The Zabbix 2.0 API does not appear to support a selectDiscoveryRule parameter for itemprototype.get to output details of the parent discovery rule. Without this there doesn't appear to be any non-trivial way to determine a itemprototype's parent discovery rule.
Both triggerprototype.get and graphprototype.get support a selectDiscoveryRule, it would appear this has been omitted for itemprototype.get