The study of biological nitric oxide

Single molecule detection of nitric oxide enabled by d(AT)15 DNA adsorbed to near infrared fluorescent single-walled carbon nanotubes

We report the selective detection of single nitric oxide (NO) molecules using a specific DNA sequence of d(AT)(15) oligonucleotides, adsorbed to an array of near-infrared fluorescent semiconducting single-walled carbon nanotubes (AT(15)-SWNT). While SWNT suspended with eight other variant DNA sequences show fluorescence quenching or enhancement from analytes such as dopamine, NADH, L-ascorbic acid, and riboflavin, d(AT)(15) imparts SWNT with a distinct selectivity toward NO. In contrast, the electrostatically neutral polyvinyl alcohol enables no response to nitric oxide, but exhibits fluorescent enhancement to other molecules in the tested library. For AT(15)-SWNT, a stepwise fluorescence decrease is observed when the nanotubes are exposed to NO, reporting the dynamics of single-molecule NO adsorption via SWNT exciton quenching. We describe these quenching traces using a birth-and-death Markov model, and the maximum likelihood estimator of adsorption and desorption rates of NO is derived. Applying the method to simulated traces indicates that the resulting error in the estimated rate constants is less than 5% under our experimental conditions, allowing for calibration using a series of NO concentrations. As expected, the adsorption rate is found to be linearly proportional to NO concentration, and the intrinsic single-site NO adsorption rate constant is 0.001 s(-1) μM NO(-1). The ability to detect nitric oxide quantitatively at the single-molecule level may find applications in new cellular assays for the study of nitric oxide carcinogenesis and chemical signaling, as well as medical diagnostics for inflammation.

Half-life of nitric oxide in aqueous solutions with and without haemoglobin

Nitric oxide (NO) has been linked to many regulatory functions in mammalian cells. Studies of NO release are hampered by the short half-life of the molecule. In the blood, NO disappears within seconds because it binds avidly with haemoglobin (Hb). The relationship between Hb concentration and NO disappearance, however, has not been described. In this study we utilized an amperometric NO sensor (WPI, Sarasota, FL) to monitor continuously the disappearance of NO from an aqueous solution when Hb (free or as red blood cells) was added. The calibration and linearity of the NO sensor was checked frequently using a chemical reaction to generate a known concentration of NO. An aliquot of NO solution (prepared from authentic gas) was added to a glass beaker containing 20 ml saline to generate NO concentration of approximately 1200 nM. Under our experimental conditions (PO2 = 40 mmHg), NO concentration fell slowly over 20 min with a half-life of 445 s. However, when haemoglobin was added, NO disappeared rapidly in proportion to Hb concentration. The results suggest that rapid binding of NO to Hb occurs in a 4:1 ratio. The maximum rate constant of NO disappearance due to binding with Hb was 2 x 10(5) M-1 s-1. The 4:1 binding ratio between NO:Hb may be used as a tool to quantitate NO release in some biological assays. The study supports the notion that NO acts as an autocoid because it disappears rapidly in the presence of Hb and is not likely to act as a circulating humoral substance. The NO sensor was useful for monitoring of NO concentration in Hb free solutions, but its response time limits its use in blood.

NO-synthase: what can research on invertebrates add to what is already known?

The present study attempts to review presently known data regarding the distribution of nitric oxide (NO) synthase and the function of NO in invertebrate species. NO is synthesized from L-arginine by the enzyme NO-synthase, and activates guanylate cyclase which in turn leads to an increase in levels of cGMP in target cells. Major contributions to the knowledge of NO as a messenger molecule in invertebrates have been made by NADPH-diaphorase histochemistry and biochemical assays. These techniques suggest the presence of a L-arginine/NO pathway in a variety of tissues, thus implicating multiple roles for NO in invertebrates.