Incorporation of 18 O into homovanillic acid of rat brain during exposure to oxygen 18 containing atmospheres

Rats were exposed to air containing ¹⁸O₂ at atmospheric pressure. $\text{In vivo}$ incorporation of ¹⁸O in brain homovanillic acid (HVA) was determined by gas chromatography-mass spectrometry. One ¹⁸O atom was incorporated into each molecule of HVA indicating that tyrosine is the predominant precursor of brain dopamine and that the oxygen in the 3-position is of atmospheric origin. Intraperitoneal administration of ¹⁸O-enriched water did not alter the ¹⁸O content of brain HVA Mass fragmentography (2) was used to measure the increase in ¹⁸O and the decrease in ¹⁶O in HVA from rat brain over several hours of exposure to an ¹⁸O enriched atmosphere. These experiments demonstrate the possibility to pulse label brain dopamine and its metabolites by $\text{in vivo}$ inhalation of stable oxygen isotopes. The procedure should be useful for quantitative determinations of the turnover of brain dopamine in animals and man.     

The fate of18O labelled phosphate in soil/plant systems

KH₂PO₄ labelled with¹⁸O and³²P was mixed with soil that was placed in pots in which grass seed was sown. Grass samples were taken after 5, 7, and 12 weeks. The dilution factor (DF) for¹⁸O in the first cut was much greater than the DF for³²P, indicating that the bulk of the¹⁸O in the absorbed phosphate was lost. The DFs for¹⁸O and³²P determined in phosphate extracted from the soil at the end of the pot experiment indicated that half the¹⁸O excess in the applied phosphate was lost.A succeeding experiment showed no loss of¹⁸O when the treated soil was shaken for 3 months with water to which a germicide was added. Thus, the loss of¹⁸O was presumably caused by biochemical processes which brought about the replacement of¹⁸O by¹⁶O. We suggest that the loss of¹⁸O from applied labelled phosphate may be used as a measure of biological activity in soil.     

Chimeric human mitochondrial PheRS exhibits editing activity to discriminate nonprotein amino acids

Mitochondria are considered as the primary source of reactive oxygen species (ROS) in nearly all eukaryotic cells during respiration. The harmful effects of these compounds range from direct neurotoxicity to incorporation into proteins producing aberrant molecules with multiple physiological problems. Phenylalanine exposure to ROS produces multiple oxidized isomers: tyrosine, Levodopa, ortho‐Tyr, meta‐Tyr (m‐Tyr), and so on. Cytosolic phenylalanyl‐tRNA synthetase (PheRS) exerts control over the translation accuracy, hydrolyzing misacylated products, while monomeric mitochondrial PheRS lacks the editing activity. Recently we showed that “teamwork” of cytosolic and mitochondrial PheRSs cannot prevent incorporation of m‐Tyr and l ‐Dopa into proteins. Here, we present human mitochondrial chimeric PheRS with implanted editing module taken from EcPheRS. The monomeric mitochondrial chimera possesses editing activity, while in bacterial and cytosolic PheRSs this type of activity was detected for the (αβ)₂ architecture only. The fusion protein catalyzes aminoacylation of tRNA^Phe with cognate phenylalanine and effectively hydrolyzes the noncognate aminoacyl‐tRNAs: Tyr‐tRNA^Phe and m‐Tyr‐tRNA^Phe.     

Effect of aromatic acids on protein synthesis in subcellular preparations from the rat brain

The incorporation of [³H]phenylalanine, [³H]tyrosine, and [³H]tryptophan into protein and amino acyl–tRNA was studied in cell-free preparations from rat brain. Tyrosine and tryptophan inhibited the incorporation of phenylalanine into protein, and tyrosine inhibited the incorporation of phenylalanine and tryptophan into amino acyl–tRNAs. In most cases, homogentisate, phenylpyruvate, and phenyllactate inhibited the incorporation of phenylalanine, tyrosine, and tryptophan into protein and amino acyl–tRNAs, and the incorporation of phenylalanine into polyphenylalanine. All other protein amino acids, and phenylacetate, salicylate, and benzoate were wholly ineffectual. The results suggest that the formation of amino acyl–tRNAs may have been the step which was affected most by the inhibitors. The incorporation data at different concentrations of the aromatic amino acids were fitted to the simple Michaelis equation. Homogentisate and phenylpyruvate generally tended to reduce both K_m and V in the incorporation of aromatic amino acids into protein and amino acyl-tRNAs, even if V decreased more than K_m.     

