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 activities that catalyse the first two steps of lysine degradation through the saccharopine pathway in upper eukaryotes. This paper describes the isolation and characterization of a cDNA clone encoding a bifunctional enzyme bearing domains corresponding to these two enzymic activities. We partly purified those activities from mouse liver and showed for the first time that both 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 in liver and kidney. The longest molecule, 3.4 kb in size, corresponds to the isolated cDNA and encodes the bifunctional enzyme. The 2.4 kb short transcript probably codes for the monofunctional dehydrogenase. Sequence analyses show that the bifunctional enzyme is likely to be a mitochondrial protein. Furthermore, enzymic and expression analyses suggest that lysine-oxoglutarate reductase/saccharopine dehydrogenase levels increase in livers of mice under starvation. Lysine-injected mice also show an increase in lysine-oxoglutarate reductase and saccharopine dehydrogenase levels.

Regulation of oxidative degradation of l-lysine in rat liver mitochondria

The generation of 14CO2 from [1-14C]lysine by hepatic mitochondria through the saccharopine pathway is controlled by intramitochondrial concentrations of lysine, 2-oxoglutarate and NADPH. Mitochondria, isolated from rats pre-treated with glucagon, exhibited higher activities of L-lysine: 2-oxoglutarate reductase, saccharopine dehydrogenase and 2-aminoadipate aminotransferase. The flux through this pathway is stimulated in liver mitochondria after glucagon treatment. Multiple regulation of lysine oxidation in liver mitochondria confirms a complex mechanism of ‘mitochondrial activation’ by glucagon.