We investigated the effect of high-pressure treatment on the properties of cytoplasmic 5′-nucleotidase (NT), which converts inosine monophosphate (IMP) into inosine. After pressure treatment at 400 MPa, the activity of purified IMP-NT remained at almost 100%, but the activity of partially purified adenosine monophosphate (AMP)-NT decreased to about 40%. These data suggest that there is a difference in the pressure stability between the enzymes. In situ fluorescence spectroscopy of IMP-NT under pressure showed that its pressure-induced denaturation was reversible. When the pressure was reduced from the highest pressure to ambient pressure, hysteresis was observed. This suggests that high pressure treatment may lead to a partial change in the affinity of the subunits for each other once they have dissociated. The activities of IMP-NT and AMP-NT extracted from pressure-treated muscles decreased remarkably between 250 and 450 MPa, but IMP-NT was more stable than AMP-NT.
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