PubMed Health⌕ Search

Biomedical subjects

D R Webster

Publications and source records attributed to D R Webster.

At least 55 records · Page 3Linked to original sources
PubMed1984

Failure of protein loading tests to identify heterozygosity for ornithine carbamoyltransferase deficiency.

Protein loading tests for the diagnosis of heterozygous ornithine carbamoyltransferase deficiency were performed on two occasions on an asymptomatic woman whose daughter and two infant sons died of the disease. Neither loading test produced the expected increases in urinary orotic acid excretion and studies of other pyrimidine and purine metabolites in urine and plasma did not suggest that these would provide better discrimination from non-carriers. The results probably reflect an extensive inactivation of the mutant X chromosome in liver cells and reinforce the need for caution in interpreting negative test results.

Adult↗
PubMed1984

Pregnancy in xanthinuria: demonstration of fetal uric acid production?

This paper reports biochemical studies in a pregnant xanthinuric female (McKusick 27830) and compares the results with findings in three other xanthinurics studied by us, including a previously unreported female who had a nephrectomy for xanthine stones. The findings of raised levels of uric acid in plasma and urine at presentation in the third trimester of pregnancy, and the subsequent fall to almost undetectable levels 6 weeks post-partum, is regarded as evidence of the extent of fetal uric acid production and clearance by the maternal circulation.

Adult↗
PubMed1983

On the metabolism of allopurinol. Formation of allopurinol-1-riboside in purine nucleoside phosphorylase deficiency.

Allopurinol-1-riboside, a major metabolite of allopurinol, is commonly thought to be directly synthesized by purine nucleoside phosphorylase (PNP) in vivo. As this enzyme is otherwise believed to function in vivo primarily in the direction of nucleoside breakdown, we have determined by high performance liquid chromatography and a conventional chromatographic method the urinary metabolites of allopurinol in a child deficient of PNP. In this patient approximately 40% of urinary allopurinol metabolites consisted of allopurinol-1-riboside, thus proving the possibility of indirect formation of allopurinol-1-riboside via allopurinol-1-ribotide in vivo, catalysed by hypoxanthine guanine phosphoribosyltransferase (HGPRT) and a phosphatase.

Allopurinol↗
PubMed1983

Methylthioadenosine phosphorylase activity in human erythrocytes.

An enzyme capable of degrading 5'-methylthioadenosine to adenine was found in the human erythrocyte. A rapid assay for this enzyme, 5'-methylthioadenosine phosphorylase, was developed using high pressure liquid chromatography. The specific activity in 24 normal subjects was 8.9 +/- 2.0 nmol . mg-1 Hb . h-1. Levels within this range were also found in erythrocyte lysates from gouty subjects and patients with a variety of inborn errors of purine metabolism, including patients with a complete deficiency of the adenine salvage enzyme--adenine phosphoribosyltransferase. Erythrocyte lysates from the latter however, were unable to convert the adenine produced to AMP in a linked assay system, in contrast to controls and other patients. These results support the suggestion that adenine, which is excreted in quantity by patients with adenine phosphoribosyltransferase deficiency is derived endogenously from 5'-methylthioadenosine as a by-product of polyamine biosynthesis.

Adenine↗
PubMed1983

Importance of platelet-free preparations for evaluating lymphocyte nucleotide levels in inherited or acquired immunodeficiency syndromes.

Low ATP/ADP ratios have been reported consistently for nucleotide levels of mononuclear cells separated from peripheral blood by conventional techniques. We have established that these low values (mean 2.3:1) were not due to cell damage or poor viability, but resulted from heavy platelet contamination, which is unavoidable when heparinized blood is used. The results reflect the low ATP/ADP ratios (mean 1.6:1) characteristic of platelets. Platelet-free extracts from defibrinated blood had very high ATP/ADP ratios (mean 17.4:1). The initial finding of detectable amounts of deoxy-ATP and deoxy-GTP in mononuclear cells from children with two distinct inherited immunodeficiency disorders [adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) deficiency respectively] many have been due to contamination by nucleated erythrocytes as well as platelets in non-defibrinated preparations. Defibrination before nucleotide extraction of mononuclear cells from a patient with T-cell leukaemic/lymphoma treated with the ADA inhibitor deoxycoformycin enabled the demonstration of grossly raised deoxy-ATP levels relative to deoxy-ADP levels (ratio 16.1:1), associated with severe ATP depletion. This reciprocal relationship between ATP and dATP was found by us previously in the erythrocytes in inherited ADA deficiency. These findings underline the importance of extracts uncontaminated by platelets, or nucleated erythrocytes, in the evaluation of lymphocyte nucleotide levels in inherited or acquired immunodeficiency syndromes.

