THERMAL INACTIVATION OF SHRIMP DEOXYRIBONUCLEASE WITH AND WITHOUT SODIUM DODECYL-SULFATE

Due to the differences in sample treatment, purified shrimp DNase migr ates to different positions in sodium dodecyl sulfate (NaDod-SO4) poly acrylamide gel electrophoresis. DNase molecules migrating to some of t hese positions are enzymatically active as revealed by the DNase activ ity stain in situ. When the sample is not heated, DNase molecules (For m I) migrate to a position corresponding to an apparent M(r) of 22000 and are stain DNase-active. When the sample is heated with NaDod-SO,, DNase molecules (Form II) migrate to the apparent M(r) of 39000 positi on and are also DNase-active. In contrast, when the sample is heated w ithout NaDod-SO4, DNase molecules (Form II') migrate to the same posit ion as Form II but are DNase-inactive. When the sample is heated in th e presence of beta-mercaptoethanol (with or without NaDod-SO4), the in active Form III is generated. Form III must be the fully extended poly peptide because its apparent M(r) is very close to the true M(r) of DN ase. These treated samples were also analyzed for DNase activity by di luting DNase-NaDod-SO4 into the DNase (hyperchromicity) assay solution . The results are consistent in that only Forms I and II are DNase-act ive, However, Form I differs from Form II in the time required for exp ression of DNase activity. After dilution of NaDod-SO4, Form II requir es approx. 60 min to attain full activity while Form I is active immed iately. Form II itself is inactive. The activity found at the M(r) 390 00 position is due to conversion of Form II to the active Form I, as r evealed by two-dimensional NaDod-SO4 gel electrophoresis. The sample h eated without NaDod-SO, soon contains active Form II, indicating that Form II is an intermediate during the production of Form II'. When For m II' is heated with NaDod-SO4, it does not refold back to the native state through Form II, suggesting that Form II' is irreversibly denatu red. Thus, the anomalous behaviors of shrimp DNase in NaDod-SO, gel el ectrophoresis have been utilized to study, in the presence and absence of NaDod-SO4, the thermal unfolding, as well as the refolding to the active enzyme during cooling and NaDod-SO4 removal.