Biosynthesis and metabolism of pterins in peripheral blood mononuclear cells and leukemia lines of man and mouse

Abstract

The cellular origin and the control of neopterin release associated with immune stimulation was studied in cell cultures. Using purified human mononuclear cells, the intracellular change in concentrations of GTP and pterins was measured under various kinds of stimulation. Three enzymes involved in tetrahydrobiopterin biosynthesis, i.e. GTP cyclohydrolase I, 6-pyruvoyl tetrahydropterin synthase and sepiapterin reductase, were also determined.Human macrophages stimulated with culture supernatant from activated T-lymphocytes were the main producers of neopterin. In these cells, GTP cyclohydrolase I activity was elevated due to high GTP levels and therefore neopterin accumulated. Human macrophages lack 6-pyruvoyl tetrahydropterin synthase activity. Exogenous tetrahydrobiopterin added to the culture medium of stimulated T cells and macrophages suppressed the elevation of GTP cyclohydrolase I activity and neopterin concentration, but not the elevation of intracellular GTP.Stimulation of macrophages with recombinant human interferon-y and neutralization of the effect of T cell supernatants by addition of a monoclonal antibody specific for human interferon-y showed that immune interferon induced the alterations in GTP cyclohydrolase I activity and neopterin concentration. In the human macrophage line U-937 and in the leukemia line HL-60, no GTP cyclohydrolase I activity or intracellular pterins were detected, but high levels of GTP. In mouse mononuclear cells, no neopterin was detected, but biopterin and pterin. After stimulation, biopterin was elevated in the same way as neopterin in human mononuclear cells. This is explained by the different regulation of the rate-limiting steps of tetrahydrobiopterin biosynthesis in man and in mouse. These results suggest that neopterin is an unspecific marker for the activation of the cellular immune system.

The Intracellular Environment of Human Macrophages That Produce Nitric Oxide Promotes Growth of Mycobacteria

“…1A and B), suggesting that available substrate is not a limitation in the production of NO by human macrophages under these conditions. It has been suggested that human macrophages are limited in the ability to generate NO as a consequence of the inability to synthesize BH 4 (38,39). Therefore, BH 4 (10 M) was added to human macrophage cultures.…”

Allosteric characteristics of GTP cyclohydrolase I from Escherichia coli

“…In resting blood cells GTP concentrations are between 100-200 pM/l [25], which is in the range of the K, for GTP cyclohydrolase. An increase in enzymic activity can therefore only be achieved by a drastic increase of the GTP concentration, as it occurs in activated cells [9].…”

“…regulation via substrate pool, e. g, intracellular GTP concentration, has been suggested in the cellular immune system [9]. In Drosophilu, the structural gene for GTP cyclohydrolase I has been cloned and the gene product was found to be essential for vital functions in embryogenesis and for eye pigmentation [Ill.…”

Inhibitory effect of neopterin on NADPH‐dependent superoxide‐generating oxidase of rat peritoneal macrophages

“…As a consequence, these cells accumulate NPH2TP and excrete 7,8-dihydroneopterin (NPH2) and NP after cleavage of the phosphate moiety by phosphatase. It has been generally accepted that NP is released from monocytes/macrophages, that interferon-y (IFN-y), one of the immuno-modulators produced by activated T cells, is the only inducer for NP secretion from macrophages [3,4], and that IFN-)I increases the intracellular concentration of GTP as well as the conversion of it to NP [5]. Although the biological function of NP remains unknown, NP is used as a clinical marker for the diagnosis of patients with various malignant disorders, as well as viral infections including AIDS [6-g].…”