Phosphorylation of argininosuccinate synthase by protein kinase A

Argininosuccinate synthase (AS) is essential for endothelial nitric oxide (NO) production and its regulation in this capacity has been studied primarily at the transcriptional level. The dynamics of vascular function suggest that an acute regulation system may mediate AS function. This premise underlies our hypothesis that AS is phosphorylated in vascular endothelium. Immunoprecipitation and immobilized metal affinity chromatography demonstrated that AS is an endogenous phosphoprotein. An in vitro kinase screen revealed that protein kinase A (PKA), a kinase that enhances NO production via eNOS activation, phosphorylated AS. Vascular endothelial growth factor (VEGF) was identified as a candidate pathway for regulating AS phosphorylation since it enhanced NO production and activated PKA and eNOS. MDLA, an AS inhibitor, diminished maximal VEGF-mediated NO production. In addition, immunoprecipitation studies suggested that VEGF enhanced AS phosphorylation. Overall, these results represent the first demonstration that vascular endothelial NO production can be regulated by dynamic AS phosphorylation.