INCORPORATION OF PHENYLALANINE, TYROSINE AND TRYPTOPHAN INTO PROTEIN OF HOMOGENATES FROM DEVELOPING RAT BRAIN: KINETICS OF INCORPORATION AND RECIPROCAL INHIBITION

The kinetics of the incorporation into protein of [³H]phenylalanine, [³H]tyrosine and [³H]tryptophan were studied with homogenates prepared from whole brain of 1-, 7-, 21- and 60-day-old rats. The maximal velocities (V_max)of incorporation of phenylalanine and tyrosine decreased and the apparent Michaelis-constants (K_m) for all three amino acids increased with increasing age of the rats. Tyrosine had the smallest and tryptophan the largest K_m values in all age groups. Phenylalanine competitively inhibited the incorporation of tyrosine, but tyrosine inhibited non-competitively the incorporation of phenylalanine. Tryptophan inhibited competitively the incorporation of phenylalanine, but at least partially non-competitively the incorporation of tyrosine. Phenylalanine and tyrosine did not significantly affect the incorporation of tryptophan in homogenates from 60-day-old rats. In 1-day-old rats only a very large excess of phenylalanine or tyrosine inhibited detectably. The K_i for phenylalanine in the incorporation of tyrosine was significantly smaller in 1- than in 60-day-old rats. In every case the inhibition presumably occurred at a single rate-limiting step in the complicated process of incorporation of amino acids into protein.     

Source of oxygen in cytochrome P-450 catalyzed carbinolamine formation

N-Methylcarbazole was incubated in H₂O¹⁸ and under an ¹⁸O atmosphere. N-Hydroxymethylcarbazole, 1-OH- and 3-OH-N-methylcarbazole were isolated by HPLC and analyzed for ¹⁸O content In incubations containing ¹⁸O, all three metabolites showed >95% ¹⁸O incorporation. In incubations containing H₂O¹⁸, the N-hydroxymethyl metabolite showed ¹⁶O incorporation equal to control incubations in 100% H₂O. These data demonstrate that the sole source of oxygen in cytochrome P-450 catalyzed, NADPH supported N-hydroxymethylcarbazole formation is atmospheric oxygen.     

Measurement of protein synthesis in rat lungs perfused in situ

Compartmentalization of amino acid was investigated to define conditions required for accurate measurements of rates of protein synthesis in rat lungs perfused in situ. Lungs were perfused with Krebs–Henseleit bicarbonate buffer containing 4.5% (w/v) bovine serum albumin, 5.6mm-glucose, normal plasma concentrations of 19 amino acids, and 8.6–690μm-[U-¹⁴C]phenylalanine. The perfusate was equilibrated with the same humidified gas mixture used to ventilate the lungs [O₂/CO₂ (19:1) or O₂/N₂/CO₂ (4:15:1)]. [U-¹⁴C]Phenylalanine was shown to be a suitable precursor for studies of protein synthesis in perfused lungs: it entered the tissue rapidly (t_½, 81s) and was not converted to other compounds. As perfusate phenylalanine was decreased below 5 times the normal plasma concentration, the specific radioactivity of the pool of phenylalanine serving as precursor for protein synthesis, and thus [¹⁴C]phenylalanine incorporation into protein, declined. In contrast, incorporation of [¹⁴C]histidine into lung protein was unaffected. At low perfusate phenylalanine concentrations, rates of protein synthesis that were based on the specific radioactivity of phenylalanyl-tRNA were between rates calculated from the specific radioactivity of phenylalanine in the extracellular or intracellular pools. Rates based on the specific radioactivities of these three pools of phenylalanine were the same when extracellular phenylalanine was increased. These observations suggested that: (1) phenylalanine was compartmentalized in lung tissue; (2) neither the extracellular nor the total intracellular pool of phenylalanine served as the sole source of precursor for protein; (3) at low extracellular phenylalanine concentrations, rates of protein synthesis were in error if calculated from the specific radioactivity of the free amino acid; (4) at high extracellular phenylalanine concentrations, the effects of compartmentalization were negligible and protein synthesis could be calculated accurately from the specific radioactivity of the free or tRNA-bound phenylalanine pool.     

