In the field of enzymology, a dimethylargininase, also known as a dimethylarginine dimethylaminohydrolase (DDAH), is an enzyme that catalyzes the chemical reaction: N-omega,N-omega'-methyl-L-arginine + H2O dimethylamine + L-citrulline Thus, the two substrates of this enzyme are N-omega,N-omega'-methyl-L-arginine and H2O, whereas its two products are dimethylamine and L-citrulline. Isozymes Dimethylarginine dimethylaminohydrolase is an enzyme found in all mammalian cells. Two isoforms exist, DDAH I and DDAH II, with some differences in tissue distribution of the two isoforms). The enzyme degrades methylarginines, specifically asymmetric dimethylarginine (ADMA) and NG-monomethyl-L-arginine (MMA). Function The methylarginines ADMA and MMA inhibit the production of nitric oxide synthase. As such, DDAH is important in removing methylarginines, generated by protein degradation, from accumulating and inhibiting the generation of nitric oxide. Clinical significance Inhibition of DDAH activity causes methylarginines to accumulate, blocking nitric oxide(NO) synthesis and causing vasoconstriction. An impairment of DDAH activity appears to be involved in the elevation of plasma ADMA, and impairment of vascular relaxation observed in humans with cardiovascular disease or risk factors (such as hypercholesterolemia, diabetes mellitus, and insulin resistance). The activity of DDAH is impaired by oxidative stress, permitting ADMA to accumulate. A wide range of pathologic stimuli induce endothelial oxidative stress such as oxidized LDL-cholesterol, inflammatory cytokines, hyperhomocysteinemia, hyperglycemia and infectious agents. Each of these insults attenuates DDAH activity in vitro and in vivo. The attenuation of DDAH allows ADMA to accumulate, and to block NO synthesis. The adverse effect of these stimuli can be reversed in vitro by antioxidants, which preserve the activity of DDAH. The sensitivity of DDAH to oxidative stress is conferred by a critical sulfhydryl in the active site of the enzyme that is required for the metabolism of ADMA. This sulfhydryl can also be reversibly inhibited by NO in an elegant form of negative feedback. Homocysteine (a putative cardiovascular risk factor) mounts an oxidative attack on DDAH to form a mixed disulfide, inactivating the enzyme. By oxidizing a sulfhydryl moiety critical for DDAH activity, homocysteine and other risk factors cause ADMA to accumulate and to suppress nitric oxide synthase (NOS) activity. The critical role of DDAH activity in regulating NO synthesis in vivo was demonstrated using a transgenic DDAH mouse. In this animal, the activity of DDAH is increased, and plasma ADMA levels are reduced by 50%. The reduction in plasma ADMA is associated with a significant increase in NOS activity, as plasma and urinary nitrate levels are