Current Drug Metabolism - Volume 3, Issue 2, 2002

Tetrahydrobiopterin plays an essential role in nitric oxide synthase catalysis, not only as an allosteric modulator but also as a cofactor involved in electron flow through the enzyme. In absence of tetrahydrobiopterin, all isoforms of nitric oxide synthases are virtually inactive. The present review focusses on attempts to inhibit nitric oxide synthase by biopterin analogues, and what is known about the pharmacological effects of these compounds. While several biopterin analogues are capable of inhibiting nitric oxide synthases, the 4-amino analogue of tetrahydrobiopterin (4-amino tetrahydrobiopterin) is the compound of which pharmacological actions in animals have been described. 4-Amino tetrahydrobiopterin inhibits all three isoforms of nitric oxide synthases in micromolar concentrations. In cultured cells and in aortic strips, a surprising selectivity of inhibition of the inducible isoform of nitric oxide synthases has been observed. When applied intramuscularly, 4-amino tetrahydrobiopterin spreads throughout the body within minutes, and is cleared with a half life of about an hour. A single bolus dose applied intravenously in a rat model of septic shock, saved the animals from the lethal effects of endotoxin. When applied three times a day intramuscularly in a murine model of cardiac allograft rejection, 4-amino tetrahydrobiopterin prolongs allograft survival as efficiently as high-dose cyclosporin A treatment does. Thus, 4-amino tetrahydrobiopterin is an effective immunosuppressant. The mechansim of its action is currently under investigation.

Tetrahydrobiopterin (BH4) deficiencies are disorders affecting phenylalanine metabolism in liver and neurotransmitters biosynthesis in brain. BH4 is the essential cofactor in the enzymatic hydroxylation of 3 aromatic amino acids (phenylalanine, tyrosine, and tryptophan). BH4 is synthesized from guanosine triphosphate (GTP) catalyzed by GTP cyclohydrolase I (GTPCH), 6-pyruvoyl-tetrahydropterin synthase, and sepiapterin reductase (SPR), and in aromatic amino acids hydoxylating system is regenerated by pterin-4a-carbinolamine dehydratase (PCD) and dihydropteridine reductase (DHPR). To date, 4 enzyme deficiencies (GTPCH, PTPS, DHPR, PCD) have been reported and they all follow an autosomal recessive mode of inheritance. The incidence of BH4 deficiency is at 1 in 1,000,000, except that in Taiwanese (much higher than in Japanese and Caucasians). BH4 deficiency has been diagnosed in patients with hyperphenylalaninemia (HPA) by neonatal mass-screening based on BH4 oral loading tests, analysis of urinary or serum pteridines, and measurement of dihydropterindine reductase (DHPR) activity in blood from a Guthrie card. BH4 deficiency without treatment causes combined symptoms of HPA and neurotransmitter (dopamine, norepinephrine, epinephrine, and serotonin) deficiency, such as red hair, psychomotor retardation, and progressive neurological deterioration. Treatment of BH4 deficiencies consists of BH4 supplementation (2-20 mg / kg per day) or diet to control blood phenylalanine concentration and replacement therapy with neurotransmitters precausers (L-dopa / CarbiDOPA and 5-hydroxytryptophan), and supplements of folinic acid in DHPR deficiency.

Ever since the discovery that (6R)-5,6,7,8-tetrahydro-L-biopterin (BH4) is a cofactor of NOS, its function has been the object of intense research and occasional controversy. Only in the last couple of years a consensus has been reached on what constitutes the main role of BH4 in NO synthesis. In this review we aim to provide an outline of the various ways in which BH4 affects NOS catalysis. First we give a brief general description of the structure and catalytic mechanism of NOS, with special emphasis on those aspects of catalysis that are actively debated, and that directly or indirectly involve BH4. Foremost among those issues is uncoupled catalysis, i.e. the NOS-catalyzed oxidation of NADPH in the absence of substrate or pterin that does not result in NO production. We also shortly discuss the ongoing debate on whether NO is the actual reaction product of NOS catalysis, as well as the phenomenon of NO-mediated autoinhibition. We describe the function of BH4 in aromatic amino acid hydroxylation, and discuss the allosteric and structural effects that BH4 exerts on NOS. Next we turn our attention to what is now becoming accepted as the central function of BH4: its capacity to act as a 1-electron donor during reductive activation of the oxyferrous complex of the heme. Finally, we illustrate how BH4 might transform the NOS dimer into an efficient S-nitrosoglutathione synthase, and briefly touch on some more speculative aspects of the role of BH4 in NO synthesis.

