattern of phenylalanine hydroxylase activity in the chicken

Experiments were conducted to determine the conditions for assay of hepatic phenylalanine hydroxylase (PAH) activity in the chicken and to determine the developmental pattern of PAH activity in liver 25,000 x g supernatant. PAH activity was detected in liver supernatant and (postnuclear) 25,000 x g particulate fraction. Optimum assay conditions differed for the two cell fractions, the most notable difference being a broad pH optimum of 7.7 to 9.2 for the supernatant and 4.7 and 5.6 for the particulate fraction. The PAH activity in the supernatant increased to a maximum as L-phenylalanine concentration in the assay medium increased from 0.02 to 0.5 mM and 1.0 mM. Activity increased in the particulate fraction as the Phe concentration increased to 0.5 mM. Substrate inhibition of PAH activity occurred at Phe concentrations of 3 to 5 mM in the supernatant but not in the particulate fraction. Concentrations of the cofactor, 6(R)-5,6,7,8-tetrahydrobiopterin, ranging from 0.09 to 0.75 mM, resulted in maximal PAH activity. The developmental pattern of PAH in supernatant was determined using a modified assay in which substrate and cofactor concentrations and pH were optimum. The PAH activity in liver supernatant was present at a low level in 11 d chick embryos and increased several fold between Days 15 and 17 to a maximum at Days 17 to 21. Activity declined at hatching to levels that were present in 11 to 15 d embryos and remained at this level in male chicks through 4 wk of age. Mature males had higher PAH activity than mature laying females.

Correlation of rat hepatic phenylalanine hydroxylase, with tetrahydrobiopterin and GTP concentrations

Hepatic phenylalanine hydroxylase is reported to be more abundant in experimentally-diabetic rats; whereas livers of animals fed a high protein diet, where gluconeogenesis also prevails, have normal amounts of this enzyme. In this study, in addition to seeking an explanation for this effect of experimental diabetes, we also examined the effects of providing alternative dietary gluconeogenic substrates. In rats fed a diet composed of 40% (w/w) glycerol, the specific activities of hepatic phenylalanine hydroxylase are decreased to about 60% of control values. There is no effect on the apparent state of phosphorylation of the enzyme. However, studies on the incorporation of radiolabelled leucine into liver phenylalanine hydroxylase suggested that there was a decreased rate of synthesis. Similarly, animals fed a diet containing 85% (w/w) fructose also have diminished phenylalanine hydroxylase activities. Under all of the above circumstances and also in streptozotocin-induced diabetic animals, alterations in the concentrations of the hydroxylase cofactor, tetrahydrobiopterin and of GTP closely correlate with the effects on the enzyme activities. They are elevated in livers of diabetic animals and significantly diminished in livers of rats fed diets rich in glycerol or fructose. These observations suggest that in adult rat both liver tetrahydrobiopterin concentrations and the expression of hepatic phenylalanine hydroxylase are regulated by GTP [210].

The role of reversible phosphorylation in the hormonal control of phenylalanine hydroxylase in isolated rat proximal kidney tubules

Reversible phosphorylation is the major mechanism underlying the short-term hormonal control of phenylalanine hydroxylase activity in the liver. We report here, for the first time, the impact of a range of hormonal effectors on both the phosphorylation state and enzymic activity of phenylalanine hydroxylase present in isolated rat proximal kidney tubules. The most potent stimulator of enzyme phosphorylation was found to be parathyroid hormone, which is known to stimulate the production of cyclic AMP in proximal-tubule cells. In addition, adrenergic amines also stimulated enzyme phosphorylation, although to a lesser extent, through interaction with a mixed alpha 1 and beta receptor population.

Characterization of phenylalanine hydroxylase from rat kidney

1. Phenylalanine hydroxylase has been purified from rat kidney using an immunoaffinity procedure. 2. SDS-PAGE and immunoblot analysis of the purified enzyme revealed subtle differences in the size and abundance of subunits for the enzyme purified from the kidney compared with enzyme purified from the liver. These differences may be explained on the basis of limited proteolysis of the enzyme, during purification from the kidney. 3. The purified renal enzyme is, like the hepatic enzyme, a target for cyclic AMP-dependent protein kinase action. 4. The extent of phosphorylation of the renal enzyme is stimulated by incubation of isolated kidney tubules in the presence of either dibutyryl-cyclic AMP or the protein phosphatase inhibitor, okadaic acid.

Phenylalanine hydroxylation in isolated rat kidney tubules

1. Phenylalanine hydroxylation has been demonstrated to occur in isolated rat kidney tubules under physiological conditions. 2. The hydroxylation flux response is hyperbolic with apparent Km and Vmax values of ca 85 microM phenylalanine and 49 nmol tyrosine formed/mg dry wt per hr respectively. 3. Hydroxylation in kidney tubules is substantially less sensitive to effectors of cyclic AMP turnover and Ca2+ mobilization than phenylalanine hydroxylation in isolated liver cells.

The effect of streptozotocin-induced diabetes on phenylalanine hydroxylase expression in rat liver

The impact of experimentally induced diabetes on the expression of rat liver phenylalanine hydroxylase has been investigated. A significant elevation in maximal enzymic activity was observed in diabetes. This was associated with significant increases in the amount of enzyme, the phenylalanine hydroxylase-specific translational activity of hepatic RNA and the abundance of phenylalanine hydroxylase-specific mRNA. These changes in phenylalanine hydroxylase expression were not observed when diabetes was controlled by daily injections of insulin. These results are discussed in relation to the hormonal control of phenylalanine hydroxylase gene expression.

