A low-molecular-weight inhibitor of the neutral proteinase from rat intestinal smooth muscle

Area of Science:

• Biochemistry
• Molecular Biology
• Physiology

Background:

• Rat intestinal smooth muscle contains endogenous inhibitory activity.
• This activity is directed against a previously described neutral trypsin-like muscle proteinase.
• Contamination by other inhibitors is unlikely.

Purpose of the Study:

• To characterize the endogenous inhibitory activity in rat intestinal smooth muscle.
• To isolate and purify the low-molecular-weight inhibitory component.
• To investigate the inhibitory mechanism and specificity.

Main Methods:

• Resolution of inhibitory activity into high- and low-molecular-weight components.
• Purification of the low-molecular-weight component to homogeneity.
• Enzyme inhibition assays to determine specificity and mechanism (competitive inhibition).

Main Results:

• A homogeneous low-molecular-weight inhibitor (approx. 9000 Da) was isolated, stable across a wide pH range (3-11).
• The inhibitor competitively inhibited the target muscle proteinase (Ki ≈ 1 µM) but not other tested proteinases.
• Leupeptin also competitively inhibited the muscle proteinase (Ki ≈ 0.3 µM), while other small proteins had minimal effect.

Conclusions:

• A specific, low-molecular-weight inhibitor of the rat intestinal muscle proteinase exists.
• This inhibitor is distinct from common proteinase inhibitors and small peptides.
• It may play a role in modulating intracellular protein degradation by the muscle proteinase.