  In this contribution we report our recent investigation of the gas
metallicity in active galactic nuclei and its dependence on luminosity and
redshift. We compile large spectroscopic datasets of broad-line and narrow-line
AGNs, and compare them with the results of our photoionization models. Through
the analysis of both the broad and the narrow emission-line regions, we find
that: (1) for a given luminosity, there is no redshift dependence of the gas
metallicity; (2) for a given redshift, there is a significant correlation
between gas metallicity and luminosity; (3) the luminosity-metallicity relation
does no show any evolution in the redshift range 2 < z < 4.
