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Abstract: Levels of Nitric Oxide Synthase Product Citrulline are Elevated in Chronic Fatigue Syndrome Patients

  [ 27 votes ]   [ Discuss This Article ] • October 9, 2002

J of Chronic Fatigue Syndrome, Vol. 10(3/4) 2002, pp. 37-41  Martin L. Pall, PhD

Affiliation: School of Molecular Biosciences, Washington State University, Pullman, WA 99164-4660 USA (E-mail: ).

This research was supported by the Air Force Office for Scientific Research (AFOSR) and its support is gratefully acknowledged. The author also thanks Scott Rigden, MD, Albert G. Corrado, MD and Nancy Gregory, ARNP, CDE for their invaluable assistance.


Serum levels of citrulline, a product of nitric oxide synthase activity, were measured in 36 CFS patients and 16 controls to determine whether synthase activity may be elevated in CFS patients. Serum citrulline levels were found to be significantly elevated in CFS patients and, in addition, there was a trend towards higher levels in CFS patients with stronger symptoms. These results provide support for the view that nitric oxide synthase activity tends to be elevated in CFS patients, thus supporting a prediction of the elevated nitric oxide/peroxynitrite theory of CFS etiology.

KEYWORDS. Clinical marker, peroxynitrite, vicious cycle mechanism, NOS, fibromyalgia, oxidative stress, reactive nitrogen species, amino acid pools, etiologic mechanism


Chronic fatigue syndrome (CFS) is reported to impact a large number of different biochemical, physiological and immunological properties in patients, ranging from mitochondrial/energy metabolism to circulatory function to neuroendocrine function to natural killer cell function (discussed in 1,2). Similarly a wide range of symptoms are commonly reported (3). One of the central questions facing CFS researchers is how these diverse characteristics may be impacted by a single etiologic mechanism. A wide-ranging theory of CFS is the elevated nitric oxide/peroxynitrite theory, which is supported by 12 different biochemical/physiological observations about CFS (1,2,4) and explains many of the diverse symptoms in this condition through known biochemical and physiological mechanisms (2). The same basic etiology may also explain the properties of three conditions that overlap with CFS: multiple chemical sensitivity, fibromyalgia and posttraumatic stress disorder (5,6). According to this theory, CFS is initiated by stresses such as infection that lead to elevated levels of nitric oxide and its oxidant product, peroxynitrite. Peroxynitrite acts through several positive feedback loops to elevate the levels of both of its precursors, nitric oxide and superoxide, which will then react with each other to form more peroxynitrite. In this way, a biochemical vicious cycle is initiated and maintained with that vicious cycle being responsible for the chronic nature of CFS.

According to this view, infection acts by inducing inflammatory cytokines which induce, in turn, the inducible nitric oxide synthase (iNOS). However, the chronic phase of CFS may be maintained through the action of both iNOS and the other nitric oxide synthase isozymes, nNOS and eNOS (1). A number of studies have reported that during the chronic phase of CFS, both inflammatory cytokines that induce the iNOS gene are elevated and that the marker for iNOS activity, neopterin levels, are also elevated in CFS (references in 1,2,4). However, because both iNOS and the other NOS isozymes are proposed to be involved in the chronic phase of CFS, it is important to measure a marker of overall nitric oxide synthesis, not just iNOS activity.

Because nitric oxide is unstable in vivo, nitric oxide synthase activity in vivo is usually measured by either of two different markers. Either nitrate/nitrite levels are measured because these are produced through the degradation of nitric oxide and its oxidant product, peroxynitrite, or L-citrulline levels are measured. Because nitrate/nitrite levels are elevated by diets including such foods as green vegetables and sausages and other preserved meat and fish products, it is essential to closely control the diet of people for whom nitrate/nitrite measurements are to reflect NOS activity. Citrulline, the coproduct of nitric oxide synthase activity, is stable and its levels are much less likely to be perturbed by diet, as compared with nitrate/nitrite, and may therefore be a preferred measurement for this reason. In a recent study by Larson et al. (7), the citrulline levels in the cerebrospinal fluid of fibromyalgia patients were reported to be elevated, allowing them to infer that nitric oxide synthesis was elevated in the central nervous systems of fibromyalgia patients as compared with controls. In the current study, the serum levels of citrulline of CFS patients are compared with those of controls and also found to be elevated.

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