  The Keplerian motion of accretion disks in Active Galactic Nuclei (AGN) is
usually believed to be generated by a heavy central mass. We investigate
accreting disk systems with polytropic gas in Keplerian rotation and obtain a
phenomenological formula that relates the Keplerian angular frequency to the
ratio of disk and central masses. Central mass approaches the Keplerian value,
if the inner boundary of a disk is close to the minimal stable orbit of a black
hole. These results are applied to NGC 4258, the unique AGN with a finely
measured Keplerian rotation curve of the central disk, with the conclusion that
its rotation curve is, in fact, determined by the central black hole. The mass
of the accretion disk exceeds 100 solar masses.