18O Labeling of Chlorophyll d in Acaryochloris marina Reveals That Chlorophyll a and Molecular Oxygen Are Precursors

The cyanobacterium Acaryochloris marina was cultured in the presence of either H₂¹⁸O or ¹⁸O₂, and the newly synthesized chlorophylls (Chl a and Chl d) were isolated using high performance liquid chromatography and analyzed by mass spectroscopy. In the presence of H₂¹⁸O, newly synthesized Chl a and d, both incorporated up to four isotopic ¹⁸O atoms. Time course H₂¹⁸O labeling experiments showed incorporation of isotopic ¹⁸O atoms originating from H₂¹⁸O into Chl a, with over 90% of Chl a ¹⁸O-labeled at 48 h. The incorporation of isotopic ¹⁸O atoms into Chl d upon incubation in H₂¹⁸O was slower compared with Chl a with ∼50% ¹⁸O-labeled Chl d at 115 h. The rapid turnover of newly synthesized Chl a suggested that Chl a is the direct biosynthetic precursor of Chl d. In the presence of ¹⁸O₂ gas, one isotopic ¹⁸O atom was incorporated into Chl a with approximately the same kinetic incorporation rate observed in the H₂¹⁸O labeling experiment, reaching over 90% labeling intensity at 48 h. The incorporation of two isotopic ¹⁸O atoms derived from molecular oxygen (¹⁸O₂) was observed in the extracted Chl d, and the percentage of double isotopic ¹⁸O-labeled Chl d increased in parallel with the decrease of non-isotopic-labeled Chl d. This clearly indicated that the oxygen atom in the C3¹-formyl group of Chl d is derived from dioxygen via an oxygenase-type reaction mechanism.     

Expression of pheA gene using PR-PL tandem promoter of bacteriophage lambda

Summary The genepheA ⁺ coding chorismate niutase P-prephenate dehydratase, one of the regulatory enzymes of phenylalanine biosynthesis, was cloned into the down-stream of P_R-P_L tandem promoter. In this construction, both the native promoter-operator region and the attenuator region ofpheA ⁺ operon were eliminated so as to avoid the repression and attenuation ofpheA ⁺ gene expression. The expression ofpheA ⁺ gene was directed by P_R-P_L tandem promoter of bacteriophage lambda and controlled by a temperature sensitive repressor, cI₈₅₇.It was shown that the expression as well as phenylalanine production was regulated by temperature. Maximum production of phenylalanine, 170 mg/l, was obtained at 40°C. The host strain, MC1065, produced a trace (4 mg/l) of phenylalanine at the same temperature.     

Cerebral lipid metabolism in experimental hyperphenylalaninaemia: incorporation of 14C-labelled glucose into total lipids

—1. Effects of the administration of phenylalanine to rats on incorporation in vivo or in vitro of [U-¹⁴C]glucose into cerebral lipids were studied during the first 5–10 days of postnatal development. In addition, the effects of added phenylalanine and its deaminated metabolites on incorporation of [U-¹⁴C]glucose by homogenates into lipids of developing rat brain were investigated. Hyperphenylalaninaemia reduced incorporation both in vivo and in vitro of [U-¹⁴C]glucose into cerebral lipids. 2. Phenylalanine or tyrosine added in vitro at concentrations equivalent to those in the brain of the hyperphenylalaninaemic rat (0-1 μmole/ml incubation medium) did not inhibit incorporation of [U-¹⁴C)glucose into lipids, although at much higher concentrations of phenylalanine (36 μumoles/ml incubation medium) slight inhibition (10 per cent) of incorporation of [U-¹⁴C]glucose into lipids was observed. 3. In contrast, the deaminated metabolites in general exerted greater inhibitory effects at lower concentrations. Phenyllactic acid, in comparison to phenylpyruvic and phenyl-acetic acid, was the most potent inhibitor of the incorporation in vitro of [U-¹⁴C]glucose into cerebral lipids. These results indicated that these metabolites of phenylalanine were the more potent inhibitors of cerebral lipid metabolism in immature animals.     

