Hormon-Rezeptor-Beziehungen: Synthese und Eigenschaften von [Phenylalanin, (4,5-dehydro-4,5-ditritio)-norvalin4]-adrenocorticotropin-(1–24)-tetrakosipeptid

Enzymatic synthesis of peptides containing unnatural amino acids

Several proteases were studied as potential catalysts for the enzymatic synthesis of oligopeptides containing the unnatural amino acid allylglycine, the overall objective being the synthesis of a reactive tetrapeptide that could be chemically polymerized into a potentially biocompatible or biodegradable material. Commercially available enzymes were screened for esterase activity toward the methyl ester of the amino acid allylglycine (DL-AgOMe) to identify potential catalysts for dipeptide synthesis. Proteases from Aspergillus oryzae and Aspergillus sojae, pronase E and protease Nagarse synthesized the protected dipeptide Cbz-allylglycine-phenylalaninamide (Cbz-L-Ag-L-PheNH2) from Cbz-DL-AgOMe and L-PheNH2. However, the same enzymes were not able to catalyze the synthesis of Cbz-phenylalanine-allylglycine ethyl ester (Cbz-L-Phe-L-AgOEt). Thus, although these enzymes could use allylglycine as the acyl donor they could not employ it as the acyl acceptor in peptide synthesis. In contrast, chymotrypsin was able to use allylglycine ethyl ester (DL-AgOEt) as the acyl acceptor in the synthesis of Cbz-L-Phe-L-AgOEt, but was not able to synthesize Cbz-L-Ag-L-PheNH2. The two dipeptides, Cbz-allylglycine-phenylalanine and phenylalanine-allylglycine ethyl ester, served as substrates for the thermolysin-catalyzed synthesis of the tetrapeptide Cbz-L-Ag-L-Phe-L-Phe-L-AgOEt.

Tritium labeling of gonadotropin releasing hormone in its proline and histidine residues

3,4-dehydroproline9-GnRH prepared by solid phase peptide synthesis was tritiated catalytically under various conditions yielding 3H-GnRH with specific radioactivities in the range from 35-60 Ci/mmol and full LH releasing activity in vitro. Using palladium/alumina catalyst, the tritiation of the double bond occurs within ten minutes. Investigation of the tritium distribution between the amino acid residues showed a remarkably high incorporation of tritium into the histidine residue (11 to 37%). On the basis of this observation, the tritium labeling of GnRH and angiotensin I by direct catalytic hydrogen-tritium exchange was found to be useful for the labeling of these peptides at remarkably high specific radioactivity.

Preparation and characterization of specifically tritiated adrenocorticotropin

The catalytic dehalogenation of iodinated derivatives of corticotropin in the presence of tritium was investigated. In 0.1 M acetic acid, complete and rapid removal of iodine was achieved in the presence of freshly prepared palladium or palladium oxide as catalyst, but the specific radioactivity of the product was only 10-20% of the theoretically attainable value. Synthetic human corticotropin containing a 3,5-diiodo tyrosine in position 23 in place of tyrosine was successfully dehalogenated in solvent mixture 0.1 M acetic acid: hexamethylphosphoramide: dimethylformamide (1 : 10 : 90, v/v) in the presence of palladium oxide and calcium carbonate. The product was obtained in 30% yield after purification by carboxymethyl cellulose chromatography. The tritiated hormone had a specific radioactivity of 46 Ci/mmol (80% of the theoretical value) and was as potent as synthetic human corticotropin in stimulating steroidogenesis and lipolysis.