The RDF*/SPARQL* approach extends RDF and SPARQL with means to capture and to query annotations of RDF triples, which is a feature that is natively available in graph databases modeled as Labeled Property Graphs (LPGs). Hence, the approach presents a step towards making the different graph database models interoperable. This paper takes this step further by providing a solid theoretical foundation for converting LPGs into RDF* data and for querying LPGs using the query language SPARQL*. Regarding the latter, the contributions in this paper consider approaches that materialize the RDF* representation of the LPGs into an RDF*-enabled triplestore as well as approaches in which the queried LPGs may reside in an LPG-specific database system.