Tetrahydrobiopterin (H4-biopterin) is an essential cofactor of a set of enzymes that are of central metabolic importance, i.e. the hydroxylases of the three aromatic amino acids phenylalanine, tyrosine, and tryptophan, of ether lipid oxidase, and of the three nitric oxide synthase (NOS) isoenzymes. As a consequence, H4-biopterin plays a key role in a vast number of biological processes and pathological states associated with neurotransmitter formation, vasorelaxation, and immune response. In mammals, its biosynthesis is controlled by hormones, cytokines and certain immune stimuli. This review aims to summarize recent developments concerning regulation of H4-biopterin biosynthetic and regulatory enzymes and pharmacological effects of H4-biopterin in various conditions, e.g. endothelial dysfunction or apoptosis of neuronal cells. Also, approaches towards gene therapy of diseases like the different forms of phenylketonuria or of Parkinson's disease are reviewed. Additional emphasis is given to H4-biopterin biosynthesis and function in non-mammalian species such as fruit fly, zebra fish, fungi, slime molds, the bacterium Nocardia as well as to the parasitic protozoan genus of Leishmania that is not capable of pteridine biosynthesis but has evolved a sophisticated salvage network for scavenging various pteridine compounds, notably folate and biopterin.

Increased amounts of neopterin are produced by human monocytes / macrophages upon stimulation with the cytokine interferon-γ. Therefore, measurement of neopterin concentrations in body fluids like serum, cerebrospinal fluid or urine provides information about activation of T helper cell 1 derived cellular immune activation. Increased neopterin production is found in infections by viruses including human immunodeficiency virus (HIV), infections by intracellular living bacteria and parasites, autoimmune diseases, malignant tumor diseases and in allograft rejection episodes. But also in neurological and in cardiovascular diseases cellular immune activation indicated by increased neopterin production, is found. Major diagnostic applications of neopterin measurements are, e.g. monitoring of allograft recipients to recognize immunological complications early. Neopterin production provides prognostic information in patients with malignant tumor diseases and in HIV-infected individuals, high levels being associated with poorer survival expectations. Neopterin measurements are also useful to monitor therapy in patients with autoimmune disorders and in individuals with HIV infection. Screening of neopterin concentrations in blood donations allows to detect acute infections in a non-specific way and improves safety of blood transfusions.As high neopterin production is associated with increased production of reactive oxygen species and with low serum concentrations of antioxidants like α-tocopherol, neopterin can also be regarded as a marker of reactive oxygen species formed by the activated cellular immune system. Therefore, by neopterin measurements not only the extent of cellular immune activation but also the extent of oxidative stress can be estimated.

Transplantation has become an established and successful therapy. Rejections and infections are the principal immune-related complications in the post-transplant course. A reliable and early diagnosis is necessary to prevent graft failure and patient morbidity. Despite the immunologic nature of these complications the diagnostic procedures still rely on functional tests and organ biopsies. Non-invasive monitoring remains to be one of the major goals in transplant medicine.Neopterin is a sensitive marker of the cellular immune response. It reflects the activation of macrophages and can be easily measured in serum, plasma, urine or other body fluids. This review summarises studies on the diagnostic value of neopterin in transplant medicine. Based on these results key factors for immune monitoring in regard to neopterin are evaluated. In particular the unspecificity of diagnostic immune markers, the kinetics of the immune response, the importance of adjustment of neopterin to kidney function, and quantitative differences in immune pathways against viruses and allografts are discussed. Reflecting these points a concept to use neopterin for non-invasive immune monitoring in the clinical routine is presented. This approach calculates probabilities for specific post-transplant complications based on daily measurements of neopterin, a combination with the acute phase reactant amyloid A, and a modification of the likelihood ratio concept.

