In the AST determination using the MDH step, which enzyme is used?

In the AST determination using the MDH step, which enzyme is used?

• A. Lactate dehydrogenase (LDH)
• B. Malate dehydrogenase (MDH)
• C. Glutamate dehydrogenase (GDH)
• D. Isocitrate dehydrogenase (IDH)

Answer: (B)

In this assay, AST activity is measured using a coupling step with malate dehydrogenase. AST converts aspartate and α-ketoglutarate to oxaloacetate and glutamate. Oxaloacetate is then reduced to malate by malate dehydrogenase, using NADH and converting it to NAD+. The decrease in NADH (monitored by absorbance at 340 nm) reflects the amount of oxaloacetate formed, and thus the AST activity. So the enzyme used in this MDH step is malate dehydrogenase. The other enzymes would not couple the AST product to NADH in the same way (lactate dehydrogenase acts on pyruvate, while GDH and IDH are not part of this coupling sequence).

In this assay, AST activity is measured using a coupling step with malate dehydrogenase. AST converts aspartate and α-ketoglutarate to oxaloacetate and glutamate. Oxaloacetate is then reduced to malate by malate dehydrogenase, using NADH and converting it to NAD+. The decrease in NADH (monitored by absorbance at 340 nm) reflects the amount of oxaloacetate formed, and thus the AST activity. So the enzyme used in this MDH step is malate dehydrogenase. The other enzymes would not couple the AST product to NADH in the same way (lactate dehydrogenase acts on pyruvate, while GDH and IDH are not part of this coupling sequence).