Elevated plasma concentrations of the endogenous nitric oxide synthase inhibitor asymmetric dimethylarginine in citrullinemia Thomas Lu ¨cke a, 4 ,1 , Dimitrios Tsikas b,1 , Nele Kanzelmeyer a , Bernhard Vaske c , Anibh Martin Das a a Department of Pediatrics, Medical School of Hannover, Carl Neuberg Str. 1, D-30625 Hannover, Germany b Institute of Clinical Pharmacology, Medical School of Hannover, D-30625 Hannover, Germany c Biometric Institute, Medical School of Hannover, D-30625 Hannover, Germany Received 10 April 2006; accepted 19 July 2006 Abstract Citrullinemia is an inborn error of the urea cycle with deficiency of the argininosuccinate synthase. It is characterized by elevated concentrations of l-citrulline and decreased levels of l-arginine in body fluids. Asymmetric dimethylarginine is an endogenous inhibitor of nitric oxide synthase that converts l-arginine to l-citrulline and nitric oxide (NO). Asymmetric dimethylarginine is hydrolyzed by the enzyme dimethylarginine dimethylaminohydrolase to l-citrulline and dimethylamine. Elevation of l-citrulline in citrullinemia prompted us to study the l-arginine/NO pathway in this disorder. In 8 children with citrullinemia (3 days to 3 years of age), elevated plasma levels of asymmetric dimethylarginine ( P = .028) were found compared with age-matched healthy children. We hypothesize that the l-arginine/NO pathway plays a role in the pathophysiology of citrullinemia. D 2006 Elsevier Inc. All rights reserved. 1. Introduction Citrullinemia (MIM 215700) is an inborn error of the urea cycle (autosomal recessive trait, chromosome 9q34) due to argininosuccinate synthase deficiency and results in dramat- ically elevated levels of l-citrulline, especially during catabolic spells. Citrullinemia is often associated with severe mental retardation after metabolic decompensation [1]. The mechanisms leading to cerebral damage in citrullinemia are incompletely understood. Ischemia, energy depletion, am- monia-associated neurotoxicity, and enhanced oxidative stress during metabolic decompensation are currently dis- cussed as potential factors contributing to the often poor neurologic outcome in citrullinemia [2]. A role of l-citrulline itself in the pathogenesis of citrullinemia is uncertain. By far the main source of l-citrulline in the body of patients with citrullinemia is the urea cycle. However, l-citrulline is also produced in the l-arginine/nitric oxide (NO) pathway [3]. The NO synthases (NOSs) are a family of enzymes that convert l-arginine to l-citrulline and NO [4]. The activity of NOS is effectively controlled by endogenous inhibitors, with the l-arginine analogue asym- metric dimethylarginine (ADMA) being the most important [5,6]. Asymmetric dimethylarginine is produced by meth- ylation of protein-associated l-arginine via N -methyl protein transferases [7]. After proteolysis, ADMA is released into the circulation from which it is eliminated in part by the kidney [7,8]. Another, much more abundant contributor to l-citrulline in the l-arginine/NO pathway involves the enzyme dime- thylarginine dimethylaminohydrolase (DDAH), which hydrolyzes ADMA to l-citrulline and dimethylamine (DMA) [7]. Rough estimates indicate that approximately 70% of daily ADMA production is eliminated by the kidney as DMA [9]. In brain tissue, ADMA and DDAH are abundantly present [7]. Nitric oxide is a gaseous, freely diffusible molecule with multiple physiologic functions including vasodilation, inhi- bition of platelet aggregation, and adhesion, as well as neurotransmission [10]. Prompted by its close connection to l-citrulline [3], we investigated in the present study the status of the l-arginine/NO pathway in citrullinemia. Nitric oxide synthesis was assessed by measuring nitrite and nitrate, the 0026-0495/$ – see front matter D 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.metabol.2006.07.020 4 Corresponding author. Tel.: +49 0511 532 3220; fax: +49 0511 532 8073. E-mail address: luecke.thomas@mh-hannover.de (T. Lu ¨cke). 1 These authors contributed equally to the study. Metabolism Clinical and Experimental 55 (2006) 1599 – 1603 www.elsevier.com/locate/metabol