Enzymological studies of melanotropins

1. 1. The relative stability of natural melanotropins and related synthetic analogues to serum and purified proteolytic enzymes was studied. 2. 2. Both α- and β-MSH were rapidly inactivated by frog serum, but much more slowly by rat serum. β-MSH was more stable than α-MSH to serum inactivation. 3. 3. Both α- and β-MSH were rapidly inactivated by α-chymotrypsin and trypsin. 4. 4. The synthetic analogues, [Nle⁴, d-Phe⁷]-α-MSH and and [Cys⁴,Cys¹⁰]-α-MSH, were totally resistant to inactivation by frog and rat serum enzymes. 5. 5. [Nle⁴, d-Phe⁷]-α-MSH was resistant to inactivation by α-chymotrypsin and trypsin, whereas [Cys⁴,Cys¹⁰]-α-MSH, was partially resistant to these enzymes under similar conditions. 6. 6. Melanotropin analogues resistant to inactivation by serum enzymes may prove useful in a variety of physiological studies wherein natural melanotropins would be rapidly inactivated.