Two endogenous inhibitors of nitric oxide synthesis, N^G-monomethylarginine (MMA) and N^G, N^G-dimethylarginine (DMA), are broken down by N^G, N^G-dimethylarginine dimethylaminohydrolase (DDAH) [EC 3.5.3.18]. This enzymatic activity is widely distributed in rat tissues, and is correlated with the distribution of free MMA and DMA, and the production of nitric oxide. In this study, immunoblotting analyses with a monoclonal antibody directed towards rat kidney DDAH showed that immunoreactive proteins exist in human pancreas, kidney, and liver. Enzyme activity was definitely detected in these tissues, but the specific activity was lower than that in the corresponding rat tissue. These results are interesting in connection with the idea that the enzyme might play a role as a regulator of nitric oxide generation in human tissues.

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