In the AST assay employing MDH, what is the reaction product?

• A. Malate plus NAD
• B. Pyruvate plus NADH
• C. Lactate plus NAD
• D. Oxaloacetate plus NADH

Answer: (A)

A key idea here is how the AST assay is read using a coupled enzyme step. AST produces oxaloacetate from aspartate via transamination. In the linked MDH step, oxaloacetate is reduced to malate using the cofactor NADH, which is oxidized to NAD+. So the products of this MDH reaction are malate and NAD+ (the oxidized form of NADH). The assay measures NADH consumption by monitoring the decrease in NADH absorbance at 340 nm, linking the enzyme activity to this coupled reaction. Therefore, the reaction product in this MDH-coupled step is malate plus NAD+.