Thermal stability of an extracellular proteinase from Pseudomonas fluorescens AFT 36

A metalloproteinase, isolated from a shaken milk culture of Pseudomonas fluorescens AFT 36 by chromatography in DEAE and CM-cellulose and Sephadex G-150, was very unstable in 0.1 M-phosphate buffer, pH 6.6, being completely denatured above 70 degrees C in 1 min. It was also unstable in a Ca-containing buffer (synthetic milk salts, SMS) between 50 and 60 degrees C (minimum at 55 degrees C), but stability was very high above 80 degrees C in this buffer. D-values were determined at 10 degrees C intervals in the range 70-150 degrees C in SMS from which a Z value of 31.9 degrees C and an Ea of 8.82 X 10(4) J mol-1 were calculated; the half-life at 150 degrees C was 9 s. Instability at 55 degrees C was due to autolysis as evidenced by gel electrophoresis, gel filtration and increase in 2,4,6-trinitrobenzenesulphonic acid-reactive amino groups. The extent of inactivation experienced at 80 degrees C was inversely related to the rate of heating to 80 degrees C, i.e. length of time spent in the neighbourhood of 55 degrees C. Addition of increasing concentrations of caseinate substrate reduced inactivation of the enzyme at 55 degrees C, presumably due to substrate binding. Attempts to stabilize the enzyme at 55 degrees C by addition of EDTA or by adjusting the reaction pH to 4.2, at which the enzyme has little proteolytic activity, were unsuccessful, although autolysis was prevented. Unlike the proteinase from Ps. fluorescens MC 60, AFT 36 proteinase did not inactivate itself on cooling to 55 degrees C from 80, 100 or 150 degrees C, but did regain autolytic activity on cooling to below 50 degrees C to an extent dependent on the duration of holding at the lower temperature. It is suggested that on heating to approximately 55 degrees C, a conformational change occurs which renders the enzyme susceptible to proteolysis by still active enzyme; at higher temperatures the enzyme, although susceptible to autolysis, is inactive; an active conformation is restored on cooling to below 50 degrees C.