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

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.

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.

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.