Citrulline occurrence

Particular ability of liver P450s3A to catalyze the oxidation of N-omega-hydroxyarginine to citrulline and nitrogen oxides and occurrence in NO synthases of a sequence very similar to the heme-binding sequence in P450s, Biochem. Biophys. Res. Commun. 192 (1993), p. 53-60... 

The ornithine cycle [Eq. (8)-(ll)] was proposed by Krebs and Henseleit (1932) to explain urea formation from ammonium derived from amino acid catabolism in the mammalian liver. Since the promulgation of the ornithine cycle was partly based on the natural occurrence of citrulline in juice of the watermelon Citrullus vulgaris) (Wada, 1930), it is not surprising that citrulline is an intermediate in arginine formation in higher plants. 

Ureides are a class of cyclic or acyclic nitrogenous organic compounds derived from or structurally related to urea. Representatives of this class which have been found in plants include allantoin, allantoic acid, citrulline, uric acid, hypoxanthine, xanthine, caffeine, hydroxycitruUine, and albizziine. The structures of allantoin (ALN), allantoic acid (ALA), and citrulline (CIT), ureides which occur throughout the plant kingdom, are presented in Fig. 1. These ureido compounds play an essential role in the assimilation, metabolism, transport, and storage of nitrogen in plants. In this chapter, attention will be focused on the occurrence, function, synthesis, and metabolism of these key metabolites. 

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