On the Cycle of Synthesis of Arginine and Nitric Oxide in the Liver

Abstract

Studies performed in the two past decades showed that the simple molecule of nitric oxide (NO) is a versatile regulator of numerous biochemical processes in humans and animals. It is known that NO is synthesized from L -arginine and oxygen by special enzymes-NO synthases. Today, the most well-known are three isoforms of this enzyme: two constitutive (endothelial and neuronal NO synthases; eNOS and nNOS, respectively) and one inducible iNOS). The first two NO synthases, eNOS and nNOS, are usually activated by increased concentrations of cytosolic Ca 2+ , which stimulates NO production within several minutes. These enzymes synthesize small quantities of NO (no more than tens micromoles). The inducible isoform of NO synthase, which is expressed predominantly in immunocompetent cells and nearly in all cells of the liver after stimulation with mediators of immune or inflammatory reactions, produces NO at millimolar concentrations [1]. In these cells, NO is used as an effector molecule that is involved in elimination of various pathogens (bacteria, viruses, tumor cells, etc.). A characteristic feature of iNOS in these cells is that its induction (and, therefore, NO synthesis) continues for a long time after stimulation-for several hours or even days.For example, in experimental models of endotoxemia in vivo , iNOS mRNA expression was detected for 3 h after the addition of a stimulator, whereas the maximal accumulation of iNOA was observed after 12-16 h [2]. In human hepatocytes stimulated by cytokines and LPSs, a single band of mRNA was detected as early as 4 h after stimulation; it reached maximum 8 h after stimulation and became negligible 48 h after stimulation. The level of mRNA was compared to the activity of iNOS, which was estimated by the accumulation of and in culture supernatant. The amount of these anions increased in 20-30 times 24 and 48 h after stimulation [3]. It was shown that the induction of iNOS in macrophages, neutrophils, and E. coli may be NO 2 -NO 3 -caused by ascorbic acid and that one peritoneal macrophage can produce as many as 1.5 × 10 9 NO molecules [4][5][6][7]. With regard for the fact that one arginine molecule is consumed for the synthesis of one NO molecule, a question arises as to where such a substantial amount of arginine can be taken. At such a high level of consumption, arginine should be synthesized as required. In mammals, arginine is synthesized in the liver in the urea cycle from citrulline and ammonia. It is believed that thus formed arginine is rapidly cleaved by arginase to urea and ornithine [8]. Published data on the withdrawal of arginine from the urea cycle and its function as a substrate of iNOS are absent [1,9,10]. In view of this, a question arises as to how the organism satisfies its increased requirements for arginine. In this study, we obtained data suggesting the existence of a special cycle of synthesis of arginine and NO in liver cells.Materials and methods. Ammonium chloride, citrulline, and arginine used in the study were from Acros (United States); human leuk...

Skin organ culture as a model to study oxidative stress, inflammation and structural alterations associated with UVB‐induced photodamage

“…But they are in apparent contradiction with earlier reports that UVB irradiation increases NO production in keratinocytes by post-transcriptional activation of constitutive NOS species (26). The conflict can be resolved, however, if we assume that ferrousascorbate complexes promote nitrite decay to form NO, a reaction known to proceed up to pH 5.5 (70) and hence likely to occur in skin tissue (71). Alternatively, high levels of peroxynitrite could also deplete nitrite contents (72).…”

On the Cycle of Synthesis of Arginine and Nitric Oxide in Macrophages

“…Data on arginine stores in mammals are absent as well. The results of our previous study [11] allowed us to assume that there exists a special cycle of arginine and NO synthesis in the liver, in which citrulline produced by iNOS may be used again for the synthesis of L -arginine and NO formation. The existence of such a cycle in macrophages might explain the mechanism whereby immunocompetent cells satisfy the high demands on L -arginine required for the synthesis of NO.…”

“…Because arginine is synthesized in the urea cycle from citrulline and a source of nitrogen, we preliminarily demonstrated the possibility of existence of a minor cycle of synthesis of arginine and NO in [11]. In this study, we tested the possibility of existence of an independent cycle of synthesis of arginine and NO in mac-…”

“…For this purpose, all portions of macrophage suspension were mixed with fresh heparinized whole blood. Then, 20 and 40 h after the beginning of incubation, samples of macrophage suspensions were prepared in the form of columns (3 mm in diameter and 30 mm in height) frozen in liquid nitrogen [11]. EPR spectra of these columns were recorded in an ESP-300 Bruker Analitische Messtechnik spectrophotometer (Germany) equipped with a minicomputer at 77 K.…”

“…Then, 20 and 40 h after the beginning of incubation, samples of macrophage suspensions were prepared in the form of columns (3 mm in diameter and 30 mm in height) frozen in liquid nitrogen [11]. EPR spectra of these columns were recorded in an ESP-300 Bruker Analitische Messtechnik spectrophotometer (Germany) equipped with a minicomputer at 77 K.Because arginine is synthesized in the urea cycle from citrulline and a source of nitrogen, we preliminarily demonstrated the possibility of existence of a minor cycle of synthesis of arginine and NO in [11]. In this study, we tested the possibility of existence of an independent cycle of synthesis of arginine and NO in mac-…”