Renal balance of pterin cofactors in the rat. A clearance and micropuncture study

Altered tetrahydrobiopterin metabolism in patients with phenylalanine hydroxylase deficiency

The tetrahydrobiopterin (BH4) cofactor is essential for the activity of various enzymes, including phenylalanine (Phe) hydroxylase. In phenylketonuria (PKU) patients, who are chronically exposed to high Phe levels, high urinary excretion of BH4 metabolites neopterin and biopterin is observed. The aim of this longitudinal study was to investigate consistence and variability of the urinary excretion of pterins (neopterin and biopterin) in PKU patients in relation to age and concomitant blood Phe and tyrosine levels. The study was based on the result of 274 pterin examinations (3-13 exams per subject) performed in 47 PKU patients (aged 6 days to 37 years). Multivariate analysis showed that urinary biopterin and neopterin excretion was affected by age and concomitant blood Phe concentration. The influence of blood Phe on both biopterin and neopterin levels was greater in patients younger than 4 months. Later on, interindividual variability was higher than intraindividual variability for both biopterin and neopterin.

CONCLUSION

Common metabolic (blood Phe levels) and individual (age) factors implicated in the assessment of PKU outcome account only marginally and transiently for the variability of neopterin and biopterin excretion in PKU patients. Other unknown homeostatic factors may probably affect the individual response to chronically elevated Phe levels. What is Known: • In PKU patients, a high urinary excretion of biopterin and neopterin is found. • Biopterin and neopterin excretion is influenced by age and phenylalanine levels. W hat is New: • Blood phenylalanine concentration is the major determinant on pterin excretion in PKU patients in the first months of life. • In older PKU patients, the influence of phenylalanine on pterin excretion is less prominent.

Tetrahydrobiopterin and nitric oxide: mechanistic and pharmacological aspects

In previous minireviews in this journal, we discussed work on induction of tetrahydrobiopterin biosynthesis by cytokines and its significance for nitric oxide (NO) production of intact cells as well as functions of H4-biopterin identified at this time for NO synthases (Proc Soc Exp Biol Med 203: 1-12, 1993; Proc Soc Exp Biol Med 219: 171-182, 1998). Meanwhile, the recognition of the importance of tetrahydrobiopterin for NO formation has led to new insights into complex biological processes and revealed possible novel pharmacological strategies to intervene in certain pathological conditions. Recent work could also establish that tetrahydrobiopterin, in addition to its allosteric effects, is redox-active in the NO synthase reaction. In this review, we summarize the current view of how tetrahydrobiopterin functions in the generation of NO and focus on pharmacological aspects of tetrahydrobiopterin availability with emphasis on endothelial function.

Differential effect of xanthopterin and biopterin on cell growth

1. Xanthopterin inhibited proliferation of primary renal proximal tubule cells (RPTC) and LLC-PK1 cells while in a growth phase but when incubated at confluence the cells were relatively insensitive. 2. The growth of malignant human prostate PC-3 cells was also inhibited by xanthopterin in a concentration and time dependent manner. 3. Dunning R3327 AT-3 rat prostate tumor cells which were exposed to xanthopterin in vitro before their in vivo inoculation resulted in smaller tumours while in vivo administration of xanthopterin following implantation also resulted in smaller tumors. 4. Xanthopterin exerts differential effects on cell growth dependent upon the cell origin and their state of proliferation.

Developmental aspects of pteridine metabolism and relationships with phenylalanine metabolism

Large variations of pteridine elimination occur in childhood, due to the ontogenic development of the metabolism of tetrahydrobiopterin. The main feature is the slow maturation of biopterin synthesis whereas neopterin synthesis is high at birth; thus a high neopterin to biopterin ratio (4.4 +/- 2.1) occurs in the neonatal period, a ratio which then decreases to adult values (0.5 +/- 0.2). Comparing pteridine elimination of PKU patients with that of controls of the same age, a high excretion of biopterin and, to a lesser extent, of neopterin is found. In normal subjects, following an oral phenylalanine load, biopterin levels in urine and serum also increase, whereas variations of neopterin concentration are small. In rats, phenylalanine also leads to an increase of serum biopterin whereas liver biopterin decreases. This suggests that the main explanation for the biopterin increase in serum and in urine by phenylalanine is a release of the intracellular biopterin by the aminoacid.

