Gestational diabetes insipidus is caused by increased activity of which enzyme?

• A. Vasopressinase
• B. Adenylyl cyclase
• C. Amylase
• D. Lipase

Answer: (A)

Gestational diabetes insipidus happens because pregnancy increases the placental enzyme that breaks down vasopressin (antidiuretic hormone). Vasopressinase accelerates the degradation of vasopressin, so circulating levels fall and less ADH is available to act on the kidney. Vasopressin normally binds to V2 receptors in the collecting ducts and stimulates a cascade that raises cAMP, leading to insertion of aquaporin-2 channels and more water reabsorption. When vasopressin is rapidly degraded, ADH signaling drops, the kidneys reabsorb less water, and dilute, high-volume urine results. The enzyme responsible is vasopressinase, the placental vasopressin-degrading enzyme. The other enzymes listed aren’t involved in this pregnancy-related condition: adenylyl cyclase is part of the signaling pathway but isn’t increased here, and amylase and lipase are unrelated pancreatic enzymes.