Photoinstability of S-nitrosothiols during sampling of whole blood: a likely source of error and variability in S-nitrosothiol measurements.

BACKGROUND The determination of reference intervals for the concentration of total S-nitrosothiols (RSNOs) in blood is a highly controversial topic, likely because of the inherent instability of these species. Most currently available techniques to quantify RSNOs in blood require considerable sample handling and multiple pretreatment steps during which light exposure is difficult to completely eliminate. We investigated the effect of brief light exposure on the stability of RSNO species in blood during the initial sampling process. METHODS A novel amperometric RSNO sensor, based on an immobilized organoselenium catalyst at the distal tip of an electrochemical nitric oxide detector, was used to determine RSNO species in diluted whole blood without centrifugation or pretreatment. Porcine blood was collected into aluminum foil-wrapped syringes via a 12-inch butterfly needle tube assembly. Two blood samples were collected from the same animal -- one with the butterfly needle tubing wrapped in aluminum foil and one with the tubing exposed to ambient room light. The RSNO concentrations in these sequential blood samples were determined by a standard addition procedure. RESULTS Eight sets of measurements were made in 6 animals. Samples exposed to light yielded RSNO concentrations only 23.6% (7.2%) [mean (SD)] of the RSNO concentrations determined in samples that were shielded from light and obtained from the same animals. CONCLUSIONS These results suggest significant photoinstablity of RSNOs in whole blood and indicate the critical importance of proper light protection during sampling and processing of blood samples for the accurate determinations of endogenous RSNO concentrations.