Thyrotrophin releasing hormone degradation by rat synaptosomal peptidases: production of the metabolite His-Pro

Multifunctional drug treatment in neurotrauma

Although the concepts of secondary injury and neuroprotection after neurotrauma are experimentally well supported, clinical trials of neuroprotective agents in traumatic brain injury or spinal cord injury have been disappointing. Most strategies to date have used drugs directed toward a single pathophysiological mechanism that contributes to early necrotic cell death. Given these failures, recent research has increasingly focused on multifunctional (i.e., multipotential, pluripotential) agents that target multiple injury mechanisms, particularly those that occur later after the insult. Here we review two such approaches that show particular promise in experimental neurotrauma: cell cycle inhibitors and small cyclized peptides. Both show extended therapeutic windows for treatment and appear to share at least one important target.

The development of two fluorimetric assays for the determination of pyroglutamyl aminopeptidase type-II activity

Two fluorimetric assays for the determination of pyroglutamyl aminopeptidase type-II activity have been developed. The assays are based on hydrolysis of the quenched-fluorimetric substrate <Glu-His-Pro-7-amino-4-methylcoumarin. Following the removal of the N-terminal <Glu by pyroglutamyl aminopeptidase type-II, liberation of 7-amino-4-methylcoumarin from the metabolite His-Pro-7-amino-4-methylcoumarin is catalyzed by one of two methods: (i) the addition of partially purified bovine serum dipeptidyl aminopeptidase type-IV or (ii) by incubating the reaction mixture for up to 2 h at 80 degrees C, thus promoting the nonenzymatic cyclization of His-Pro-7-amino-4-methylcoumarin to cyclo His-Pro and free 7-amino-4-methylcoumarin. Pyroglutamyl aminopeptidase type-II from bovine brain is used to establish appropriate assay conditions. These fluorimetric assays offer expeditious alternatives to the existing radiolabeled thyrotropin-releasing hormone assays for the determination of PAPII activity.

Pharmacological and biochemical comparison of thyrotropin releasing hormone (TRH) and di-methyl proline-TRH on pituitary GH3 cells

1. The binding of [3H]-thyrotropin releasing hormone ([3H]-TRH) and [3H]-RX77368 (di-methyl proline TRH) and the ability of these peptides to stimulate phosphoinositide hydrolysis were investigated in the GH3 pituitary cell line. 2. For both peptides binding was found to be saturable with a single component (Hill slopes were, for TRH, 0.98 and for RX77368, 1.13). TRH bound with greater affinity than RX77368 Kd values were 16 nM and 144 nM respectively. Bmax values were 227 fmol mg-1 protein for TRH and 123 fmol mg-1 protein for RX77368. 3. The rank order of potency of a series of TRH analogues to inhibit binding was the same versus each peptide. However, unlike with saturation analysis, Hill slopes of all displacing ligands were less than 1.0 against both TRH and RX77368 suggesting either multiple binding sites, alteration of affinity state, negative co-operativity or some allosteric interaction. 4. Both peptides stimulated phosphoinositide hydrolysis in a dose-dependent fashion. TRH was more potent than RX77368, EC50 values were 7.9 +/- 1 nM and 96.3 +/- 3 nM respectively. 5. These in vitro data suggest that the greater in vivo potency of RX77368 is not the result of enhanced receptor affinity but is more probably due to its greater metabolic stability.

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.

Degradation of thyrotropin-releasing hormone and luteinising hormone-releasing hormone by enzymes of brain tissue

In this article, the enzymes of brain and associated tissues that can degrade thyrotropin-releasing hormone (TRH) and luteinising hormone-releasing hormone (LH-RH) are reviewed. As both TRH and LH-RH are considered to act as neurotransmitters or neuromodulators in the CNS, attention is paid to the subcellular location of the enzymes described and how their topographies and substrate specificities fit them to playing roles as inactivating agents for TRH and LH-RH or as regulators of intracellular concentrations of TRH and LH-RH. Consideration is also given to enzymes involved in biotransformation of TRH to secondary metabolites that exhibit biological activity and to enzymes involved in the metabolism of secondary metabolites.

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

Reversed-phase high-performance liquid chromatography with radioactive flow detection was utilized to investigate the catabolism of thyrotropin-releasing hormone (TRH) in central nervous system (CNS) tissues. Two different column/gradient solvent systems were tested: (1) octadecylsilane (ODS) with an acetic acid-acetonitrile gradient and (2) poly(styrenedivinylbenzene) (PRP-1) with a trifluoroacetic acid-acetonitrile gradient. Both systems used 1-hexanesulfonic acid as the second ion-pairing reagent and yielded excellent separation of TRH and its catabolic products, TRH acid, cyclo(histidyl-proline), histidyl-proline, proline, and prolinamide, produced in CNS tissue homogenates. The PRP-1 column with a trifluoroacetic acid-acetonitrile solvent system produced a better and more reproducible separation of TRH catabolic products than the ODS column with the acetic acid-acetonitrile solvent system. This PRP-1 technique was utilized to demonstrate different rates and products of TRH catabolism in mouse and human spinal cord compared with cerebral cortex.

Proline residues in the maturation and degradation of peptide hormones and neuropeptides

The proteases involved in the maturation of regulatory peptides like those of broader specificity normally fail to cleave peptide bonds linked to the cyclic amino acid proline. This generates several mature peptides with N-terminal X-Pro-sequences. However, in certain non-mammalian tissues repetitive pre-sequences of this type are removed by specialized dipeptidyl (amino)peptidases during maturation. In mammals, proline-specific proteases are not involved in the biosynthesis of regulatory peptides, but due to their unique specificity they could play an important role in the degradation of them. Evidence exists that dipeptidyl (amino)peptidase IV at the cell surface of endothelial cells sequesters circulating peptide hormones which are then susceptible to broader aminopeptidase attack. The cleavage of several neuropeptides by prolyl endopeptidase has been demonstrated in vitro, but its role in the brain is questionable since the precise localization of the protease is not clarified.