Mo. Rev. Stat. § 408.070

In actions for the enforcement of liens upon personal property subjected to a security agreement to secure indebtedness, or to maintain or secure possession of property so subjected to a security agreement, or in any other case when the validity of such lien is drawn in question, proof upon the trial that the party holding or claiming to hold the lien has received or exacted usurious interest for the indebtedness shall render any security agreement of personal property, or any lien whatsoever thereon given to secure the indebtedness, invalid and illegal.