The Szasz assay for GGT uses which substrates?

• A. Gamma-glutamyl-p-nitroanilide and glycylglycine
• B. Gamma-glutamyl-p-nitroanilide alone
• C. p-nitroanilide and glycylglycine
• D. Gamma-glutamylglycine and p-nitroaniline

Answer: (A)

GGT activity is measured by a transfer reaction: the gamma-glutamyl group is moved from a donor substrate to an amino-acid acceptor, and the amount of product formed reflects the enzyme's activity. In the Szasz assay, the donor is gamma-glutamyl-p-nitroanilide and the acceptor is glycylglycine. When GGT acts, the gamma-glutamyl group is transferred to glycylglycine, producing gamma-glutamylglycine and releasing p-nitroaniline. The liberated p-nitroaniline is what’s measured spectrophotometrically, and its amount is proportional to GGT activity. Therefore, both gamma-glutamyl-p-nitroanilide and glycylglycine are the substrates used in this assay. Using only the donor would leave the transfer incomplete, and using other substances that aren’t the proper donor or acceptor does not reflect the enzyme’s activity in this assay.