Effects of phenylalanine loading on protein synthesis in the fetal heart and brain of rat: an experimental approach to maternal phenylketonuria

Pregnant rats were loaded with L-phenylalanine, and the distributions of [14C]leucine and [14C]urea into fetal plasma and tissues were examined. Uptake of [14C]leucine into the supernatant and protein fractions of fetal plasma and tissues was low in the rats loaded with phenylalanine. In contrast, [14C]urea was distributed identically in both groups, indicating that maternal hyperphenylalaninemia did not affect blood flow across the placenta. Administration of phenylalanine and p-chlorophenylalanine produced amino acid imbalance in fetal tissues. Along with these changes, polysomes of the affected fetal heart and brain disaggregated without changes in the ribonuclease activity. These results indicate that high phenylalanine levels in maternal plasma disturb the active transport of amino acids across the placenta, causing an amino acid imbalance and disaggregation of polysomes in fetal heart and brain. These changes may contribute to the congenital heart disease and mental retardation of maternal phenylketonuria.

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.

Distribution of phenylalanine hydroxylase (EC 1.14.3.1) in liver and kidney of vertebrates

The range of phenylalanine hydroxylase activity was determined by measuring the conversion of radioactive phenylalanine to tyrosine in liver and kidney of various vertebrates. Rodents (rats, mouse, gerbil, hamster and guinea pig) were found to have the highest liver phenylalanine hydroxylase activity among all animals studied. They are also the only species that possessed a significant kidney phenylalanine hydroxylase activity which was about 25% of that found in the liver of the same animal. The synthetic dimethyl-tetrahydro-pteridine, used as a cofactor for the enzyme assay in most studies, catalyzed non-enzymatic hydroxylation of phenylalanine to tyrosine. Inclusion of boiled-blank and strict control of timing between incubation and product measurement were essential precautions to minimize erroneous results from substrate contamination and non-enzymatic hydroxylation.

beta-2-Thienyl-DL-alanine as an inhibitor of phenylalanine hydroxylase and phenylalanine intestinal transport

The inhibitory properties of beta-2-thienyl-dl-alanine on rat phenylalanine hydroxylase from crude liver and kidney homogenates were assessed in vitro and in vivo, as well as its effects on the intestinal transport of phenylalanine, by using a perfusion procedure in vivo. The apparent K(m) for liver phenylalanine hydroxylase changed from 0.61mm in the absence of the inhibitor to 2.70mm in the presence of 24mm-beta-2-thienyl-dl-alanine, with no significant change in the V(max.). For kidney the corresponding values were 0.50 and 1.60mm respectively. A single dose of beta-2-thienyl-dl-alanine (2mmol/kg) failed to inhibit phenylalanine hydroxylase in either organ. Repeated injections during a 4-day period caused a decline of the enzymic activity to about 40% of controls. Intestinal absorption of phenylalanine when perfused at 0.2-2.0mm concentration was also competitively inhibited by beta-2-thienyl-dl-alanine. Its K(i) value was estimated at 81mm. The limited inhibitory effects of beta-2-thienyl-dl-alanine towards hepatic phenylalanine hydroxylase and phenylalanine intestinal transport, and its rapid metabolism, as suggested by the small elimination of this compound in the urine and its virtual absence from animal tissues, are factors that restrict its potential usefulness as an inducer of phenylketonuria in rats or as an effective blocker of phenylalanine absorption by the gut.

Net placental transfer of free amino acids against varying concentrations

1. The patterns of the free plasma amino acids in the pregnant guinea-pig and her foetuses, near term, are described. The concentration of each amino acid was higher in the foetal plasma than in the maternal. The foetal:maternal gradients (F:M) varied for each amino acid; the straight chain amino acids had the highest F:M ratios.2. Net transfer of endogenous plasma amino acids, from the maternal circulation across the placental membrane, was studied. The foetus was removed and the foetal placenta perfused in situ via the umbilical arteries, with an artificial fluid containing varying concentrations of amino acids.3. All the amino acids, both essential and non-essential, could be transferred from the maternal to the foetal circulation against the F:M gradients. With ;closed circuit' perfusion, this transport increased the concentration of total amino N in the perfusate until it was twice that of the normal F:M gradient of 5. The concentrations of the individual amino acids was increased to 1.7-4.2 times those normally present in foetal plasma, and the final values reached were similar to the concentrations of free amino acid found in placental tissue.4. The umbilical vein-artery differences were small, with the placenta perfused ;open circuit' in the steady state, using physiological flow rates and amino acid concentrations. The average net placental transfer of amino N found was 1.14 m-mole min(-1). This is about 60% of the calculated net rate of accumulation of N by the 60 g guinea-pig foetus.5. The influence of foetal placental perfusion concentration on transfer was small but significant. In the steady state, the transfer of amino N, and each individual amino acid, was found to be inversely proportional to the concentrations in the perfusate when the placenta was perfused ;open circuit'. The slopes of the regression of transfer on concentration had an average value of 0.13 n-mole min(-1) g(-1) per mumole. No significant difference in the slopes was found between the three amino acid transport groups.6. Net transfer was independent of perfusate flow, within the physiological range, which suggests a secretory process across the membrane from maternal to foetal circulation.