[Purine nucleoside phosphorylase]

Purine nucleoside phosphorylase (PNP) is one of the most important enzymes of the purine metabolism, wich promotes the recycling of purine bases. Nowadays is the actual to search for effective inhibitors of this enzyme which is necessary for creation T-cell immunodeficient status of the organism in the organs and tissues transplantation, and chemotherapy of a number pathologies as well. For their successful practical application necessary to conduct in-depth and comprehensive study of the enzyme, namely a structure, functions, and an affinity of the reaction mechanism. In the review the contemporary achievements in the study of PNP from various biological objects are presented. New data describing the structure of PNP are summarised and analysed. The physiological role of the enzyme is discussed. The enzyme basic reaction mechanisms and actions are considered. The studies on enzyme physicochemical, kinetic, and catalytic research are presented.

Purine nucleoside phosphorylase: immunodetection and characterization of the human enzyme

An antibody prepared against human placental purine nucleoside phosphorylase (PNP) has been used to characterize the subunit structure of the normal enzyme by two-dimensional gel electrophoresis. The antibody against the placental enzyme reacts with PNP present in human lymphocytes, fibroblasts, erythrocytes and placenta. The enzyme can be detected in intact cells by immunofluorescence with the same antibody preparation. The techniques allow the identification of cross-reacting material in a small number of lymphocytes and could therefore be used to detect abnormal enzyme protein in immunodeficiency.

Purine nucleoside phosphorylase deficiency: a new case report and identification of two novel mutations (Gly156A1a and Val217Ile), only one of which (Gly156A1a) is deleterious

Purine nucleoside phosphorylase (PNP) deficiency results in an autosomal recessive immunodeficiency disease characterized by initial involvement of cellular immunity and neurological manifestations with subsequent abnormalities of humoral immunity. The initial presentation and clinical course has varied widely in the relatively few published cases. The molecular basis has been reported in only 10 patients, precluding evaluation of phenotype-genotype relationships. We now report clinical, immunologic, and molecular findings in a new case of relatively early onset that emphasizes hypotonia and developmental delay as early manifestations. The patient carried two novel missense mutations (Gly56A1a and Val217Ile) on the same allele in apparent homozygosity. Expression of each of the mutant enzymes in vitro demonstrated that the Gly156A1a mutation abolished enzyme activity while the Val217Ile mutation was without obvious effect and is therefore a normal variant. Such "normal" polymorphisms might be associated with a variable response to the immunosuppressive PNP inhibitors currently in clinical trials.

The partial purification and characterization of purine nucleoside phosphorylase from mammalian mitochondria

Cytosolic purine nucleoside phosphorylase (PNPase) is a well known, and described enzyme which exists in a variety of organisms, both procaryotic and eucaryotic. More recently this enzyme was found in bovine liver mitochondria. The mitochondrial purine nucleoside phosphorylase was purified 63 fold and has a molecular weight of 48-60 kD. From Lineweaver-Burk plots apparent KM's of 23 microM for inosine, 42 microM for deoxyinosine, 40 microM for phosphate, 2 microM for hypoxanthine, and 163 microM for ribose-1-phosphate were calculated. Both 8-aminoguanosine (Ki = 0.5 microM) and araG (Ki = 381 microM) are inhibitors of the enzyme. The protein's isoelectric point (pI) was calculated at a pH of 4.2. Preliminary immunological work showed no cross-reactivity between epitopes on the mitochondrial protein and those on PNPase from human erythrocytes. The apparent KM's calculated for the mitochondrial enzyme are, with the exception of that using hypoxanthine, within the range commonly associated with KM's from the cytosolic species. The mitochondrial enzyme's molecular weight and pI are less than normally described. The enzyme's isolation from mitochondria, together with several unique characteristics, suggest that it is a separate protein from that found in the cytosol.

Deficiency of 5'-deoxy-5'-methylthioadenosine phosphorylase activity in malignancy. Absence of the protein in human enzyme-deficient cell lines

The absence of 5'-deoxy-5'-methylthioadenosine phosphorylase (MTAase) activity in malignant cells, and the putative localization of its gene, suggest that this enzyme deficiency might be due to a genomic alteration also involving a tumour-suppressor gene. We studied the possible occurrence of inactive forms of the protein in two MTAase-negative cell lines, namely K562 and Jurkat, by immunochemical methods. Two highly specific antisera, directed against different epitopes of the phosphorylase [Della Ragione, Oliva, Gragnaniello, Russo, Palumbo & Zappia (1990) J. Biol. Chem. 265, 6241-6246], were used to carry out immunotitration and immunoblotting analyses, as well as to investigate the biosynthesis of the enzyme. No MTAase protein was detected by Western-blotting technique performed under conditions where all the phosphorylase-positive samples gave a clear band at the MTAase subunit molecular mass. No cross-reacting material was observed by a sensitive immunotitration method which permitted the detection of as low as 0.5 ng of protein. Moreover, the results obtained by [35S]methionine-labelling experiments ruled out phosphorylase biosynthesis in the negative cell lines. Altogether, these data suggest that an alteration at the gene level hampering the specific mRNA biosynthesis or resulting in an untranslatable mRNA is the cause of the enzyme deficiency in the MTAase-negative cell lines studied.

