Classical and Slow-Binding Inhibitors of Human Type II Arginase

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Abstract
Arginases catalyze the hydrolysis of l-arginine to yield l-ornithine and urea. Recent studies indicate that arginases, both the type I and type II isozymes, participate in the regulation of nitric oxide production by modulating the availability of arginine for nitric oxide synthase. Due to the reciprocal regulation between arginase and nitric oxide synthase, arginase inhibitors have therapeutic potential in treating nitric oxide-dependent smooth muscle disorders, such as erectile dysfunction. We demonstrate the competitive inhibition of the mitochondrial human type II arginase by Nω-hydroxy-l-arginine, the intermediate in the reaction catalyzed by nitric oxide synthase, and its analogue Nω-hydroxy-nor-l-arginine, with Ki values of 1.6 μM and 51 nM at pH 7.5, respectively. We also demonstrate the inhibition of human type II arginase by the boronic acid-based transition-state analogues 2(S)-amino-6-boronohexanoic acid (ABH) and S-(2-boronoethyl)-l-cysteine (BEC), which are known inhibitors of type I arginase. At pH 7.5, both ABH and BEC are classical, competitive inhibitors of human type II arginase with Ki values of 0.25 and 0.31 μM, respectively. However, at pH 9.5, ABH and BEC are slow-binding inhibitors of the enzyme with Ki values of 8.5 and 30 nM, respectively. The findings presented here indicate that the design of arginine analogues with uncharged, tetrahedral functional groups will lead to the development of more potent inhibitors of arginases at physiological pH.