  Context.- This paper is part of a series involving the AMIGA project
(Analysis of the Interstellar Medium of Isolated GAlaxies), which identifies
and studies a statistically-significant sample of the most isolated galaxies in
the northern sky. Aims.- We present a catalogue of nuclear activity, traced by
optical emission lines, in a well-defined sample of the most isolated galaxies
in the local Universe, which will be used as a basis for studying the effect of
the environment on nuclear activity. Methods.- We obtained spectral data from
the 6th Data Release of the Sloan Digital Sky Survey, which were inspected in a
semi-automatic way. We subtracted the underlying stellar populations from the
spectra (using the software Starlight) and modelled the nuclear emission
features. Standard emission-line diagnostics diagrams were applied, using a new
classification scheme that takes into account censored data, to classify the
type of nuclear emission. Results.- We provide a final catalogue of
spectroscopic data, stellar populations, emission lines and classification of
optical nuclear activity for AMIGA galaxies. The prevalence of optical active
galactic nuclei (AGN) in AMIGA galaxies is 20.4%, or 36.7% including transition
objects. The fraction of AGN increases steeply towards earlier morphological
types and higher luminosities. We compare these results with a matched analysis
of galaxies in isolated denser environments (Hickson Compact Groups). After
correcting for the effects of the morphology and luminosity, we find that there
is no evidence for a difference in the prevalence of AGN between isolated and
compact group galaxies, and we discuss the implications of this result.
Conclusions.- We find that a major interaction is not a necessary condition for
the triggering of optical AGN.
