  We produce a heteronuclear quantum degenerate mixture of two bosonic species,
$^{87}$Rb and $^{41}$K, in a three-dimensional optical lattice. On raising the
lattice barriers, we observe the disapperance of the inference pattern of the
heavier $^{87}$Rb, shifting toward shallower lattice depths in the presence of
a minor fraction of $^{41}$K. This effect is sizable and requires only a
marginal overlap between the two species. We compare our results with similar
findings reported for Fermi-Bose mixtures and discuss the interpretation
scenarios proposed to date, arguing that the explanation may be linked to the
increased effective mass due to the interspecies interactions.
