The invention provides a composition comprising a proteinase or an enzymatically active fragment thereof, said proteinase comprising the amino acid sequence of SEQ ID NO: 1 or comprising an amino acid sequence which is at least about 70% identical to SEQ ID NO: 1, wherein i) the concentration of free calcium in said composition is ≤about 80 μM; or ii) the concentration of monovalent salt in said composition is ≥about 20 mM. Under such conditions, the proteinases and enzymatically active fragments thereof are inducibly thermolabile. The invention further provides samples comprising one or more polypeptides and a proteinase or an enzymatically active fragment thereof, said proteinase comprising the amino acid sequence of SEQ ID NO: 1 or comprising an amino acid sequence which is at least about 70% identical to SEQ ID NO: 1, wherein i) the concentration of free calcium in said sample is ≤about 80 μM; or ii) the concentration of monovalent salt in said sample is ≥about 20 mM. The invention further provides methods comprising the inactivation of such proteinases or enzymatically active fragments thereof, wherein said method comprises the step of heating the sample to inactivate said proteinase or enzymatically active fragment, and wherein i) the concentration of free calcium in said sample is ≤about 80 μM; or ii) the concentration of monovalent salt in said sample is ≥about 20 mM.