Which pair of urea cycle enzymes functions within the mitochondrion?

• A Argininosuccinate lyase and arginase
• B Ornithine transcarbamylase and argininosuccinate synthetase
• C Carbamoyl phosphate synthetase I and ornithine transcarbamylase ✓
• D Carbamoyl phosphate synthetase I and argininosuccinate synthetase

Explanation

The first two reactions occur in the mitochondrion: CPS-I condenses ammonia with bicarbonate to form carbamoyl phosphate, and ornithine transcarbamylase transfers the carbamoyl group to ornithine forming citrulline, which then exits the mitochondrion. The remaining three enzymes, argininosuccinate synthetase, lyase, and arginase, are cytosolic. This compartmentation explains why OTC deficiency produces urinary orotic acid while CPS-I deficiency does not.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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