Substance P Hydrolysis by Human Serum Cholinesterase

Abstract: Highly purified human serum cholinesterase (EC 3.1.1.8, also known as pseudocholinesterase and butyrylcholinesterase) had peptidase activity toward substance P. Digestion of substance P was monitored by high performance liquid chromatography, which separated three product peptides. The cleavages occurred sequentially. The first peptide to appear was Arg¹‐Pro². The K_m for this hydrolysis was 0.3 mM; maximum activity was 7.9 nmol min⁻¹ mg⁻¹ of protein, which corresponded to a turnover number of 0.6 min⁻¹. A second cleavage yielded Lys³‐Pro⁴. A third cleavage occurred at the C‐terminal, where the amide was removed from Met¹¹ to yield a peptide containing residues 5–11. Both the peptidase and esterase activities of the enzyme were completely inhibited by the anticholinesterase agent, diisopropyl‐fluorophosphate. Substance P inhibited the hydrolysis of benzoylcholine (a good ester substrate) with a K_I of 0.17 mM, indicating that substance P interacted with cholinesterase rather than with a trace contaminant. Peptidase and amidase activities for serum cholinesterase are novel activities for this enzyme. It was demonstrated previously that the related enzyme acetylcholinesterase (EC 3.1.1.7) catalyzed the hydrolysis of substance P, but at entirely different cleavage sites from those reported in the present work. Since butyrylcholinesterase is present in brain and muscle, as well as in serum, it may be involved in the physiological regulation of substance P.