Phosphoribosylpyrophosphate synthetase in human erythrocytes: assay and kinetic studies using high-performance liquid chromatography

Identification of a new point mutation in hypoxanthine phosphoribosyl transferase responsible for hyperuricemia in a female patient

A 29-year-old woman was referred to our department because of gout. Routine laboratory data showed hyperuricemia, a high level of plasma oxypurines, increased urinary uric acid excretion, and increased urinary oxypurine excretion, with decreased hypoxanthine phosphoribosyl transferase (HPRT) activity in the erythrocytes. From these findings, the patient was diagnosed with a partial deficiency of HPRT. To determine its properties, a cDNA sequence encoding HPRT and the androgen receptor AR XIST minimal promoter gene, as well as methylation of the AR gene were investigated. The HPRT cDNA sequence revealed a point mutation of G to A in nucleotide 40, which changed codon 14 from GAA (Glu) to AAA (Lys) in the mutant gene. In addition, the HPRT genomic DNA sequence, including the mutation site, revealed the same point mutation, indicating that the patient was heterozygote. Further analysis of the AR gene on the X chromosome suggested nonrandom X-chromosome inactivation, whereas the AR XIST minimal promoter gene was normal. Such results have not been previously reported in a female with partial HPRT deficiency.

Red blood cells mediated delivery of 9-(2-phosphonylmethoxyethyl)adenine to primary macrophages: efficiency metabolism and activity against human immunodeficiency virus or herpes simplex virus

Red blood cells (RBC) may act as selective carriers of drugs to macrophages, an important reservoir of viruses such as human immunodeficiency virus (HIV) and herpes simplex virus type 1 (HSV-1). We therefore assessed the incorporation of 9-(2-phosphonylmethoxyethyl)adenine (PMEA), a potent inhibitor of HIV and HSV-1) into RBC, its delivery to macrophages and its activity against HIV or HSV-1. Loading of PMEA in artificially aged opsonized RBC affords significant levels of intracellular PMEA. RBC metabolize PMEA to its active congener PMEA-diphosphate, although with low efficiency. Exposure of macrophages to RBC-encapsulated PMEA inhibits the replication of both HIV and HSV-1 (about 90% inhibition at the highest RBC:macrophages ratios) even if RBC were removed before virus challenge. By contrast, the antiviral activity of free PMEA removed before virus challenge was irrelevant at concentrations up to 150-fold higher than the 50% effective concentration (EC50). Finally, the antiviral effect of RBC-encapsulated PMEA correlates with PMEA levels in macrophages about 500-fold higher than those achieved by free PMEA (at concentrations 10-fold higher than the EC50). The efficacy of RBC-mediated delivery to macrophages of PMEA (and perhaps of compounds with shorter intracellular half-lives) warrants further studies in infectious diseases involving phagocytizing cells as main targets of the pathogen.

High-performance liquid chromatography-based assays of enzyme activities

Interest in using HPLC to assay enzymatic reactions continues to grow as evidenced by the more than 100 papers published during the early 1990s. HPLC can be used for any enzymatic assay that requires separation of substrates and products before quantifying the extent of the reaction. The popularity of HPLC-based assays is due to several reasons: (1) HPLC offers unsurpassed precision, specificity, sensitivity, and reproducibility. (2) Powerful microcomputers and user-friendly software automate the running of samples and collection and processing of data. (3) Current columns, especially C18 packings, separate a very wide variety of samples, and (4) A variety of on-line detectors provide a means to detect virtually any compound. This review surveys recent papers on the development of HPLC-based assays for enzymes that degrade or otherwise modify macromolecules. Methods for assaying enzymes involved in metabolic pathways are also reviewed. Work by the authors in developing HPLC-based assays for mitochondrial enzymes that use GTP/GDP and other nucleotides that cannot be or are not easily assayed by enzyme-coupled assays is discussed. These enzymes include nucleoside diphosphate kinase, succinate thiokinase, and GTP-AMP phosphotransferase. The assays are suitable for determining the submitochondrial compartmentation of enzyme activities. Finally, current and anticipated trends in HPLC technology, including new column packings and the trend toward smaller columns that give faster separations, are reviewed in relation to enzyme assays.

An HPLC-linked assay of phosphoribosylpyrophosphate synthetase activity in the erythrocytes of adults and children with neurological disorders

A two-step non-radioactive method that uses reverse-phase high-performance liquid chromatography (RP-HPLC) is described for the determination of phosphoribosylpyrophosphate synthetase (EC 2.7.6.1) activity in human erythrocytes. The method is accurate and easily reproducible in different chromatographic systems; it is based on the quantification of phosphoribosylpyrophosphate by conversion into orotidine monophosphate and uridine monophosphate. Phosphoribosylpyrophosphate synthetase activity was determined in the erythrocytes of healthy adults and children, the latter showing significantly higher activity than the former. The enzyme activity assayed in children with different neurological disorders was significantly lower in patients with Rett syndrome than in control children or in autistic or mentally retarded patients.

A simplified method for the determination of phosphoribosylpyrophosphate synthetase activity in hemolysates

Methods currently employed for measurement of phosphoribosylpyrophosphate synthetase (ribophosphate pyrophosphokinase; PRPPs) activity are cumbersome and expensive, requiring an auxiliary enzyme reaction, a radioactive nucleobase and multiple incubations, or do not allow a complete kinetic study. The aim of the present study is to describe a simplified, single step, non-isotopic method for determination of PRPPs in hemolysate, appropriate for a complete screening of PRPPs activity disorders. Briefly, the charcoal-treated hemolysate is incubated with saturating amounts of substrates and P1 P5 di(adenosine 5') pentaphosphate (Ap5A), an inhibitor of human adenylate kinase, to prevent conversion of AMP to ADP. AMP generated in this reaction is then measured by HPLC. Adenylate kinase activity was fully inhibited by Ap5A, allowing the accurate determination of AMP. The method was sensitive and reproducible and mean and variance values compared closely with those reported using other, more complicated, assay procedures. The hyperbolic curve relating Pi concentration to initial reaction velocity was shifted to sigmoidal by addition of 0.02 mmol/l GDP which inhibited PRPPs activities only at inorganic phosphate concentrations < 2 mmol/l. This suggests that this method should provide sensitive and accurate screening for regulatory, as well as catalytic, defects underlying PRPPs superactivity.