Enzymatic measurement of saccharopine with saccharopine dehydrogenase.

We have developed an enzymatic method for measuring saccharopine, a key intermediate in lysine metabolism. With the enzyme saccharopine dehydrogenase, saccharopine can be oxidized to lysine and alpha-ketoglutarate with the corresponding conversion of NAD to NADH. The natural equilibrium favors saccharopine formation, but using hydrazine to trap one of the products, alpha-ketoglutarate, shifts the reaction toward quantitative oxidation of saccharopine. A stable endpoint is reached in 15-20 min, and although high concentrations of alpha-ketoglutarate slow the reaction, the end product is fully recovered. Unlike previous assays this technique is specific, convenient, and capable of measuring saccharopine directly in protein-free biological fluids or extracts.