Istanbul vs metropolis

Istanbul and metropolis both are nouns.

Word	Noun	Adjective	Verb	Adverb
Istanbul	Yes	No	No	No
metropolis	Yes	No	No	No

Istanbul (noun)	metropolis (noun)
the largest city and former capital of Turkey; rebuilt on the site of ancient Byzantium by Constantine I in the fourth century; renamed Constantinople by Constantine who made it the capital of the Byzantine Empire; now the seat of the Eastern Orthodox Church	people living in a large densely populated municipality
	a large and densely populated urban area; may include several independent administrative districts