Nitric oxide synthase products signal through interactions with heme centers in metalloproteins, such as guanylate cyclase and through covalent reactions with protein cysteine thiols. These nitrosylation reactions have broad relevance to eukaryotic cell biology in health and disease. However, nitrosylated proteins have often proven difficult to detect with the sensitivity and specificity needed for clinical and reseach samples. We are developing cavity ring-down spectroscopy to measure these protein modifications. We believe that this technology will dramatically improve both sensitivity and specificity it has the potential to will revolutionize the field. Here, we propose to design and fabricate an improved cavity ring-down instrument with a sample photolysis interface. This instrument will use advanced optical spectroscopy techniques to measure S- nitrosothiols in biological specimens. It will be used to studying a broad range of diseases and organ systems. It will be more powerful and more precise than previous technologies, measuring NO-modified proteins accurately in pM concentrations with minimal sample disruption. In the second Aim, it will also be applied to proteomic microarrays for rapid, high-throughput discovery.