Adenosine Deaminase↗
PubMed1982

An X-linked syndrome characterised by hyperuricaemia, deafness, and neurodevelopmental abnormalities.

Purine overproduction, with normal levels of the purine salvage enzymes, has been found in a 3-year-old boy and his mother. Both have high-frequency hearing loss from infancy. The child is hypotonic and shows locomotor delay. Two male siblings with similar neurodevelopmental problems died in early childhood. Biochemical studies suggest a combined syndrome associated with a superactive PP-ribose-P synthetase which in its severest form may produce neurodevelopmental complications, as well as uric-acid overproduction. Severe depletion of erythrocyte nicotinamide adenine dinucleotide and guanosine triphosphate appears to be associated with the neurological abnormalities and may be useful in diagnosis.

Adenine Phosphoribosyltransferase↗
PubMed1982

GTP depletion and other erythrocyte abnormalities in inherited PNP deficiency.

GTP levels were low and NAD+ levels high in purine nucleoside phosphorylase (PNP) deficient erythrocytes, in addition to the raised deoxy-GTP (dGTP) levels previously noted by others. dGTP was also identified in the PNP deficient child's lymphocytes. A further novel finding was the conversion of hypoxanthine to inosine by the PNP deficient red cells, as compared to inosine monophosphate (IMP) in controls. This has been attributed to IMP formation with subsequent breakdown, and raises interesting questions regarding the controls which normally maintain erythrocyte nucleotide pools. These findings may also explain the gross purine overproduction seen in this defect; they may likewise be related to the associated immunodeficiency, anaemia, and other clinical manifestations. The results may also have important implications for the development and clinical use of PNP inhibitors.

Adenosine Deaminase↗
PubMed1982

Reciprocal relationship between erythrocyte ATP and deoxy-ATP levels in inherited ADA deficiency.

A reciprocal relationship between erythrocyte ATP and deoxy-ATP levels has been noted in an immunodeficient child with adenosine deaminase (ADA) deficiency during therapy with red cell transfusions. The sum of red cell ATP plus deoxy-ATP equalled the normal complement of ATP prior to any form of therapy. dATP, dADP and dAMP levels were found in the same ratio (10:1:0.1) as the adenine nucleotides ATP, ADP and AMP. Red cell ATP levels were low, not high or normal as found by others in ADA deficiency, but no deoxyadenosine nucleotides could be found in peripheral blood mononuclear cells. Erythrocyte ATP depletion has recently been identified as a serious consequence of anti-leukaemic therapy with ADA inhibitors; it may thus be an important but hitherto unrecognised contributing factor in the clinical expression of inherited ADA deficiency.

Adenosine↗
PubMed1982

Formation and degradation of deoxyadenosine nucleotides in inherited adenosine deaminase deficiency.

dATP, dADP, and dAMP equalled or exceeded the depleted levels of ATP, ADP, and AMP in erythrocytes from two children with adenosine deaminase (ADA; EC 3.5.4.4) deficiency. dATP and dADP were identified in the mononuclear cells of only one child. The levels of deoxyadenosine compounds fell dramatically after enzyme replacement therapy and were no longer detectable in the urine or in mononuclear cells. Erythrocyte adenosine nucleotide levels showed a corresponding increase. Intact erythrocytes prior to treatment contained adenine, presumed to be from deoxyadenosine degraded during extraction. Adenosine at high concentrations in vitro increased both dATP and ATP levels and decreased intracellular deoxyadenosine levels. There was no significant deamination of either [8-14C]adenosine or deoxyadenosine by intact ADA-deficient erythrocytes. About 90% of adenosine was metabolized to ATP at substrate concentrations from 10-100 microM, compared to 40-60% of deoxyadenosine metabolized to dATP. These studies suggest that (i) high intracellular deoxyadenosine levels may be necessary in vivo to sustain the raised dATP levels in ADA deficiency. (ii) When ADA is inhibited or absent, deoxyadenosine is removed rapidly from the circulation by the human erythrocyte utilizing an adenosine transport system linked to both ADA and adenosine kinase (EC 2.7.1.20).

Adenosine↗