Fig. 1. Pathway for NO synthesis. This pathway produces around 60% of the arginine in the kidney, compared to the endothelium’s 15% de novo arginine synthesis [28,32]. Arginine not only serves as a substrate for the production of NO, but it also aids the kidney’s production of urea [33]. The process of converting arginine to citrulline and urea is catalyzed by the enzyme arginase. As a component of the urea cycle, the cytosolic enzyme arginase is mostly expressed in the liver. A mitochondrial enzyme predominantly expressed in the kidneys is arginine II. Arginase I is present in human erythrocytes and is still active in plasma. In addition, urea and arginine are separated by arginase, resulting in ornithine [34]. At the same time, citrulline generated in the urea cycle can be converted back to arginine through the processes of argininosuccinate synthase and argininosuccinate lyase (ASL) [32]. Abbreviations mentioned in Fig. 1., ASL: Argininosuccinate Lyase; ASS: Argininosuccinate Synthase; BH4: Tetrahydrobiopterin; FNM: Flavin Mononucleotide; FAD: Flavin Adenine Dinucleotide; NO2- : KG: Kinase G Nitrite; NO3-: Nitrate; TCA: Tricarboxylic Acid; OTC: Ornithine Transcarbamylase; NADPH: Nicotinamide Adenine Dinucleotide Phosphate.

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