In the last years, several distributed system paradigms have emerged, aiming at the share of a large amount of resources among a large number of users. The Virtual Organizations concept in a multi-institutional context presents obviously numerous opportunities for business applications as well as for scientific ones. Nevertheless, the current trends seem to lead to several independent specialized grids in opposition to the early vision of one generic world wide grid. In such a context, large scale heterogeneous databases must be seen as several specialized databases homed in different grids. The challenge is to allow users of these grids to access all databases with the same efficiency that if they are supported by the same grid. Our proposal is an overlay software architecture built on top of grid middlewares which acts on behalf of the user to facilitate the use of inter-grid databases. The proposed mediation solution deals with database heterogeneity and access rights management and is capable of making decisions regarding the selection of resources according to their homing grid and infrastructure performances.