Lysine degradation through the saccharopine pathway in mammals: involvement of both bifunctional and monofunctional lysine-degrading enzymes in mouse

Lysine-oxoglutarate reductase and saccharopine dehydrogenase are enzymic ac tivities that catalyse the first two steps of lysine degradation through th e saccharopine pathway in upper eukaryotes. This paper describes the isolat ion and characterization of a cDNA clone encoding a bifunctional enzyme bea ring domains corresponding to these two enzymic activities. We partly purif ied those activities from mouse liver and showed for the first time that bo th a bifunctional lysine-oxoglutarate reductase/saccharopine dehydrogenase and a monofunctional saccharopine dehydrogenase are likely to be present in this organ. Northern analyses indicate the existence of two mRNA species i n liver and kidney. The longest molecule, 3.4 kb in size, corresponds to th e isolated cDNA and encodes the bifunctional enzyme. The 2.4 kb short trans cript probably codes for the monofunctional dehydrogenase. Sequence analyse s show that the bifunctional enzyme is likely to be a mitochondrial protein . Furthermore, enzymic and expression analyses suggest that lysine-oxogluta rate reductase/saccharopine dehydrogenase levels increase in livers of mice under starvation. Lysine-injected mice also show an increase in lysine-oxo glutarate reductase and saccharopine dehydrogenase levels.