Contributions of evaporation, isotopic non-steady state transpiration and atmospheric mixing on the delta18O of water vapour in Pacific Northwest coniferous forests

Changes in the ²H and ¹⁸O of atmospheric water vapour provide information for integrating aspects of gas exchange within forest canopies. In this study, we show that diurnal fluctuations in the oxygen isotope ratio (δ¹⁸O) as high as 4‰ were observed for water vapour (δ¹⁸O_vp) above and within an old‐growth coniferous forest in the Pacific Northwest region of the United States. Values of δ¹⁸O_vp decreased in the morning, reached a minimum at midday, and recovered to early‐morning values in the late afternoon, creating a nearly symmetrical diurnal pattern for two consecutive summer days. A mass balance budget was derived and assessed for the ¹⁸O of canopy water vapour over a 2‐d period by considering the ¹⁸O‐isoflux of canopy transpiration, soil evaporation and the air entering the canopy column. The budget was used to address two questions: (1) do δ¹⁸O values of canopy water vapour reflect the biospheric influence, or are such signals swamped by atmospheric mixing? and (2) what mechanisms drive temporal variations of δ¹⁸O_vp? Model calculations show that the entry of air into the canopy column resulted in an isotopically depleted ¹⁸O‐isoflux in the morning of day 1, causing values of δ¹⁸O_vp to decrease. An isotopically enriched ¹⁸O‐isoflux resulting from transpiration then offset this decreased δ¹⁸O_vp later during the day. Contributions of ¹⁸O‐isoflux from soil evaporation were relatively small on day 1 but were more significant on day 2, despite the small H₂¹⁶O fluxes. From measurements of leaf water volume and sapflux, we determined the turnover time of leaf water in the needles of Douglas‐fir trees as ≈ 11 h at midday. Such an extended turnover time suggests that transpiration may not have occurred at the commonly assumed isotopic steady state. We tested a non‐steady state model for predicting δ¹⁸O of leaf water. Our model calculations show that assuming isotopic steady state increased isoflux of transpiration. The impact of this increase on the modelled δ ¹⁸O_vp was clearly detectable, suggesting the importance of considering isotopic non‐steady state of transpiration in studies of forest ¹⁸O water balance.     

Influence of analogue-resistant mutants ofAzotobacter chroococcum solubilizing phosphate on yield and quality of sunflower (Helianthus annuus)

Two phosphate-solubilizing analogue-resistant mutants ofAzotobacter chroococcum showed a positive interactive effect on sunflower (Helianthus annuus). Plant height, leaf area, seed mass, seed yield, protein and oil content increased significantly by inoculation withA. chroococcum mutants. Both levels of fertilizers applied (80N+30P₂O₅ and 80N+60P₂O₅ kg/hm² showed increased seed yield. Likewise, inoculation resulted in a significantly higher yield. Application ofP-solubilizingA. chroococcum strains with the lower fertilizer level showed no significant difference on plant growth parameters when compared with the higher fertilizer level, indicating that inoculation of P-solubilizingAzotobacter strains can reduce the necessary levels of fertilization from 60P₂O₅ to 30P₂O₅ kg/hm².     

Role of the second coordination sphere residue tyrosine 179 in substrate affinity and catalytic activity of phenylalanine hydroxylase

Phenylalanine hydroxylase converts phenylalanine to tyrosine utilizing molecular oxygen and tetrahydropterin as a cofactor, and belongs to the aromatic amino acid hydroxylases family. The catalytic domains of these enzymes are structurally similar. According to recent crystallographic studies, residue Tyr179 in Chromobacterium violaceum phenylalanine hydroxylase is located in the active site and its hydroxyl oxygen is 5.1 Å from the iron, where it has been suggested to play a role in positioning the pterin cofactor. To determine the catalytic role of this residue, the point mutants Y179F and Y179A of phenylalanine hydroxylase were prepared and characterized. Both mutants displayed comparable stability and metal binding to the native enzyme, as determined by their melting temperatures in the presence and absence of iron. The catalytic activity (k_cat) of the Y179F and Y179A proteins was lower than wild-type phenylalanine hydroxylase by an order of magnitude, suggesting that the hydroxyl group of Tyr179 plays a role in the rate-determining step in catalysis. The K_M values for different tetrahydropterin cofactors and phenylalanine were decreased by a factor of 3–4 in the Y179F mutant. However, the K_M values for different pterin cofactors were slightly higher in the Y179A mutant than those measured for the wild-type enzyme, and, more significantly, the K_M value for phenylalanine was increased by 10-fold in the Y179A mutant. By the criterion of k_cat/K_Phe, the Y179F and Y179A mutants display 10% and 1%, respectively, of the activity of wild-type phenylalanine hydroxylase. These results are consistent with Tyr179 having a pronounced role in binding phenylalanine but a secondary effect in the formation of the hydroxylating species. In conjunction with recent crystallographic analyses of a ternary complex of phenylalanine hydroxylase, the reported findings establish that Tyr179 is essential in maintaining the catalytic integrity and phenylalanine binding of the enzyme via indirect interactions with the substrate, phenylalanine. A model that accounts for the role of Tyr179 in binding phenylalanine is proposed.Electronic Supplementary Material Supplementary material is available in the online version of this article at Abbreviations AAAHs aromatic amino acid hydroxylases - BH₂ 7,8-dihydro-l-biopterin - BH₄ (6R)-5,6,7,8-tetrahydro-l-biopterin - CD circular dichroism - cPAH Chromobacterium violaceum phenylalanine hydroxylase - DMPH₄ 6,7-dimethyl-5,6,7,8-tetrahydropterin - DTT dithiothreitol - EDTA ethylenediaminetetraacetic acid - ES-MS electrospray ionization mass spectrometry - hPAH human phenylalanine hydroxylase - ICP-AE inductively coupled plasma atomic emission - 6-MPH₄ 6-methyl-5,6,7,8-tetrahydropterin - PAH phenylalanine hydroxylase - PH₄ tetrahydropterin - PKU phenylketonuria - RDS rate-determining step - TH tyrosine hydroxylase - THA 3-(2-thienyl)-l-alanine - TPH tryptophan hydroxylase - wt wild-type     