Use of site-directed mutagenesis to enhance the epitope-shielding effect of covalent modification of proteins with polyethylene glycol

Modification by covalent attachment of polyethylene glycol (PEG) can reduce the immunogenicity and prolong the circulating life of proteins, but the utility of this approach for any protein is restricted by the number and distribution of PEG attachment sites (e.g., epsilon-amino groups of lysine residues). We have developed a strategy for introducing additional sites for PEG attachment by using site-directed mutagenesis to selectively replace arginine with lysine codons and tested it with purine nucleoside phosphorylase (PNP) from Escherichia coli, an extremely stable but immunogenic enzyme, that could potentially be used to treat an inherited deficiency of PNP. A triple mutant, RK3, possessing three Arg----Lys substitutions was constructed that increased the number of lysines per PNP subunit from 14 to 17, providing an additional 18 potential PEG attachment sites per hexameric enzyme molecule. The wild-type and RK3 enzymes had similar catalytic activity, antigenicity, and immunogenicity. After PEG modification, both enzymes retained catalytic activity, the plasma half-life of both enzymes in mice increased from approximately 4 hr to 4 days, and the binding of both enzymes by antisera raised against each unmodified enzyme was markedly diminished. However, antibody raised against wild-type PEG-PNP did not bind the PEG-RK3 enzyme. PEG-RK3 PNP was also substantially less immunogenic than wild-type PEG-PNP. Accelerated antibody-mediated clearance of PEG-PNP occurred in 2 of 12 mice treated with PEG-RK3 PNP, compared with 10 of 16 mice treated with the modified wild-type enzyme. This combined use of directed mutagenesis and PEG modification is aimed at permitting the widest choice of proteins, including products of genetic and chemical "engineering," to be used for therapy of inherited and acquired disorders.

Physicochemical and immunological studies on mammalian 5'-deoxy-5'-methylthioadenosine phosphorylase

5'-Deoxy-5'-methylthioadenosine phosphorylase (MTAase) was purified to homogeneity (10,000-fold) from bovine liver with a recovery of 12%. The pure protein shows a molecular weight of about 98,000 +/- 3,000 and is composed of three apparently identical subunits. Several physicochemical features have been investigated including hydrodynamic properties, amino acid composition, and secondary structure. In particular, the CD spectrum of the protein indicates a very low alpha-helical content and a large percent of beta-structure and random coil. The pure protein was used to raise specific rabbit antisera but, because of the scarce antigenic properties of the native enzyme, different chemically modified forms were prepared and employed as immunogens. Among the antibodies obtained, those to keyhole limpet hemocyanin-MTAase recognize both the native and the denatured enzyme and are also active against the human protein. Therefore, they were employed as a tool to investigate the occurrence of inactive forms of MTAase in two human malignant cell lines lacking this enzymatic activity. The results obtained with K562 and Jurkat cells indicate that the protein is absent in these phosphorylase-deficient cell lines.

Purine nucleoside phosphorylase synthesis and turnover in human lymphoid cells

We have studied the turnover and synthesis of purine nucleoside phosphorylase by using a polyclonal rabbit antiserum to this protein. The turnover of purine nucleoside phosphorylase was studied in the B lymphoblast cell, WI-L2, by specific immunoprecipitation of [3H]leucine-labeled proteins. The half-lives for total protein and purine nucleoside phosphorylase were 14.5 and 14.1 hr, respectively. For cells cultured in the presence of inosine the half-life of purine nucleoside phosphorylase was reduced to 11.2 hr. The synthesis of purine nucleoside phosphorylase was analyzed during phytohemagglutinin-stimulated T cell transformation by pulse labeling cells with [35S]methionine. Purine nucleoside phosphorylase synthesis increased greater than 10-fold during the first 12 hr of transformation and continued to a maximum of 30-fold. The relative rate of purine nucleoside phosphorylase labeled to total proteins was 0.04% in unstimulated T cells and increased to 0.18% 12 hr after stimulation. These studies identify some preferential synthesis of purine nucleoside phosphorylase during the early stages of T cell transformation.

Common variable immunodeficiency and purine nucleotidase and nucleoside phosphorylase deficiency. A case report

The results of immunological and purine enzyme investigations in an adult male patient with common variable immunodeficiency, recurrent lymph node granuloma and splenomegaly are presented. Serum immunoglobulins were present in trace amounts only and a progressive loss of Ig-bearing peripheral lymphocytes were demonstrated. Furthermore, the mitogenic responses to PHA. ConA and PWM were markedly reduced and the ratio of T.m/T.g cells was decreased. Finally, a combined deficiency of lymphocyte purine 5-nucleoside phosphorylase was demonstrated in the patient.

Immunofluorescence: a sensitive and rapid method for the detection of purine nucleoside phosphorylase in single cells

Indirect immunofluorescence was used as a rapid, sensitive and specific method for the visualization of the enzyme purine nucleoside phosphorylase in single cells. The enzyme was localized throughout cytoplasm of human lymphoblasts and fibroblasts but not in cell nuclei. This method is valuable for the detection of mutant enzyme protein in cell-mediated immunodeficiencies caused by purine nucleoside phosphorylase deficiency since it does not rely on enzyme activity. It requires only a limited number of cells and can therefore be used for the rapid screening for the presence of cross-reactive protein in immunodeficiency diseases.

Impaired defense against vaccinia in a child with T-lymphocyte deficiency associated with inosine phosphorylase defect

A case of progressive vaccinia associated with a profound deficiency of cellular immunity and a defect in inosine phosphorylase is described. A striking contrast was observed between humoral immunity, which showed little if any impairment, and a severe cellular defect affecting both markers and functions of T lymphocytes. The child died despite treatment with methosazone, levamisole, transfer factor, irradiated blood transfusions, and a thymus graft. An adequate serum level of antibody to vaccinia virus was obtained by transfer of specific immunoglobulins, but this failed to stop the progression of the disease. This observation suggests that host defense against vaccinia infection is mainly mediated by cellular immunity.