[Protein binding of thromboxane--one of the sources of error in thromboxane determination]

The determination of thromboxane A2 (TxA2), the main metabolite of arachidonic acid (AA) in platelets, is very difficult due to extreme instability of this compound. Only stable end products such as TxB2, the hydrolysis product of TxA2, and malondialdehyde (MDA) can be analysed by means of routine techniques. Since the presence of proteins interferes with the photometric determination of MDA, these have to be eliminated by acidification. The TxB2 concentration was measured in the supernatant in parallel with MDA determinations before and after protein precipitation. A significant difference was found between these two TxB2 concentrations. The TxB2 values after precipitation were significantly lower than before precipitation, indicating that a high percentage (about 70%) of TxB2 is bound to proteins. Protein precipitation in a biological sample might, thus, cause misinterpretation of the measured value.