Select the coupling enzyme used in the kinetic AST reaction of Henry.

In the kinetic AST (aspartate aminotransferase) reaction, malate dehydrogenase serves as the coupling enzyme, which is instrumental in the assay's methodology. The role of malate dehydrogenase in this context is crucial because it catalyzes the conversion of oxaloacetate to malate, utilizing NADH in the process. This reaction is coupled with the AST reaction to continuously regenerate the substrates needed for measuring AST activity.

The use of malate dehydrogenase allows for the accurate monitoring of the decrease in absorbance associated with the oxidation of NADH, which correlates directly with the AST activity in the sample being tested. This coupling effectively enhances the sensitivity and specificity of the assay, making it a reliable method for determining enzyme activity.

Other enzymes mentioned, like lactate dehydrogenase or glutamate dehydrogenase, do not play a role in the kinetic measurement of AST within this specific methodology, and glucose-6-phosphate dehydrogenase is not involved in the reactions relevant to AST coupling for the kinetic approach. Therefore, malate dehydrogenase is the key coupling enzyme for the kinetic AST reaction described.