Which carcinoplacental alkaline phosphatase isoenzyme is inhibited by phenylalanine?

• A. Regan
• B. Nagao
• C. Kasahara
• D. None of the above

Answer: (A)

Phenylalanine sensitivity is a diagnostic clue for identifying placental-type alkaline phosphatase isoenzymes. The Regan isoenzyme, a placental-type ALP associated with certain carcinomas, is inhibited by L-phenylalanine, so introducing phenylalanine reduces its activity. This specific inhibition profile helps distinguish Regan from other placental-like isoenzymes such as Nagao and Kasahara, which do not respond to phenylalanine in the same way. Therefore, observing a drop in ALP activity when phenylalanine is added points to the Regan isoenzyme being present, making it the best answer.