Citrulline formation

Hobbs and coworkers111 used a specific chemiluminescent reaction to measure NO. With this they determined that the reaction measured by citrulline formation was not affected by SOD but that NO was increased. 

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). 

Also, in this case, research results indicate the need for caution in the selection of starter cultures for MLF in wine, since citrulline formation from arginine degradation could result in ethyl carbamate production, even at normal temperatures, during prolonged storage. In addition, spontaneous MLF by undeflned strains should be avoided, as this may lead to formation of ethyl carbamate precursors (Liu etal. 1994). 

Mira de Orduna, R., Liu, S.-Q., Patchett, M.L., and Pilone, GJ. 2000. Ethyl carbamate precursor citrulline formation from arginine degradation by malolactic wine lactic acid bacteria. FEMS Microbiol. Lett. 183, 31-35. 

Although the equilibrium constant strongly favors citrulline formation, the reaction is reversible. Citrulline... 

In a recent study carried out in our laboratory, it was demonstrated that preincubation of brain cytosolic preparations in a complete reaction mixture led to a time-dependent loss of NOS activity (Mittal and Jadhav, 1994). This inhibition was not attributable to denaturation or proteolysis, since the preparations contained protease inhibitors and the rates of l-citrulline formation were linear up to 8 min at 37°C (Fig. 5). The omission of individual ingredients of the reaction mixture from the preincubation phase revealed the requirement of calcium ion for the inhibitory phenomenon (Table I). However, the concentrations required for the optimal inhibitory effect were significantly higher (1 mM) than those required for the catalytic activity (100 /zM) of NOS. 

The question as to the number of moles of ATP required for each step and the over-all reaction is still unsettled. Ratios of citrulline formation to inorganic phosphate formed have been reported for the reactions 33 and... 

L-Ornithine transcarbamoylase catalyzes transfer of the carbamoyl group of carbamoyl phosphate to ornithine, forming citrulline and orthophosphate (reaction 2, Figure 29-9). While the reaction occurs in the mitochondrial matrix, both the formation of ornithine and the subsequent metabolism of citmlline take place in the cytosol. Entry of ornithine into mitochondria... 

Argininosuccinate synthase links aspartate and citrulline via the amino group of aspartate (reaction 3, Figure 29-9) and provides the second nitrogen of urea. The reaction requires ATP and involves intermediate formation of citrullyl-AMP. Subsequent displacement of AMP by aspartate then forms citrulline. 

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

Cohen and Grisolia then concentrated on the first step in the reaction, obtaining citrulline from ornithine. The reaction appeared to depend on oxygen, a requirement traced to the need for high concentrations of ATP. Physiologically the formation of urea occurs at very low levels of ammonia, which is extremely toxic as it is also lipid soluble and enters cells very easily. Cells are not very effectively buffered against OH. ... 

An anomaly associated with citrulline that became evident when detailed kinetic studies were made in the 1950s (R.B. Fisher and J.R. Bronk) was the irreproducibility of its catalytic activity in liver slices on the formation of urea, despite the clear evidence from Ratner and Petrack of its importance in arginine synthesis. Initially the discrepancy in catalytic activity between ornithine and citrulline was ascribed to the possible impermeability of the liver cell plasma membrane to the latter intermediate, a hypothesis which was rapidly disproved experimentally. Only recently has it been shown that ornithine transcarbamylase is clearly associated with the ornithine/... 

Figure 1.1 PTMs found on the first 30 residues of the human histone proteins. Only acetylation, methylation, deimination (formation of citrulline) and phosphorylation are shown.

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. 

Several lines of investigation assert to the inability of canal ine to function as an effective ornithine antagonist. Ornithine interaction with canaline has been evaluated with the ornithine carbamoyl transferase (EC 2.1.3.3) of human liver. Neither canal ine nor ornithine inhibited this enzyme when the other member of this set served as the carbamoyl group recipient (29). The ornithine antagonist, 2,4-diaminobutyric acid drastically reduced urea production in the rat this reflected curtailment of the ornithine carbamoyl transferase-mediated conversion of ornithine to citrulline. Yet, canaline had no such effect on urea formation in this mammal (30). 

Canaline is a potent inhibitor of all seven pyridoxal phosphate-containing enzymes studied by Rahiala et (27) but it lacks adverse effects on three ornithine-utiTTzing enzymes lacking a Bg cofactor. Finally, in jack bean, Canavalia ensiformis, ornithine carbamoyl transferase can form 0-ureido-L-homoserine from canaline and carbamoyl phosphate as it does citrulline from ornithine and carbamoyl phosphate. Nevertheless, neither compound inhibited formation of the reaction products (31). 

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