Effect of oxygen on ascorbic acid uptake and concentration in embryonic chick brain

The effects of oxygen on ascorbic acid concentration and transport were studied in chick embryo (Gallus gallus domesticus). During normoxic incubations, plasma ascorbic acid concentration peaked on fetal day 12 and then fell, before increasing again on day 20 when pulmonary respiration began. In contrast, cerebral ascorbic acid concentration rose after day 6, was maintained at a relatively high level during days 8–18, and then fell significantly by day 20. Exposure of day 16 embryos for 48 h to 42% ambient O₂ concentration decreased ascorbic acid concentration by four-fifths in plasma and by one-half in brain, compared to values in normoxic (21% O₂) or hypoxic (15% O₂) controls. Hyperoxic preincubation of embryos also inhibited ascorbic acid transport, as evidenced by decreased initial rates of saturable and Na⁺-dependent [¹⁴C]ascorbic acid uptake into isolated brain cells. It may be concluded that changes in ascorbic acid concentration occur in response to oxidative stress, consistent with a role for the vitamin in the detoxification of oxygen radicals in fetal tissues. However, changing O₂ levels have less effect on ascorbic acid concentration in brain than in plasma, indicating regulation of the vitamin by brain cells. Furthermore, the effect of hyperoxia on cerebral vitamin C may result, in part, from inhibition of cellular ascorbic acid transport.     

Self-hydroxylation of taurine/α-ketoglutarate dioxygenase: evidence for more than one oxygen activation mechanism

2-Aminoethanesulfonic acid (taurine)/α-ketoglutarate (αKG) dioxygenase (TauD) is a mononuclear non-heme iron enzyme that catalyzes the hydroxylation of taurine to generate sulfite and aminoacetaldehyde in the presence of O₂, αKG, and Fe(II). Fe(II)TauD complexed with αKG or succinate, the decarboxylated product of αKG, reacts with O₂ in the absence of prime substrate to generate 550- and 720-nm chromophores, respectively, that are interconvertible by the addition or removal of bound bicarbonate and have resonance Raman features characteristic of an Fe(III)–catecholate complex. Mutagenesis studies suggest that both reactions result in the self-hydroxylation of the active-site residue Tyr73, and liquid chromatography nano-spray mass spectrometry/mass spectrometry evidence corroborates this result for the succinate reaction. Furthermore, isotope-labeling resonance Raman studies demonstrate that the oxygen atom incorporated into the tyrosyl residue derives from H₂ ¹⁸O and ¹⁸O₂ for the αKG and succinate reactions, respectively, suggesting distinct mechanistic pathways. Whereas the αKG-dependent hydroxylation likely proceeds via an Fe(IV)=O intermediate that is known to be generated during substrate hydroxylation, we propose Fe(III)–OOH (or Fe(V)=O) as the oxygenating species in the succinate-dependent reaction. These results demonstrate the two oxygenating mechanisms available to enzymes with a 2-His-1-carboxylate triad, depending on whether the electron source donates one or two electrons.     

