David had played an important role throughout the reign of his older brother and predecessor John IV. He had been given the courtly title of "despotes", which in Trebizond designated the heir to the throne. David had participated in his brother's expeditions against the Genoese, and also fulfilled various diplomatic tasks. In 1458 he ratified his brother's treaty with the Ottoman Sultan Mehmed II in Adrianople, and later the same year he conveyed his niece Theodora to her husband, Uzun Hassan of the Ak Koyunlu.