During starvation, skeletal muscle releases alanine and glutamine as the major amino acids for interorgan nitrogen transport. Alanine reaches the liver where it undergoes transamination. The nitrogen eventually enters the urea cycle. Which two enzymes in the urea cycle are located in the mitochondrial matrix, while the remaining enzymes are cytosolic?

• A Arginosuccinate synthetase and arginosuccinate lyase
• B Arginase and ornithine transcarbamylase
• C Carbamoyl phosphate synthetase I and arginosuccinate synthetase
• D Carbamoyl phosphate synthetase I and ornithine transcarbamylase ✓

Explanation

The first two steps of the urea cycle occur in the mitochondrial matrix: (1) carbamoyl phosphate synthetase I (CPS I) converts NH4+ + CO2 + 2ATP to carbamoyl phosphate, and (2) ornithine transcarbamylase (OTC) transfers the carbamoyl group to ornithine forming citrulline. Citrulline then exits the mitochondria; subsequent steps (arginosuccinate synthetase, arginosuccinate lyase, arginase) are cytosolic.

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

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