Biosynthesis in isolatedAcetabularia chloroplasts II. Plastid pigments

Summary The plastid pigments — chlorophylls and carotenoids — of the alga,Acetabularia, have been chromatographically separated and identified. These pigments were found to become radio-active during incubations of an isolated chloroplast fraction with¹⁴CO₂. Specific activity calculations indicate that appreciable amounts of synthesis were occurringin vitro. The phytol and porphyrin moieties of chlorophyll a were both radioactive; thus the pigments were being formed completely from recent photosynthetic products. A comparison of the incorporation of¹⁴CO₂ into plastid pigmentsin vivo andin vitro suggests that the isolated chloroplasts form the pigments at their normalin vivo rates.     

Trichothecene Biosynthesis in Fusarium sporotrichioides: Origin of the Oxygen Atoms of T-2 Toxin

Mass spectral analysis of T-2 toxin formed during the growth of Fusarium sporotrichioides (ATCC 24043) in the presence of H₂¹⁸O showed incorporation of up to three ¹⁸O atoms per toxin molecule. The carbonyl oxygens of the acetates at C-4 and C-15 and of the isovalerate at C-8 were derived from H₂O. Toxin formed in the presence of ¹⁸O molecular oxygen incorporated up to six ¹⁸O atoms per toxin molecule. The overall incorporation was 78 and 92% of toxin molecules labeled for H₂¹⁸O and ¹⁸O₂ labeled samples, respectively. The oxygens of position 1, the 12,13-epoxide, and the hydroxyl groups at C-3, C-4, C-8, and C-15 were all derived from molecular oxygen.     

Enrichment of oxygen heavy isotopes during photosynthesis in phytoplankton

Some of the oxygen produced during oxygenic photosynthesis is consumed but little is known about the extent of the processes involved. We measured the ¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios in O₂ produced by certain marine and freshwater phytoplankton representing important groups of primary producers. When the cells were performing photosynthesis under very low dissolved oxygen concentrations (<3 μM), we observed significant enrichment in both ¹⁸O and ¹⁷O with respect to the substrate water. The difference in δ¹⁸O between O₂ and water was about 4.5, 3, 5.5, and 7‰ in the diatom Phaeodactylum tricornutum, Nannochloropsis sp. (Eustigmatophyceae), the coccolithophore Emiliania huxleyi and the green alga Chlamydomonas reinhardtii, respectively. The difference in δ¹⁷O was about 0.52 that of δ¹⁸O. As explained, the observed enrichments most probably stem from considerable oxygen consumption during photosynthesis even when major O₂-consuming reactions such as photorespiration were minimized. These enrichments increased linearly with rising O₂ levels but with different δ¹⁷O/δ¹⁸O slopes for the various organisms, suggesting engagements of different O₂-consuming reactions with rising O₂ levels. Consumption of O₂ may be important for energy dissipation during photosynthesis. The isotope enrichment observed here, not accounted for in earlier assessments, closes an important gap in our understanding of the difference between the isotopic compositions of atmospheric oxygen and that of seawater, i.e., the Dole effect.     

O-Atom Scrambling in the Aqueous Isomerization of Pernitrous Acid

Raman spectroscopy has been used to determine the isotopic distribution of oxygen during the isomerization of pernitrous acid to nitrate in¹⁸OH₂. Decomposition of unlabeled pernitrous acid in carbon-dioxide-free phosphate-buffered solutions of¹⁸OH₂at pH 6.8 results in the incorporation of 13 ± 1%¹⁸O into the nitrate product with no detectable double-isotopic incorporation and 83% of the product having complete retention of its oxygen atoms. The mechanistic implications of this are contrasted for three frequently considered pathways for peroxide bond cleavage and it is concluded that the results are most consistent with the formation of reactive intermediate which can either rearrange or undergo hydrolytic O-atom exchange with water.     

ELEVATED PLASMA PHENYLALANINE CONCENTRATION AND LYSINE INCORPORATION INTO RIBOSOMAL PROTEIN OF DEVELOPING BRAIN

Hyperphenylalaninemia was induced in 7-day-old rabbits over a 6-hr period by intraperitoneal injection of phenylalanine. l -[U-¹⁴C]Lysine was injected intraperitoneally into these rabbits and into a control group. The rate of incorporation of l -[U-¹⁴C]lysme into brain ribosomal protein was decreased during a 5-hr period in the presence of elevated plasma phenylalanine concentrations. Lysine transport from the peritoneum to the plasma was unaffected by the high plasma phenylalanine concentrations.