There is no direct competition between arginase and nitric oxide synthase for the common substrate L-arginine

Aims: Extrahepatic arginases are postulated to be involved in cardiovascular-related pathologies by competing with nitric oxide synthase (NOS) for the common substrate L-arginine, subsequently decreasing nitric oxide production. However, previous models used to study arginase and NOS competition did not account for steady state level of L-arginine pool, which is dependent on conditions of L-arginine supply and utilization pathways. This work aimed at revisiting the concept of NOS and arginase competition while considering different conditions of L-arginine supply and L-arginine utilization pathways. Methods and results: Mouse macrophage-like RAW cells and human vascular endothelial cells co-expressing NOS and arginase were used to reevaluate the concept of substrate competition between arginase and NOS under conditions of L-arginine supply that mimicked either a continuous (similar to in vivo conditions) or a limited supply (similar to previous in vitro models). Enzyme kinetics simulation models were used to gain mechanistic insight and to evaluate the tenability of a substrate competition between the two enzymes. In addition to arginase and NOS, other L-arginine pathways such as transporters and utilization towards protein synthesis were considered to understand the intricacies of L-arginine metabolism. Our results indicate that when there is a continuous supply of L-arginine, as is the case for most cells in vivo, arginase does not affect NOS activity by a substrate competition. Furthermore, we demonstrate that L-arginine pathways such as transporters and protein synthesis are more likely to affect NOS activity than arginase. Conclusions: Arginase does not outcompete NOS for the common substrate L-arginine. Findings from this study should be considered to better understand the role of arginase in certain pathologies and for the interpretation of in vivo studies with arginase inhibitors.

Automated summary

Under continuous L-arginine supply conditions, arginase does not affect NOS activity through substrate competition. The study also demonstrates that L-arginine pathways such as transporters and protein synthesis are more likely to affect NOS activity than arginase.