Proteins are carriers of biological functions and the effects of atmospheric-pressurenon-thermal plasmas on proteins are important to applications such as sterilizationand plasma-induced apoptosis of cancer cells. Herein, we report our detailedinvestigation of the effects of helium-oxygen non-thermal dielectric barrierdischarge (DBD) plasmas on the inactivation of lactate dehydrogenase (LDH) enzymesolutions. Circular dichroism (CD) and dynamic light scattering (DLS) indicate thatthe loss of activity stems from plasma-induced modification of the secondarymolecular structure as well as polymerization of the peptide chains. Raising thetreatment intensity leads to a reduced alpha-helix content, increase in thepercentage of the beta-sheet regions and random sequence, as well as graduallydecreasing LDH activity. However, the structure of the LDH plasma-treated for 300seconds exhibits a recovery trend after storage for 24 h and itsactivity also increases slightly. By comparing direct and indirect plasmatreatments, plasma-induced LDH inactivation can be attributed to reactive species(RS) in the plasma, especially ones with a long lifetime including hydrogenperoxide, ozone and nitrate ion which play the major role in the alteration of themacromolecular structure and molecular diameter in lieu of heat, UV radiation, andcharged particles.