Pteridine derivatives which have a widespread occurrence in nature have been investigated upon their interactions with free radicals and free radical mediated reactions utilizing a number of different experimental systems. Searching for biological functions, which are still unknown for the majority of pteridine compounds, the effect of pteridines in systems like luminol-induced chemiluminescence, enzyme activity, DNA photodamage, EPR experiments or radical induced injury - just to name a few - have been investigated. The general view during the initial phase of investigations on this special field was, that reduced pterins, i. e., tetra- as well as dihydropterins, generally act as radical scavengers, while aromatic pterins, if not inactive, exert radical promoting activity. Meanwhile the data available provide a more complex view: pteridines of all oxidation states have been shown to act anti- or prooxidatively, depending on the special conditions of the experiment. The reason is that reduced pterins, besides of being scavengers of free radicals, also are strongly reducing agents and therefore, in the presence of transition metal ions promote Fenton chemistry. Aromatic pterins have been described as inhibitors or substrates of enzymes involved - in vitro and in vivo - in free radical generation. Together with the unknown local concentrations of, e.g., neopterin and dihydroneopterin occurring in vivo, these reasons make it impossible to unequivocally predict a physiological net effect of pterins of different oxidation states concerning free radical mediated reactions in real biological systems.

Folates are important cofactors in the transfer and utilization of one-carbon-groups and play a key role in the remethylation of methionine thus providing essential methyl groups for numerous biological reactions. Furthermore, folates donate one-carbon units in the process of DNA-biosynthesis with implications for the regulation of gene expression, transcription, chromatine structure, genomic repair and genomic stability. As the role of folate deficiency in atherosclerotic cardiovascular disease, neurological and neuropsychiatric disorders, in congenital defects and carcinogenesis has become better understood, folate has been recognized as having great potential to prevent these many disorders through folate supplementation for the general population. Folate acts directly to produce antioxidant effects, interactions with enzyme endothelial NO synthase (eNOS) and effects on cofactor bioavailability of NO. Folate acts indirectly to lower homocysteine levels and insure optimal functioning of the methylation cycle. Folate metabolism provides an interesting example of gene-environmental interaction. A great part of the population, especially subgroups with higher demand, appears to have suboptimal folate intake, as determined through more sensitive parameters now widely determined. The available data strongly suggest that criteria for ”folate deficiency“ may have to be redefined.

Tetrahydrofolate is an essential cofactor for the conversion of homocysteine to methionine, and hyperhomocysteinemia is considered as a risk factor for cardiovascular and cerebrovascular diseases. In subjects with hyperhomocysteinemia usually an inverse relationship exists to folic acid levels, and supplementation with folic acid is able to lower homocysteine concentrations. The pathogenesis of most if not all diseases which are accompanied with moderate hyperhomocysteinemia involves cellular immune activation and therefore in patients very often exists also a positive correlation between homocysteine concentrations and the degree of immune activation which is indicated, e.g. by increased neopterin concentrations. Since neopterin concentrations also serves as an estimate of oxidative stress merging from immune system activation, this association suggests that cellular immune activation and oxidative stress could be involved in the development of hyperhomocysteinemia. Because tetrahydrofolate is very susceptible to oxidation, an increased oxidative degradation of tetrahydrofolates may become relevant under oxidative stress conditions. In this way folate deficiency may develop despite normal dietary intake of the vitamin. In our patients, hyperhomocysteinemia is considered as an indirect consequence of hyperconsumption of antioxidant vitamins during prolonged states of immune activation.