A method for measuring the unstable thromboxane A2: radioimmunoassay of the derived mono-O-methyl-thromboxane B2

A radioimmunoassay was developed for a mono-O-methyl derivative of thromboxane B2. The antibodies showed high specificity for this compound and cross reacted only 1.2% with thromboxane B2 and less than 0.1% with prostaglandins and prostaglandin metabolites. The method had a sensitivity of 7 picog. The radioimmunoassay was employed in studies where thromboxane A2 was generated in human platelets and immediately converted into mono-O-methyl thromboxane B2 by treatment of the sample with a large volume of methanol. In some of the experiments, thromboxane B2 was simultaneously measured by a separate radioimmunoassay. Using these two assays it was demonstrated that thromboxane A2 could be detected only during the earlier stages of the platelet aggregation, whereas thromboxane B2 rapidly reached a constant level. In a separate experiment, the half-life of thromboxane A2 in buffer was found to be 32.5 + 2.5 (S.D.) sec at 37 degrees C; the compound was more stable at lower temperatures. The t1/2 for thromboxane A2 was also considerably longer in plasma.