Lack of evidence for thyrotropin-releasing hormone deamidation in normal and hyperthyroid human sera

Two-hour incubations of human serum with 50 ng TRH or pyroglutamyl-histidyl-proline (TRH-OH) were performed under substrate conditions of 0.2 microgram substrate/ml serum. During incubations with normal serum, 46.3 +/- 1.3 (SEM) ng TRH were degraded while only 14.2 +/- 5.1 ng TRH-OH were degraded (P less than 0.001). During incubations with serum from patients with hyperthyroidism, 42.7 +/- 2.6 ng TRH were degraded compared to only 19.6 +/- 2.0 ng TRH-OH (P less than 0.001). Despite the fact that TRH degradation was significantly greater than TRH-OH degradation in both normal and hyperthyroid serum, no formation of TRH-OH (less than 3.1 ng/incubation tube) from TRH was detected. Formation of TRH-OH from TRH was also not noted when normal or hyperthyroid serum was incubated with TRH at substrate concentrations of 62.5 microgram/ml serum. These data confirm other reports that TRH deamidation does not occur in normal serum and extends this observation to hyperthyroid serum.