  We have selected a sample of 292 SDSS Narrow-Emission-Line galaxies known to
have formed and evolved in relative isolation to study the nature and origin of
the AGN phenomenon. The galaxies in our sample have line fluxes with S/N>3 and
were separated using a standard diagnostic diagram into Star Forming Galaxies
(SFGs; 36.0%), Transition type Objects (TOs; 28.4%) and Narrow-Line AGNs
(NLAGNs; 35.6%). Having found a strong correlation between the bulge mass and
the NLAGN phenomenon, we have applied the same relation as for the Broad-Line
AGNs to estimate their black hole (BH) masses. The BH in the NLAGNs are 2 to 3
orders lower in mass than the BHs found in BLAGNs, but are comparable to those
observed in Narrow-Line Seyfert~1, although none of our objects can be
classified as such. To determine the metallicities, [O/H], of the NLAGNs we
calibrated the standard diagnostic diagram [OIII]/Hb vs. [NII]/Ha using similar
relation as for the SFGs, which reproduce the values obtained with CLOUDY
simulations developed for Bennert et al. [3]. For some individual objects we
compared our line ratios with other CLOUDY similations by different authors.
This suggests we achieve a typical uncertainty of 0.2 dex on [O/H], increasing
to 0.3-0.5 in the Seyfert~2 (S2). This calibration suggests the metallicities
of the NLAGNs are subsolar, varying between 1 and 0.3 $Z_\odot$. We find two
statistically significant positive correlations: for [O/H] with the BH mass and
for [O/H] with the luminosity at 5100\AA, $\lambda L(5100$\AA$)$. No
correlation is found between [O/H] and the accretion rate, L_bol/L_Edd.
However, comparisons with the BLAGNs suggest the NLAGNs extend the
metallicity-accretion rate relationship [24] to the low metallicity regime.
Although the NLS1 have similar BH masses as the NLAGNs they show higher
accretion rates, which is consistent with their higher metallicities.
