L-citrulline

Three years later Robert F Furchgott discov ered that the relaxing of smooth muscles such as blood vessel walls was stimulated by an unknown substance produced in the lining of the blood vessels (the endothelium) He called this substance the endothelium-dependent relaxing factor or EDRF and in 1986 showed that EDRF was NO Louis J Ignarro reached the same conclusion at about the same time Further support was provided by Salvador Moncada who showed that endothelial cells did in deed produce NO and that the l arginine to l citrulline conversion was responsible... 

FIGURE 22.4 Mechanism of nitric oxide and L-citrulline production by NO synthases through the intermediate formation of iV-hydroxyl-L-arginine. 

L-arginine A/G-hydroxy-L-arginine L-citrulline nitric oxide... 

Ginseng s mechanism of vasorelaxation and nitric oxide release is probably by conversion of L-arginine to L-citrulline (Kim H et al. 1992). Ginseng saponin induces relaxation of the corpus cavernosum smooth muscle in a dose-dependent manner (Kim HJ et al. 1998). This effect is... 

Fig. 1. The overall reaction catalyzed by NOS and a schematic representation of the three main NOS isoforms. L-Arg is first converted to JV-hydroxy-L-Arg in a P450-like monooxygenation reaction requiring two electrons and one O2 molecule. In the second step, Ai-hydroxy-L-Arg to NO and L-citrulline, only one electron and one O2 molecule are required. nNOS, neuronal NOS (neural system) iNOS, inducible NOS (immune system) eNOS, endothelial NOS (cardiovascular system).

This enzyme [EC 1.14.13.39] catalyzes the reaction of l-arginine with 1.5 NADPH, 0.5 H, and two dioxygen to produce L-citrulline, nitric oxide, 1.5 NADP, and two water. The enzyme in brain, but not that induced in lung or liver by endotoxin, requires calcium ions. 

This enzyme [EC 2.1.3.3], also known as ornithine car-bamoyltransferase and citrulline phosphorylase, catalyzes the reaction of carbamoyl phosphate with L-orni-thine to produce L-citrulline and orthophosphate. 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS CITRAMALATE LYASE CITRAMALYL-GoA LYASE GITRATE LYASE GITRATE SYNTHASE L-Citrulline,... 

C. Nitric oxide is an important compound that acts as a biological messenger in many physiological responses. L-Citrulline is a product of the oxidation of L-argenine in the formation of nitric oxide. Bradykinin is formed from a precursor kininogen. 

Schematic illustration of the conversion of L-arginine to NO plus L-citrulline by the enzyme NO synthase. Conversion requires the presence of NADPH, calcium (Ca), calmodulin (CM), and O2. Calcium complexes with CM and the Ca-CM complex binds to the enzyme. The asterisk signifies the basic amino nitrogen atom that undergoes oxidation and cleavage to form NO. Both of the basic amino nittogens are equivalent and either nitrogen can be incorporated into NO.

There are cytosolic and membrane-bound isoforms of NO synthase. Certain soluble and particulate isoforms are constitutive and other soluble isoforms are inducible. The constitutive enzyme is, by definition, present in the catalytically active form and needs only to be stimulated by an appropriate chemical species, following which there is immediate formation of NO plus L-citrulline. This form of NO synthase requires calcium, and for the most part calmodulin, for stimulation of enzymatic activity. It is likely that an increase in intracellular free calcium in the presence of calmodulin is the signal for stimulation of NO synthase, and therefore, the production of NO. This view is consistent with the general understanding that, in vascular tissue, all endothelium-dependent vaso-... 

This observation led to the finding that a soluble enzyme from rat forebrain catalyzes the NADPH-dependent formation of NO and L-citrulline from L-ar-ginine by calcium-dependent mechanisms (Knowles et al., 1989). A subsequent study revealed that glutamate and NMDA stimulate NO and L-citrulline formation in rat cerebellar slices and that this is associated with a concomitant stimulation of cyclic GMP formation (Bredt and Snyder, 1989), thus establishing a link between NMDA-receptor activation and cyclic GMP formation (Fig. 4). These investigators went on to purify and characterize NO synthase from cerebellum (Bredt and Snyder, 1990 Bredt et al., 1991) and to show the localization of NO synthase in the brain to discrete neuronal populations (Bredt et al., 1990). 

Schematic illustration of the interrelationships between glutamate and NO in synaptic function in the cetebellum. The presynaptic nerve terminal synthesizes, stores, and releases glutamate (G) as the neurotransmitter by exocytosis as illustrated. The glutamate diffu.ses across the synaptic cleft and interacts with postsynaptic NMDA recepti>rs ( ) that are coupled to calcium (Ca ) channels. Ca influx occurs and the free intracellular Ca complexes with calmtxlulin and activates NO synthase. NADPH is also required hir conversion, and the products of the reaction are NO plus L-citrulline. NO diffuses out of the piistsynaptic cell to interact with nearby target cells, one of which is the presynaptic neuron that released the glutamate in the first place. NO stimulates cytosolic guanylate cyclase and cyclic GMP (cGMP) formation presynaptically, hut the consequence of this pre.synaptic modification is unknown.

In 1987, Hibbs published a landmark paper in the nitric oxide field (Hibbs et al., 1987a), by demonstrating that the L-arginine that is required for CAM cytostatic activity is metabolized to L-citrulline (similar to L-arginine but less one nitrogen atom from the guanidinium group) and nitrite plus nitrate. In addition,... 

Nitric oxide synthase is an unusual enzyme in that substrates with even numbers of electrons (L-arginine, O2, NADPH) are transformed into an odd-electron product (NO) in addition to even-electron products (L-citrulline, NADP,... 

---