Macroenzymes have a higher half-life because they are less excreted in the urine. Which statement is true?

• A. Less excreted in urine
• B. More excreted in urine
• C. Equally excreted
• D. Excretion not related

Answer: (A)

Macroenzymes are large complexes, typically formed when an enzyme binds to a large protein such as an immunoglobulin. Their increased size prevents efficient renal filtration, so they are cleared from the bloodstream more slowly and remain in circulation longer, giving them a longer half-life. That’s why the true statement is that they are less excreted in urine. If they were excreted more, clearance would be faster and half-life shorter; if excretion were the same or not related, it wouldn’t explain the extended presence of macroenzymes in serum.