Endothelial nitric oxide production is tightly coupled to the citrulline-NO cycle.
Nitric oxide (NO) is an important vasorelaxant produced along with L-citrulline from L-arginine in a reaction catalyzed by endothelial nitric oxide synthase (eNOS). Previous studies suggested that the recycling of L-citrulline to L-arginine is essential for NO production in endothelial cells. However, there is no direct evidence demonstrating the degree to which the recycling of L-citrulline to L-arginine is coupled to NO production. We hypothesized that the amount of NO formed would be significantly higher than the amount of L-citrulline formed due to the efficiency of L-citrulline recycling via the citrulline-NO cycle. To test this hypothesis, endothelial cells were incubated with [14C]-L-arginine and stimulated by various agents to produce NO. The extent of NO and [14C]-L-citrulline formation were simultaneously determined. NO production exceeded apparent L-citrulline formation of the order of 8 to 1, under both basal and stimulated conditions. As further support, alpha-methyl-DL-aspartate, an inhibitor of argininosuccinate synthase (AS), a component of the citrulline-NO cycle, inhibited NO production in a dose-dependent manner. The results of this study provide evidence for the essential and efficient coupling of L-citrulline recycling, via the citrulline-NO cycle, to endothelial NO production.
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nitric oxide NO L-citrulline L-arginine NO L-citrulline L-arginine NO NO L-citrulline Nitric oxide L-citrulline citrulline NO L-citrulline [14C]-L-arginine NO NO [14C]-L-citrulline NO L-arginine argininosuccinate L-citrulline alpha-methyl-DL-aspartate nitric oxide citrulline-NO argininosuccinate synthase AS endothelial nitric oxide synthase eNOS citrulline NO NO L-citrulline citrulline NO NO