Rapid separation of tritiated thyrotropin-releasing hormone and its catabolic products from mouse and human central nervous system tissues by high-performance liquid chromatography with radioactive flow detection

High-pressure liquid chromatographic assay for the determination of thyrotropin-releasing hormone and its common metabolites in a physiological salt solution circulated through the isolated perfused rat lung

A specific and sensitive assay for TRH (L-pyroglutamyl-L-histidyl-L-proline amide), 3H-TRH (L-proline 3,4-3H(N), histidyl-3-3H(N)), and four possible metabolites of TRH, present in the recirculated perfusate of an isolated perfused rat lung preparation, was developed. Unlike previous methods, the method developed does not require extraction of the analytes from the biological matrix. The crude sample was adjusted to a pH of 3.2 with concentrated trifluoroacetic acid and injected on to a PRP-1 (polystyrene divinylbenzene) column (10 microns, 25 cm x 4.6 mm i.d.). The mobile phase was 10% v/v acetonitrile and 90% v/v 0.75 g l-1 1-hepantanesulfonic acid in 0.004 M trifluoroacetic acid, adjusted to a pH of 2.4 with concentrated NaOH. The flow rate was 0.5 ml min-1 and the analytes were detected by UV absorption at a wavelength of 26 nm and by radiochemical detection utilizing a liquid scintillation counter. The nominal retention times for L-PRO, L-PRO-NH2, TRH, cyclo(HIS-PRO) and TRH-OH were 4.0 +/- 0.9, 10.0 +/- 0.2, 15.5 +/- 0.4, 19.2 +/- 0.5 and 25.3 +/- 0.5 min respectively. The assay performs well in terms of precision and accuracy as indicated by linear regression and intra-assay variability analysis.

Kinetics and pattern of degradation of thyrotropin-releasing hormone (TRH) in human plasma

The kinetics and mechanism of degradation of the tripeptide TRH (pGlu-His-Pro-NH2) and its various primary and secondary degradation products (TRH-OH, His-Pro-NH2, and His-Pro) have been determined in human plasma at 37 degrees C. The rates of degradation of both TRH and TRH-OH (pGlu-His-Pro) showed mixed zero-order and first-order kinetics. At low substrate concentrations first-order kinetics occurred, with TRH and TRH-OH degradation half-lives of 9.4 and 27 min, respectively. The initial step in the plasma-catalyzed degradation of TRH is due to hydrolysis of the pGlu-His bond by the TRH-specific pyroglutamyl aminopeptidase serum enzyme, resulting in the exclusive formation of histidyl-proline amide (His-Pro-NH2). Using specific HPLC methods the major degradation route (67%) of this dipeptide in human plasma was hydrolysis of the peptide bond to yield His and Pro-NH2, whereas deamidation to yield His-Pro accounted for 29% of the total degradation. A minor pathway (less than or equal to 4%) was spontaneous cyclization to yield cyclo(His-Pro). Both His-Pro-NH2 and His-Pro degraded by first-order kinetics and faster than TRH, with half-lives of 5.3 and 2.2 min, respectively, in 80% plasma.