The BRENDA tissue ontology (BTO) represents a comprehensive structured encyclopedia. It provides terms, classifications, and definitions of tissues, organs, anatomical structures, plant parts, cell cultures, cell types, and cell lines of organisms from all taxonomic groups (animals, plants, fungi, protozoon) as enzyme sources. The information is connected to the functional data in the BRENDA ("BRaunschweig ENzyme DAtabase“) enzyme information system.
BTO is one of the first tissue-specific ontologies in life sciences, not restricted to a specific organism or a specific organism group providing a user-friendly access to the wide range of tissue and cell-type information. Databases, such as Ontology Lookup Service or ses, such as MIRIAM Registry or  of the EBI-EMBL, the TissueDistributionDB, including the Tissue Synonym Library of the German Cancer Research Center (DKFZ) in Heidelberg or the Bioportal platform of the National Center for Biomedical Ontology in Stanford, USA rely on BTO and implement the encyclopedia as an essential repository of information into their respective platform.
BTO enables users from medical research and pharmaceutical sciences to search for the occurrence and histological detection of disease-related enzymes in tissues, which play an important role in diagnosis, therapies, and drug development. In biochemistry and biotechnology the organism-specific tissue terms linked to enzyme functional data are an important resource for the understanding of the metabolism and regulation in life sciences. Ontologies represent classification systems that provide controlled and structured vocabularies. They are important tools to illustrate and to link evolutionary correlations.
Development of BTO started in 2003, aimed to connect the biochemical and molecular biological enzyme data of BRENDA with a hierarchical and standardized collection of tissue-specific terms. The functional enzyme data and information in BRENDA have been manually annotated and structured by experts from biochemistry, biology, and chemistry. By October 2022, the BTO contained over  6,527 terms, linked to 6,065 synonyms and 5,474 definitions. The terms are classified under generic categories, rules, and formats of the Gene Ontology Consortium (GO,), organized as a directed acyclic graph (DAG) created using the open-source OBO-Edit. All terms from each level are directly connected the enzyme data in BRENDA. BTO is a suitable tool to distinguish between different enzymes which are expressed in a tissue-specific manner.