Irreversible stimulation of Xenopus melanophores by photoaffinity labelling with p‐azidophenylalanine¹³‐α‐melanotropin

Synthesis and biological properties of p‐azidophenylalanine13‐β‐melanotropin, a potent photoaffinity label for MSH receptors

“…An other explanation, however, assuming a higher degree of labelling and hence either a more rapid receptor turnover or desensitization or an increased inactivation of cyclic AMP, would have to postulate a relatively large number of spare receptors that would have to be labelled for the generation of the longlasting stimulation. Although the existence of spare receptors cannot be ruled out, it appears unlikely that even at low temperatures receptor turnover is up to lOxhigher than originally estimated [3]. Whether the longlasting stimulation may produce a higher phosphodiesterase activity and hence an increased inactivation of cyclic AMP (which has to be compensated by labelling of spare receptors), cannot finally be decided at present.…”

“…In a first series of photoaffinity experiments, a specific and irreversible stimulation of the melanophores was demonstrated which lasted for several hours [3]. By increasing the temperature during the wash phase, covalent hormone-receptor complexes were progressively inactivated, which made an estimation of the receptor turnover possible [3]. We then examined in more detail how the longlasting stimulation is dependent on the hormone concentration used during photolysis.…”

“…Tail-fins of this species are very transparent because in that tissue the melanophores are the only cells that absorb light. In a first series of photoaffinity experiments, a specific and irreversible stimulation of the melanophores was demonstrated which lasted for several hours [3]. By increasing the temperature during the wash phase, covalent hormone-receptor complexes were progressively inactivated, which made an estimation of the receptor turnover possible [3].…”

“…Under these conditions [Papi3]-a-MSH was covalently inserted into MSH-receptors which produced a longlasting pigment dispersion in Xenopus melanphores (see [3]). The extent of this prolonged stimulation depended on the hormone concentration used during photolysis.…”

Covalent linking of photoreactive gonadotropin-releasing hormone to gonadotropes produces a prolonged signal.

“…2 and 3) (11). Photoinduced covalent binding of a photoreactive insulin derivative to rat adipocytes caused a prolonged signal of lipogenesis (12), and an irreversible stimulation of melanophores was observed upon photolysis of Xenopus skins in the presence of a photoreactive a-melanocyte-stimulating hormone (13). These studies (11)(12)(13) and the report that removal of surface-bound human choriogonadotropin from cultured Leydig tumor cells resulted in a rapid cessation of both cAMP and progesterone production (14) suggest that the formation of the hormone-receptor complex on the cell surface is the primary event responsible for triggering the physiological response.…”