Dependency of proximal tubular fluid transport on the load of glomerular filtrate

In hydropenic rats, the reabsorption of glomerular filtrate by the proximal convoluted tubules was measured before and after reduction of its intratubular flow rate. Three different protocols were used. (1) In 26 tubules (14 rats), nephron glomerular filtration rate (SNGFR) was varied from 37.2 +/- 7.3 to 20.4 +/- 7.1 nl/min by microperfusing their loops of Henle at 0 to 5 nl/min and 40 nl/min, respectively. This 43% reduction of SNGFR was followed by a 36.0 +/- 23.3% reduction of volume reabsorption rate (P less than 0.001). Between both parameters a linear regression line can be calculated, which is given by y = 0.92 chi + 0.0017. (2) In 17 tubules (14 rats), SNGFR was altered again by feedback from 46.0 +/- 9.7 to 28.8 +/- 9.3 nl/min. The volume resorption from the first half of the proximal convoluted tubule was compared with the reabsorption in its late proximal segments, which were microperfused with proximal tubular fluid at a rate of 20 nl/min. The 36.8% reduction of SNGFR was followed by only a 28.2% reduction of volume reabsorption rate in the first half of the tubule. In the microperfused segments, however, reabsorption remained unaltered. (3) In 29 tubules (21 rats), at the midpoint of proximal convolutions, some of the tubule fluid was removed by a suction pump, and volume reabsorption rate in the late segments was compared with that in the early parts of this tubule, when SNGFR remained stable. The reduction of intratubular flow from 27.7 +/- 8.5 to 14.7 +/- 5.8 nl/min, which is 53% of control, were followed by a reduction of volume reabsorption rate in the late segment to 60.6% control. Between both parameters a regression line was calculated, which is given by y = 0.76 chi +/- 0.01. We conclude that the rate of volume reabsorption by the proximal tubule depends on its intratubular load of glomerular filtrate and, further, that this dependency accounts predominantly for the maintenance of glomerular tubular balance under conditions of hydropenia.

Biopterin in human brain and urine from controls and parkinsonian patients: application of a new radioimmunoassay

Total biopterin concentrations in the post-mortem human brain (caudate nucleus) and in the urine of controls and parkinsonian patients were measured by a newly developed radioimmunoassay. There was good correlation between the total biopterin level and tyrosine hydroxylase activity in the human brain. Biopterin concentrations in the caudate nucleus were greatly reduced in parkinsonian patients. In contrast, the reduction of urinary biopterin in parkinsonian patients was slight and not statistically significant, as compared with normal controls.

Altered urinary excretion of pteridines in neoplastic disease. Determination of biopterin, neopterin, xanthopterin, and pterin

Urinary pteridine concentrations in healthy control subjects and patients with cancer and non-malignant diseases were determined by HPLC and TLC after partial purification by ion exchange and Sephadex chromatography. Elevated concentrations of neopterin were found in 70% of the 50 cancer patients investigated. In patients with non-Hodgkin's lymphoma (seven cases) or with liver metastases (12 cases) neopterin concentrations were significantly higher than in control subjects (p < 0.01). Biopterin was less frequently increased (22%). Xanthopterin was generally raised when neopterin and/or biopterin excretion was high. Neopterin/biopterin ratios were higher in some patients with cancer or with severe renal insufficiency than in controls. These findings suggest that alterations in pteridine metabolism are common in malignant disease. The pathogenic, diagnostic and therapeutic significance of these changes remains to be established.

Micropuncture studies on the renal handling of 2,4-diamino-6,7-dimethylpteridine

The excretion of the diuretic substance DADMP (I,4-diamino-6.7-dimethylpteridine) and of DMP (6.7-dimethylpterin) was studied on single nephrons of the rat kidney using microperfusion and microinjection techniques. In the proximal tubule only DADMP was reabsorbed to a significant degree. Fractional reabsorption rate was independent of the load applied and the permeability constant was found to be 2.2 . 10(-4) cm . s-1. Similar results were obtained in nephrons in which the substances, with inulin, were injected from middle proximal tubular puncture sites and recovered in the urine. DMP appeared in the urine quantitatively and simultaneously with the injected inulin. DADMP recovery, however, was only 20--30% of the injected load during the injection period and after 2 h some 70% was recovered from the urine of both kidneys. The reabsorbed fractions were independent of the loads applied, which varied between 2 . 10(-13) mol . min-1 and 10(-9) mol . min-1. A comparison of the microperfusion and the microinfusion data suggests that the reabsorption of DADMP occurs predominantly in the proximal convolution, and it appears that the differences between the renal handling of DMP and DADMP are explicable by their different lipid solubilities.