Red cell cyanide.

The amount of cyanide released on acidifying whole blood was much greater than the total from the plasma and red cells assayed separately, and varied directly with plasma thiocyanate concentrations. This artifactual formation of cyanide by whole blood was reduced by exposure to carbon monoxide. Incubation of haemolysates of washed red blood cells showed that optimum cyanide production from thiocyanate occurred at pH 4.5 with none at pH 7.4; spectrophotometric study confirmed the involvement of haemoglobin. It is doubtful whether this activity of haemoglobin is of normal physiological importance and it is probable that the evidence for the enzyme, 'thiocyanate oxidase' was based on this artifact of assay. Plasma cyanide is probably the metabolically relevant measurement, since the red blood cells act as a 'cyanide sink'. Where values for whole blood cyanide are required we recommend separate assay of plasma and saline washed red blood cells.