Which of the following best describes the classification of enkephalin?

• A. Exogenous stimulant
• B. Endogenous opioid
• C. Intestinal peptide
• D. Enzyme substrate

Answer: (B)

Enkephalins are endogenous opioid peptides produced in the body that act as natural ligands for opioid receptors to modulate pain signals. They come from the precursor proenkephalin and include forms like met-enkephalin and leu-enkephalin, which primarily bind to delta and mu receptors to produce analgesia and other opioid-like effects. This makes them best described as endogenous opioids. They’re not external stimulants, not simply intestinal peptides, and while enzymes can degrade them, that doesn’t define their primary classification.