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Supplement: 6^th Amino Acid Assessment Workshop: SESSION 1

Regulation of Nitric Oxide Synthesis and Apoptosis by Arginase and Arginine Recycling^1–3,

Laboratory of Molecular Genetics, Faculty of Pharmaceutical Sciences, Sojo University, Kumamoto 860-0082, Japan

^* To whom correspondence should be addressed. E-mail: mori-m{at}ph.sojo-u.ac.jp.

Nitric oxide (NO) is synthesized from arginine and O₂ by NO synthase (NOS). Citrulline formed as a by-product of the NOS reaction can be recycled to arginine by argininosuccinate synthetase (AS) and argininosuccinate lyase (AL). We found that AS and sometimes AL are coinduced with inducible NOS (iNOS) in various cells. In these cells, NO was synthesized from citrulline (via arginine) as well as from arginine, indicating operation of the citrulline-NO cycle. On the other hand, we found that arginase isoforms (types I and II) are coinduced with iNOS by LPS in rodent tissues and cultured macrophages. K_m values for arginine of arginase I and II (10 mmol/L) are much higher than that of iNOS (5 µmol/L), whereas V_max of arginase I and II were 10³–10⁴ times higher than that of iNOS in activated macrophages. Thus, V_max/K_m values of arginases were close to that of iNOS, and these enzymes were expected to compete for arginine in the cells. In fact, NO production by iNOS in activated macrophages was decreased by coinduction of arginase I or arginase II. Low concentrations of NO protect cells from apoptosis, whereas excessive NO causes apoptosis. We found that NO causes endoplasmic reticulum (ER) stress, induces a transcription factor, CAAT/enhancer binding protein (C/EBP) homologous protein (CHOP), and leads to apoptosis. These results suggest that the arginine metabolic enzymes and the ER stress-CHOP pathway can be good targets to regulate NO production and NO-mediated apoptosis in diseases associated with overproduction or impaired production of NO.

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