  The strangeness-exchange reaction pi + Y -> K- + N is shown to be the
dynamical origin of chemical equilibration for K- production in heavy-ion
collisions up to beam energies of 10 A GeV. The hyperons occurring in this
process are produced associately with K+ in baryon-baryon and meson-baryon
interactions. This connection is demonstrated by the ratio K-/K+ which does not
vary with centrality and shows a linear correlation with the yield of pions per
participant. At incident energies above AGS this correlation no longer holds
due to the change in the production mechanism of kaons.
