Direct tritium-labelling into histidine of luteinizing hormone-releasing hormone agonists and use of the tracers in studies on proteolytic breakdown.

Analogs of luteinizing hormone-releasing hormone (LHRH) having higher biological activity than LHRH itself are being mainly used to study the biological effects and the mechanism of action of LHRH. In the present study, conditions for the direct 3H-labelling at the histidine residue of analogs of LHRH were worked out, circumventing the synthesis of precursor peptides for labelling. [D-Phe6,desGly10]-LHRH ethylamide and [D-Ser(But)6,desGly10]-LHRH ethylamide were tritiated by tritium gas and a 10% Pd/Al2O3 catalyst to high specific radioactivities. The labelled peptides are sufficiently stable to be used in biochemical studies. The degradability of the analogs by homogenates of various tissues of rats was compared with that of the native LHRH. The analogs were shown to be distinctly degradable, but to a lower extent. The kidney homogenate degrades the analogs [D-Phe6,desGly10]- and [D-Ser(But)6,desGly10]-LHRH ethylamide with 35 and 50%, respectively, of the velocity observed with LHRH, whereas the degradation velocity of the analogs by a homogenate of the hypothalamus and pituitary is only 10% of that of LHRH. It is suggested that the lower degradability of the analogs at peripheral sites and target sites (pituitary, ovary) explains partly their higher biological activity.