Which compound is present in saliva at approximately 50 μM in free form and is an amino acid?

• A. Alanine
• B. Glycine
• C. Arginine
• D. Proline

Answer: (C)

The question is about which amino acid appears in saliva as a free molecule at about 50 μM. Arginine fits this profile well because it is commonly found in the free amino acid pool of saliva at tens of micromolar concentrations. This reflects its entry into saliva through glandular secretion and its relatively stable free form in the oral environment, where it can serve as a substrate for bacterial arginine deiminase pathways (which raise pH and influence caries risk) and as a potential contributor to nitric oxide–related processes in the oral cavity. In contrast, the other amino acids, while present in saliva, typically do not have as consistent a free-form concentration near 50 μM and may be found more as part of peptides or in variable, context-dependent amounts. Thus, arginine is the best match for a free-form salivary amino acid at about 50 μM.