Humans lack efficient ________, so we rely on commensal bacteria to convert dietary nitrate into nitrite, which is then further reduced to NO.

Humans lack efficient ________, so we rely on commensal bacteria to convert dietary nitrate into nitrite, which is then further reduced to NO.

• A. Nitrite reductase enzymes
• B. Nitrate transporters
• C. Nitrate reductase enzymes
• D. Nitrile hydratase

Answer: (C)

The essential idea is that the first step in converting dietary nitrate to nitric oxide is the reduction of nitrate to nitrite, and that this step is carried out by nitrate reductase enzymes in commensal bacteria. Humans don’t have efficient nitrate reductase activity in our own tissues, so oral bacteria that express nitrate reductases perform NO3− → NO2−. From there, nitrite can be further reduced to NO by other pathways, including bacterial nitrite reductases or chemical reduction in the stomach, which then provides nitric oxide for signaling. Nitrite reductase would act downstream, turning nitrite into NO, not initiating the process. Nitrate transporters help bring nitrate into cells but don’t catalyze reduction. Nitrile hydratase targets nitriles, not nitrate or nitrite, so it’s not involved here.

The essential idea is that the first step in converting dietary nitrate to nitric oxide is the reduction of nitrate to nitrite, and that this step is carried out by nitrate reductase enzymes in commensal bacteria. Humans don’t have efficient nitrate reductase activity in our own tissues, so oral bacteria that express nitrate reductases perform NO3− → NO2−. From there, nitrite can be further reduced to NO by other pathways, including bacterial nitrite reductases or chemical reduction in the stomach, which then provides nitric oxide for signaling.

Nitrite reductase would act downstream, turning nitrite into NO, not initiating the process. Nitrate transporters help bring nitrate into cells but don’t catalyze reduction. Nitrile hydratase targets nitriles, not nitrate or nitrite, so it’s not involved here.