Mo. Rev. Stat. § 408.060
Usury may be pleaded as a defense in civil actions in the courts of this state, and upon proof that usurious interest has been paid, the same, in excess of the legal rate of interest, shall be deemed payment, shall be credited upon the principal debt, and all costs of the action shall be taxed against the party guilty of exacting usurious interest, who shall in no case recover judgment for more than the amount found due upon the principal debt, with legal interest, after deducting therefrom all payments of usurious interest made by the debtor, whether paid as commissions or brokerage, or as payment upon the principal, or as interest on said indebtedness; provided, however, that no corporation shall, after this section takes effect, interpose the defense of usury in any such action, nor shall any bond, note, debt, contract or obligation of any corporation or any security therefor, be set aside, impaired or adjudged invalid by reason of the rate of interest which the corporation may have paid or agreed to pay hereon.
§ 408.060, RSMo