Nitric Oxide Supplementation in Argininosuccinic Aciduria

This is a study involving a dietary supplement. Patients with argininosuccinic aciduria will be randomly assigned to receive either a nitric oxide dietary supplement or placebo for 2 weeks, and then crossed-over to receive the other treatment for two weeks. The investigators expect to see that : 1) Patients with ASA will have a decreased ability for their arteries to dilate due to nitric oxide deficiency, 2) Treatment of ASA with the nitric oxide supplement will improve the ability of their arteries to dilate, and 3) Through the testing of subjects\' fibroblasts (cells in connective tissue that produce collagen and other fibers), the investigators hope to predict which patients may respond NO supplementation.