Ubiquitous presence of argininosuccinate at millimolar levels in the central nervous system of Aplysia californica

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Endogenous nitric oxide (NO) is generated by nitric oxide synthases (NOSs), which convert arginine (Arg) and oxygen to citrulline (Cit) and NO. Cit can be enzymatically transformed back to Arg by argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL) via a pathway involving argininosuccinate (ArgSuc). Arg, Cit, and ArgSuc levels have been measured in single neurons, neuronal clusters, and neuropil from the nervous system of the common neurobiological model Aplysia californica. Using capillary electrophoresis with laser-induced fluorescence detection, ArgSuc was found to be present in the nervous system in millimolar concentrations at levels significantly exceeding Cit levels (p<0.01). ArgSuc levels are proportional to Arg concentrations in single neurons, whereas they have no clear correlation to the Cit or Arg/Cit ratio. NOS-expressing neurons often exhibit fixative-resistant nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) staining. Incubation of ganglia with Arg results in an increase in Cit and ArgSuc levels in the NADPH-d-positive neuropil with no effect on ArgSuc levels in NADPH-d-negative neurons, suggesting NOS activity in the neuropil. Similar incubation with Cit leads to decreased ArgSuc levels in NADPH-d-negative neurons. These results can be explained by localization of NOS and ASS in different neurons; therefore, the complete Arg-Cit-NO cycle may not be present in the same neuron. The surprisingly high intracellular ArgSuc concentration suggests alternative sources of ArgSuc and that at least a portion may be formed by the reverse reaction of ASL (catalyzing the conversion of Arg to ArgSuc), which can be inhibited by Cit.